Liquid crystal display device

Abstract

To provide a liquid crystal display device capable of suppressing reduction in image display function and sufficiently reducing power consumption, and to provide a method of driving the liquid crystal display device.SOLUTION: In the liquid crystal display device, in order to prevent application of an electric field on a liquid crystal before power is turned off, a fixed potential is input to a capacitive element to eliminate a potential difference between the electrodes of the capacitive element (sets capacity almost to zero), and the liquid crystal is set to an initial state. When power is stopped after an initial state image is displayed, the liquid crystal, in an off state, is stably kept in the initial state without any continued application of unnecessary electric fields. Thus, deterioration of the liquid crystal is prevented.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for driving a liquid crystal display device and a liquid crystal display device. [Background technology]

[0002] Thin film transistors (TFTs) are fabricated using semiconductor thin films formed on substrates with insulating surfaces. Thin film transistors are used in integrated circuits (ICs) and image display devices ( It is widely applied in electronic devices such as LCDs.

[0003] Electrical devices using thin film transistors include mobile phones and laptop personal computers. For such portable electronic devices, The problem of power consumption, which affects continuous operation time, is a major issue. It is important for all parties to curb the increase in power consumption that accompanies larger sizes.

[0004] In a display device, when rewriting image data input to pixels, the image of a continuous period is Even if the data is the same, the same image data is written again. Therefore, even if the image data is the same, the power consumption is reduced by writing the image data multiple times. In order to suppress the increase in power consumption of such a display device, In image display, after scanning the screen once and writing image data, there is a non-scanning period. There have been reported techniques for providing a rest period longer than the normal period (for example, Patent Document 1 and Non-Patent Document 2). (See Reference 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] U.S. Patent No. 7,321,353 [Non-patent literature]

[0006] [Non-Patent Document 1] K. Tsuda et al. IDW'02 Proc., p.295-298 Summary of the Invention [Problem to be solved by the invention]

[0007] However, after scanning the screen once and writing the image data, there is a pause longer than the scanning period. In the display method where a still image is displayed for a certain period of time, a certain voltage is applied to the liquid crystal. This causes the liquid crystal to deteriorate, resulting in a problem of a decrease in image display function. Furthermore, if the image data remains, the image may not be displayed even if the power of the display device is turned off. There is a risk that image data may remain on the surface.

[0008] Therefore, it is an object of the present invention to prevent the deterioration of the image display function of a liquid crystal display device. It shall be one.

[0009] Also provided is a liquid crystal display device that can achieve low power consumption and a method for driving a liquid crystal display device. One of our goals is to provide [Means for solving the problem]

[0010] The liquid crystal display device operates when power is supplied and becomes inoperative when power is stopped. In this specification, the state in which power is supplied to the liquid crystal display device (power-on state) is referred to as The ON state is called the ON state, and the OFF state is called the OFF state when the power supply is stopped (power off state). A control signal for turning on the display device is called a start signal, and a control signal for turning off the display device is called a stop signal. Boo.

[0011] The liquid crystal element provided in the liquid crystal display device is composed of a pixel electrode, a common electrode, and a liquid crystal layer provided between the electrodes. By applying different potentials to the pixel electrode and the common electrode, a voltage is applied to the liquid crystal element. When a voltage is applied to the liquid crystal element, an electric field is formed, and the liquid crystal is Respond by displaying an image.

[0012] On the other hand, if the same potential is applied to the pixel electrode and the common electrode, no potential difference occurs between the electrodes, and the liquid crystal Therefore, no electric field is formed in the liquid crystal element, and no electric field is applied to the liquid crystal. In this specification, the liquid crystal is referred to as a state where no electric field is applied (non-responsive state). The liquid crystal in this state (answer state) is called the initial state (initial state of liquid crystal).

[0013] The liquid crystal in the initial state is turned on by receiving a start signal, and the display device is turned on. In a display device that responds to a field by displaying an image and then switches off after receiving a stop signal, Return to the initial state.

[0014] The liquid crystal display device disclosed in this specification stores electric charges in a capacitance element, and the electric charges are used to charge a liquid crystal element. The liquid crystal display device has a pixel configuration in which a voltage applied to the pixel is maintained to maintain a displayed image. In the ON state of the device, the switching element electrically connected to the capacitance element and the liquid crystal element is It is preferable that the semiconductor element has a low current value in the off state (off current value).

[0015] If the semiconductor element has a low off-state current, electric charges are less likely to leak from the capacitor element via the semiconductor element. Therefore, the voltage applied to the liquid crystal element can be maintained for a long time. It may be a display device.

[0016] On the other hand, in the pixel where the supply of power is stopped and the pixel is in the OFF state, the electric field is applied and the liquid crystal is in the responsive state. For the crystal to return to its initial state, the charge held in the capacitance element must be released through the semiconductor element. Since an electric field continues to be applied to the liquid crystal while the charge in the capacitance element is being discharged, If the time is too long, the liquid crystal will deteriorate quickly. The image is also retained, so the image is not lost, especially in the case of reflective LCD devices that use external light as a light source. This can also result in a decrease in display quality, with the image remaining even after the power is turned off (appearing to the human eye as an afterimage). Jiru.

[0017] Applying an unnecessary electric field to the liquid crystal in the off state where no image is displayed causes This may result in a deterioration in the image display function and reliability of the liquid crystal display device.

[0018] In the liquid crystal display device disclosed in this specification, before the power is turned off, an electric field is applied to the liquid crystal. To prevent this, a fixed potential is input to the capacitance element to eliminate the potential difference between the electrodes of the capacitance element (to reduce the capacitance). The liquid crystal is in the initial state. The image displayed on the LCD screen is called the initial state image. The initial state image is displayed on a normally white LCD screen, for example. In the case of a normally black LCD display, the screen will be completely black. In the case of a normally white LCD display, a monochrome screen is displayed by a color filter or a light source. It can also be done as follows.

[0019] If the power is turned off after the initial image is displayed, the LCD will not generate unnecessary electric fields in the off state. It does not continue to be affected and can remain in a stable initial state.

[0020] In addition, the display will turn off after displaying an initial image such as an all-white screen or all-black screen. The information on the image just before the power is turned off may be leaked to others by displaying a message such as It can be prevented.

[0021] Therefore, it is possible to provide a liquid crystal display device that maintains good image display function for a long time and also has high security. It is possible.

[0022] One embodiment of a method for driving a liquid crystal display device disclosed in the present specification is a method for driving a liquid crystal display device, comprising: A screen having a plurality of pixels including a semiconductor element, a liquid crystal element, and a semiconductor element is provided with the liquid crystal element. The liquid crystal is made to respond and display an image, and a stop signal is supplied by the stop means, and the stop signal causes the display to display a plurality of images. A fixed potential is written to the element capacitance element, and the liquid crystal that responded is put into a non-responsive state, and the initial state image is displayed on the screen. The image is displayed, and the supply of power supply potential from the power supply is stopped.

[0023] One embodiment of the method for driving a liquid crystal display device disclosed in this specification is to supply a power supply potential from a power supply to a drive circuit unit. and a display screen provided with a plurality of pixels including a capacitor element, a liquid crystal element, and a semiconductor element. The liquid crystal of the liquid crystal element is made to respond to display an image, and a stop signal is supplied by the stop means. A fixed potential is written to the capacitance elements of multiple pixels, and the liquid crystal that responded is put into a non-responsive state. The initial state image is displayed on the LCD panel, and the supply of power supply potential from the power supply to the drive circuit unit is stopped.

[0024] One embodiment of a method for driving a liquid crystal display device disclosed in the present specification is a method for driving a liquid crystal display device, comprising: A power supply potential is supplied to the light section, and a plurality of pixels including a capacitance element, a liquid crystal element, and a semiconductor element are The liquid crystal of the liquid crystal element is made to respond on the provided screen to display an image, and a stop signal is sent by the stop means. The power supply voltage from the power supply to the backlight section is stopped, and a stop signal is sent to the A fixed potential is written to the pixel's capacitance element, causing the responding liquid crystal to become non-responsive, and the screen returns to its initial state. An image is displayed, and the supply of power supply potential from the power supply to the drive circuit unit is stopped.

[0025] One embodiment of a method for driving a liquid crystal display device disclosed in the present specification is a method for driving a liquid crystal display device, comprising: A power supply potential is supplied to the light section, and a plurality of pixels including a capacitance element, a liquid crystal element, and a semiconductor element are The liquid crystal of the liquid crystal element is made to respond on the provided screen to display an image, and a stop signal is sent by the stop means. A fixed potential is written to the capacitance elements of multiple pixels by a stop signal, and the liquid crystal that responded The initial state image is displayed on the screen in the non-responsive state, and the drive circuit and backlight unit from the power supply are turned off. The supply of power supply potential to the

[0026] In the above structure, the capacitor element and the liquid crystal element are electrically connected to function as a switching element. A transistor including an oxide semiconductor layer can be used as the semiconductor element. [Effects of the Invention]

[0027] Before turning off the liquid crystal display device, a fixed potential is written so that no voltage is applied to the liquid crystal element. This prevents the liquid crystal element from deteriorating and allows for a long period of good image display. This allows display functionality to be maintained while also increasing security.

[0028] Therefore, it is possible to achieve higher reliability and lower power consumption in the liquid crystal display device. become. [Brief explanation of the drawings]

[0029] [Figure 1] 1A and 1B illustrate one embodiment of a liquid crystal display device. [Figure 2] 1A and 1B illustrate one embodiment of a liquid crystal display device. [Figure 3] 1A and 1B illustrate one embodiment of a liquid crystal display device. [Figure 4] 1 is a timing chart illustrating one mode of a method for driving a liquid crystal display device. [Figure 5] 1 is a timing chart illustrating one mode of a method for driving a liquid crystal display device. [Figure 6] 1A to 1C illustrate one embodiment of a driving method of a liquid crystal display device. [Figure 7] 1A to 1C illustrate one mode of a transistor that can be used in a liquid crystal display device. [Figure 8] 1A to 1C illustrate one mode of a manufacturing method of a transistor that can be used in a liquid crystal display device. [Figure 9] 1A and 1B are a diagram and a block diagram illustrating one embodiment of a liquid crystal display device. [Figure 10] 1A and 1B are diagrams illustrating electronic devices. [Figure 11] 1A and 1B illustrate one embodiment of a liquid crystal display device. [Figure 12] Photographs showing images displayed by a liquid crystal display device. [Figure 13] Photographs showing images displayed by a liquid crystal display device. DETAILED DESCRIPTION OF THE INVENTION

[0030] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. The present invention is not limited to the following description, and various modifications in form and details are possible by those skilled in the art. Furthermore, the present invention should not be construed as being limited to the description of the following embodiments. It is not something that can be done.

[0031] (Embodiment 1) In this embodiment, one embodiment of a liquid crystal display device and a driving method thereof will be described with reference to FIGS. 1 and 2. This will be used to explain.

[0032] The liquid crystal display device of this embodiment will be described with reference to the flowchart of FIG.

[0033] As shown in FIG. 1, it is assumed that image A is displayed on the display screen of the liquid crystal display device. When the image displayed by the supplied image signal is no longer required (when use is finished), the stop means is selected. When the stop means is selected, a stop signal is input and a fixed potential is written to the capacitance elements of all pixels. By writing a fixed potential to the capacitance element, the potential difference between the electrodes of the capacitance element is eliminated ( The capacitance is set to almost zero, and the liquid crystal that was in a responsive state is set to the initial state of a non-responsive state. The initial state image S displayed on the display screen is the initial state image S displayed by the LCD in the initial state. For example, in the case of a normally white LCD display device, the screen will be completely white, and in the case of a normally black LCD device, the screen will be completely white. In the case of a liquid crystal display device, the screen will be completely black. In the case of a normally white liquid crystal display device, can be made into a monochrome screen by using a color filter or light source.

[0034] After displaying the initial state image S, the power is turned off to stop the supply of power potential to the display panel, The liquid crystal display device is turned off. Therefore, an unnecessary electric field is not applied to the liquid crystal in the off state. It does not continue and can remain in a stable initial state.

[0035] Also, since the initialization image such as a completely white screen or a completely black screen is displayed before turning off, Information on the image immediately before the power is turned off may be leaked to others due to afterimages or other issues being displayed on the screen. This can be prevented.

[0036] Therefore, it is possible to provide a liquid crystal display device that maintains good image display function for a long time and also has high security. It is possible.

[0037] Each component of the liquid crystal display device 100 of this embodiment will be described with reference to the block diagram of FIG. The liquid crystal display device 100 includes a power supply 116, a stop means 117, a display control circuit 113, a display panel 1 20. In the case of a transmissive liquid crystal display device or a semi-transmissive liquid crystal display device, it further has a light source and It is advisable to provide a backlight unit.

[0038] The liquid crystal display device 100 receives an image signal (image signal data) from a connected external device. The power supply potentials (high power supply potential Vdd, low power supply potential Vss, and common potential Vco) are m) is supplied by turning on the power supply 116 of the liquid crystal display device to start power supply, The control signals (start pulse SP and clock signal CK) are generated by the display control circuit 113. In addition, power supply potentials (high power supply potential Vdd, low power supply potential Vss, and common potential Vc The supply of the image data is stopped by the control of the stop means 117, and the initial state image is displayed. After that, the power supply 116 is turned off to stop the supply of the power supply potential to the display panel.

[0039] The high power supply potential Vdd is a potential higher than the reference potential, and the low power supply potential Vss is The high power supply potential Vdd and the low power supply potential Vss are both potentials below the reference potential. It is desirable that the potential be such that the transistor can operate. The difference in the low power supply potential Vss is sometimes called the power supply voltage.

[0040] The common potential Vcom is a reference for the potential of the image signal Data supplied to the pixel electrode. Any fixed potential may be used, and as an example, it may be the ground potential.

[0041] The image signal data can be generated by dot inversion driving, source line inversion driving, gate line inversion driving, If the signal is inverted appropriately according to frame inversion driving or the like and input to the liquid crystal display device 100, If the image signal data is an analog signal, it can be converted via an A / D converter, etc. The signal may be converted into a digital signal and supplied to the liquid crystal display device 100.

[0042] In this embodiment, the common electrode 128 and one electrode of the capacitor element 210 are connected to a power source 116. A common potential Vcom, which is a fixed potential, is applied via a display control circuit 113 .

[0043] The display control circuit 113 supplies the display panel 120 with a display panel image signal (Data) and a control signal. Control signals (specifically, start pulse SP and clock signal CK), power supply potential (high power The power supply voltage Vdd, the low power supply voltage Vss, and the common voltage Vcom are supplied to the power supply voltage Vcc.

[0044] The display panel 120 has a liquid crystal element 215 sandwiched between a pair of substrates (a first substrate and a second substrate). The first substrate is provided with a driver circuit section 121 and a pixel section 122. The second substrate is provided with a common connection portion (also called a common contact) and a common electrode 128 (common The common connection portion is connected to the first substrate. The common connection portion is provided on the first substrate and electrically connects the first substrate to the second substrate. It's fine.

[0045] The pixel section 122 includes a plurality of gate lines 124 (scanning lines) and source lines 125 (signal lines). A plurality of pixels 123 are surrounded by gate lines 124 and source lines 125. In the display panel exemplified in this embodiment, The gate line 124 extends from the gate line driving circuit 121A, and the source line 125 extends from the source line driving circuit 121B. Extending from path 121B.

[0046] The pixel 123 also includes a transistor 214 as a switching element. 2, and a liquid crystal element 215.

[0047] The liquid crystal element 215 is an element that controls the transmission or non-transmission of light by the optical modulation action of the liquid crystal. The optical modulation of the liquid crystal is controlled by the electric field applied to the liquid crystal. The field direction varies depending on the liquid crystal material, driving method, and electrode structure, and can be selected appropriately. For example, when using a driving method that applies an electric field in the thickness direction (so-called vertical direction) of the liquid crystal layer, Pixel electrodes are provided on the first substrate and a common electrode is provided on the second substrate so as to sandwich the liquid crystal. In addition, the driving method is to apply an electric field to the liquid crystal in the in-plane direction of the substrate (so-called horizontal electric field). When using this method, it is sufficient to provide a structure in which the pixel electrode and the common electrode are on the same surface as the liquid crystal. The pixel electrodes and the common electrodes may have various opening patterns. In the embodiment, an element that controls the transmission or non-transmission of light by optical modulation action The liquid crystal material, driving method, and electrode structure are not particularly limited.

[0048] The transistor 214 is connected to one of the gate lines 124 provided in the pixel section 122. The gate electrode is connected to one of the source electrode and the drain electrode. The other of the source electrode and the drain electrode is connected to one of the capacitor elements 210. The electrode is connected to one electrode (pixel electrode) of the liquid crystal element 215.

[0049] The transistor 214 is preferably a transistor with low off-state current. When the transistor 214 is in the off state, the liquid crystal element connected to the transistor 214 has a low off-state current. 215 and the charge stored in the capacitor element 210 is unlikely to leak through the transistor 214. Therefore, the state written before the transistor 214 is turned off can be maintained for a long time. do.

[0050] With this configuration, the capacitor element 210 holds the voltage applied to the liquid crystal element 215. In addition, the electrodes of the capacitor 210 can be connected to a separately provided capacitor line. Good too.

[0051] The driving circuit section 121 includes a gate line driving circuit 121A and a source line driving circuit 121B. The gate line driving circuit 121A and the source line driving circuit 121B are connected to a pixel section 1 having a plurality of pixels. 22 is a driving circuit for driving the shift register circuit (also called a shift register) It has.

[0052] The gate line driving circuit 121A and the source line driving circuit 121B are the same as the pixel section 122. They may be formed on the same substrate or on different substrates.

[0053] The drive circuit unit 121 receives a high power supply potential Vdd, A low power supply potential Vss, a start pulse SP, a clock signal CK, and an image signal Data are supplied. can be.

[0054] The terminal unit 126 receives a predetermined signal (high power supply potential Vdd, low power supply potential Vdd) output from the display control circuit 113. Potential Vss, start pulse SP, clock signal CK, image signal Data, common potential Vc om, etc.) to the drive circuit unit 121.

[0055] The common electrode 128 is a common potential Vcom controlled by the display control circuit 113. The wires are electrically connected at a common connection.

[0056] A specific example of the common connection part is a conductive particle in which an insulating sphere is coated with a metal thin film. By doing so, it is possible to electrically connect the common electrode 128 to the common potential line. The common connection section may be provided at multiple locations within the display panel 120.

[0057] The liquid crystal display device may also have a photometric circuit. The brightness of the environment in which the LCD device is placed can be detected. As a result, the photometry circuit is connected. The display control circuit 113 controls the backlight, the sub-light, and the like in response to a signal input from the photometry circuit. It is possible to control the driving method of a light source such as an LED.

[0058] In addition, color display is possible by combining color filters. In addition, other optical films (polarizing film, retardation film, anti-reflection film, etc.) In the case of a transmissive liquid crystal display device or a semi-transmissive liquid crystal display device, The light source used in this case, such as a backlight, is selected and assembled according to the application of the liquid crystal display device 100. Cold cathode fluorescent lamps and light emitting diodes (LEDs) can be used. It is possible to create a surface using multiple LED light sources or multiple electroluminescence (EL) light sources. The surface light source may be configured using three or more types of LEDs, or may be configured using white light. LEDs may also be used. Note that RGB light-emitting diodes may be placed in the backlight, By using the field sequential color mixing method (field sequential method) to display colors by dividing the image, In some cases, a color filter may not be provided.

[0059] As described above, when the liquid crystal display device is turned on and power is supplied, the ON state By using semiconductor elements with low off-state current, it is possible to reduce power consumption. Before the LCD panel is turned on, a fixed potential is written to the LCD panel to prevent voltage from being applied to the LCD panel. By displaying the image, deterioration of the liquid crystal element is prevented, and good image display function is maintained for a long time. It can also improve the reliability.

[0060] Therefore, a liquid crystal display device with higher reliability and lower power consumption is achieved, and It is possible to provide a driving method.

[0061] (Embodiment 2) In this embodiment, a liquid crystal display device that can achieve further reduction in power consumption by combining it with the first embodiment is provided. The display device driving method is shown. The same parts as those in the first embodiment or parts having similar functions, and The steps can be performed in the same manner as in the first embodiment, and the repeated explanation will be omitted. A detailed description of the location will be omitted.

[0062] LCD displays combine moving images and still images on the screen. Moving images are divided into multiple frames. By switching between multiple different corresponding images at high speed, the image is perceived as a moving image by the human eye. Specifically, images are switched at least 60 times (60 frames) per second. This means that the human eye perceives it as a moving image with less flicker. Unlike partial video, multiple images corresponding to multiple frame periods for time division can be switched at high speed. Even if the operation is switched between consecutive frames, for example, the nth frame and the (n+1)th frame, This refers to an image that does not change from frame to frame.

[0063] The liquid crystal display device according to the present invention can display both moving images and still images. When the display mode is set to "0", the video display mode and the still image display mode are selected. In this specification, the image displayed during still image display is referred to as a still image. Call.

[0064] The image signals of consecutive frames are different (for example, the first and second consecutive frames In this system, the first image signal of the first frame and the second image signal of the second frame are different. In the case of moving image display, a display mode in which an image signal is written for each frame is used. In this case, the image signals of consecutive frames are the same (for example, the first and second consecutive frames). In the frame, the first image signal of the first frame and the second image signal of the second frame are simultaneously output. In the case of a still image display, no new image signal is written, and a voltage is applied to the liquid crystal element. The voltage applied to the liquid crystal element when the potential of the pixel electrode and common electrode is floating In this case, a display mode is used in which a still image is displayed without supplying a new potential, and the voltage is maintained.

[0065] Liquid crystal display device according to the present embodiment, and moving image display mode and still image display mode of the liquid crystal display device The mode switching will be described with reference to FIGS. 3 to 6 and 11. FIG.

[0066] Each component of the liquid crystal display device 200 of this embodiment will be described with reference to the block diagram of FIG. The liquid crystal display device 200 is a transmissive liquid crystal display that displays images by using the transmission and non-transmission of light in pixels. This is an example of a display device or a semi-transmissive liquid crystal display device, and includes an image processing circuit 110, a power supply 116, a stop The display device includes a stopper 117, a display panel 120, and a backlight unit 130. In the case of the device, since external light is used as a light source, the backlight unit 130 can be omitted. Cut.

[0067] The liquid crystal display device 200 receives an image signal (image signal data) from a connected external device. The power supply potentials (high power supply potential Vdd, low power supply potential Vss, and common potential Vco) are m) is supplied by turning on the power supply 116 of the liquid crystal display device to start power supply, The control signals (start pulse SP and clock signal CK) are generated by the display control circuit 113. In addition, power supply potentials (high power supply potential Vdd, low power supply potential Vss, and common potential Vc The supply of the image data is stopped by the control of the stop means 117, and the initial state image is displayed. After that, the power supply 116 is turned off to stop the supply of the power supply potential to the display panel.

[0068] In addition, if the image signal data is an analog signal, it is converted to digital form via an A / D converter, etc. The image signal is converted into a digital signal and supplied to the image processing circuit 110 of the liquid crystal display device 200. This is preferable because it allows easy detection of the difference between image signals later.

[0069] The configuration of the image processing circuit 110 and the procedure by which the image processing circuit 110 processes signals are explained. do.

[0070] The image processing circuit 110 includes a memory circuit 111, a comparison circuit 112, a display control circuit 113, and a selection circuit. The image processing circuit 110 has a selection circuit 115. The display panel image signal and the backlight signal are generated from the The image signal controls the backlight unit 120, and the backlight signal controls the backlight unit 130. Also, a signal for controlling the common electrode 128 is output to the switching element 127. do.

[0071] The memory circuit 111 includes a plurality of frame memory circuits for storing image signals relating to a plurality of frames. The number of frame memories included in the memory circuit 111 is not particularly limited. Any element that can store image signals relating to multiple frames can be used. For example, DRAM (Dynamic Random Access Memory) , SRAM (Static Random Access Memory) and other storage elements It may be configured using the following.

[0072] The frame memory may be configured to store an image signal for each frame period. The number of frame memories is not particularly limited. It is selectively read out by a comparison circuit 112 and a display control circuit 113. The frame memory 111b in the figure conceptually illustrates a memory area for one frame. do.

[0073] One of these frame memories is set to the initial state where the liquid crystal is in a non-responsive state as shown in the first embodiment. It is possible to store the image signal of the initial state image (for example, all white screen display or all black screen display). The image signal of the initial state image is inputted to the display control circuit 113. It is read and written to the screen.

[0074] The comparison circuit 112 selects image signals for successive frame periods stored in the memory circuit 111. The image signals are read out sequentially, and the difference between successive frames is detected for each pixel. This is a circuit for outputting

[0075] In this embodiment, the display control circuit 113 and the operation of the selection circuit 115. If a difference is detected in the original signal (if there is a difference), the comparison circuit 112 determines whether the image signal is for a still image. The period of consecutive frames in which the difference is detected is determined to be a period for displaying moving images. It is determined that

[0076] On the other hand, when the comparison circuit 112 compares the image signals, if no difference is detected in any of the pixels, (If there is no difference), the period of consecutive frames in which the difference is not detected displays a still image. That is, the comparison circuit 112 determines that the period is a period showing the image of the consecutive frame periods. By detecting the presence or absence of a signal difference, it is possible to determine whether the signal is an image signal for displaying a moving image or not. The image signal is used to determine whether it is an image signal for displaying a still image or not.

[0077] The standard for detecting a "difference" through this comparison is that the magnitude of the difference must be at a certain level. It may be set so that when the difference exceeds the threshold, it is determined that a difference exists. The difference detected by the circuit 112 may be determined based on the absolute value of the difference.

[0078] In this embodiment, the comparison circuit 112 provided inside the liquid crystal display device 200 By detecting the difference between the image signals during successive frames, the image can be determined as a moving image or a still image. We have shown a configuration for determining whether an image is a video or a still image, but The signal may be supplied in this manner.

[0079] The selection circuit 115 is configured to include a plurality of switches formed of, for example, transistors. If the comparator circuit 112 detects a difference between successive frames, that is, if the image is a moving image, , a moving image signal is selected from the frame memory in the storage circuit 111 and output to the display control circuit 113. Output to.

[0080] If the comparator circuit 112 does not detect a difference between consecutive frames, the selector circuit 115 That is, when the image is a still image, the frame memory in the storage circuit 111 is read from the display control circuit 113 The image signal is not output from the frame memory to the display control circuit 113. By adopting such a configuration, the power consumption of the liquid crystal display device can be reduced.

[0081] In the liquid crystal display device of this embodiment, the comparison circuit 112 converts the image signal into a signal for displaying a still image. The operation performed when the comparator 112 determines that the image signal is a moving image display mode is a still image display mode. The operation performed when the signal is judged to be a moving image display signal is the moving image display mode.

[0082] The display control circuit 113 outputs the image signals selected by the selection circuit 115 to the display panel 120. supply of control signals (specifically, start pulse SP and clock signal CK) or stop switching), power supply potential (high power supply potential Vdd, low power supply potential and a common potential Vcom) to the backlight unit 130. Specifically, the backlight control circuit 131 controls the turning on and off of the backlight. This is a circuit that supplies a signal to

[0083] The image processing circuit exemplified in this embodiment has a display mode switching function. The display mode switching function may be performed by the user of the liquid crystal display device either manually or by an externally connected device. By using the device to select the operation mode of the liquid crystal display device, the moving image display mode or the still image display mode can be selected. This function switches the display mode.

[0084] The selection circuit 115 selects an image signal to be displayed in accordance with the signal input from the display mode switching circuit. It can also be output to the control circuit 113.

[0085] For example, when operating in a still image display mode, the display mode switching circuit selects the When a mode switching signal is input to the input terminal 15, the comparator circuit 112 outputs the Even if the difference between the image signals is not detected, the selection circuit 115 A mode in which signals are sequentially output to the display control circuit 113, that is, a moving image display mode, can be executed. In addition, when operating in the moving image display mode, the display mode switching circuit selects the When a mode switching signal is input to the comparator 112, the comparator 112 compares the image quality of the image in successive frame periods. Even when the difference between the image signals is detected, the selection circuit 115 selects one frame of image. A mode in which only the image signal is output, i.e., a still image display mode, can be executed. When the liquid crystal display device of this embodiment is operating in a moving image display mode, Of the images corresponding to the frames, the image corresponding to one frame is displayed as a still image. .

[0086] The liquid crystal display device may also have a photometric circuit. The LCD device can detect the brightness of the environment in which it is placed. As a result, the photometry circuit is connected. The display control circuit 113 controls the backlight and other components in response to a signal input from the photometry circuit. The way the light source is driven can be controlled.

[0087] For example, the photometric circuitry detects that the LCD display is being used in a dimly lit environment. Then, the display control circuit 113 controls the backlight 132 to increase the light intensity, It ensures good visibility of the screen, and conversely, it is difficult for the LCD display to be displayed in extremely bright external light (for example, outdoors). When it is determined that the display is being used in an environment where the backlight is in direct sunlight, the display control circuit 113 The intensity of the light from the backlight 132 is controlled to be reduced, thereby reducing the power consumed by the backlight 132.

[0088] The backlight unit 130 includes a backlight control circuit 131 and a backlight 132. The backlight 132 may be selected and combined depending on the intended use of the liquid crystal display device 200. Cold cathode fluorescent lamps and light emitting diodes (LEDs) can be used. In this case, a color filter can be used to display the image. For example, a white light emitting element (e.g., an LED) can be arranged on the backlight. 132 RGB light-emitting diodes are arranged, and the color is displayed in time-division additive color mixing. When using the field sequential method, if no color filter is used, The backlight control circuit 131 controls the backlight from the display control circuit 113. A backlight signal and a power supply potential are supplied.

[0089] In this embodiment, the display panel 120 includes a pixel section 122 and a switching element 127. In this embodiment, the display panel 120 has a first substrate and a second substrate. The substrate is provided with a driving circuit section 121, a pixel section 122, and a switching element 127. There are.

[0090] The pixel 123 also includes a transistor 214 as a switching element. 3, the capacitor element 210 connected to the liquid crystal element 215 is included.

[0091] The transistor 214 is preferably a transistor with low off-state current. When the transistor 214 is in the off state, the liquid crystal element connected to the transistor 214 has a low off-state current. 215 and the charge stored in the capacitor element 210 is unlikely to leak through the transistor 214. Therefore, the state written before the transistor 214 is turned off can be maintained for a long time. do.

[0092] In this embodiment, the liquid crystal is provided on a second substrate opposite to the pixel electrodes provided on the first substrate. The electrodes are controlled by a longitudinal electric field generated by a common electrode.

[0093] Examples of liquid crystals that can be used in liquid crystal elements include nematic liquid crystals, cholesteric liquid crystals, and smectic liquid crystals. tic liquid crystal, discotic liquid crystal, thermotropic liquid crystal, lyotropic liquid crystal, low molecular weight Liquid crystal, polymer liquid crystal, polymer dispersed liquid crystal (PDLC), ferroelectric liquid crystal, antiferroelectric liquid crystal, main chain type Examples of such liquid crystals include liquid crystals, side chain polymer liquid crystals, and banana-shaped liquid crystals.

[0094] One example of a liquid crystal driving method is the TN (Twisted Nematic) mode. STN (Super Twisted Nematic) mode, OCB (Optical Elly Compensated Birefringence mode, ECB (El Electrically Controlled Birefringence mode, FLC (Ferroelectric Liquid Crystal) mode, AFL C (AntiFerroelectric Liquid Crystal) mode, P DLC (Polymer Dispersed Liquid Crystal) mode , PNLC (Polymer Network Liquid Crystal) mode , guest host mode, etc.

[0095] The switching element 127 switches between a common potential and a common voltage in response to a control signal output from the display control circuit 113. Vcom is supplied to the common electrode 128. The switching element 127 is a transistor The gate electrode and the source electrode or the drain electrode of the transistor can be formed by One side is connected to the display control circuit 113, and the other side is connected to the source electrode or the drain electrode. The common potential Vcom is supplied from the display control circuit 113 via 26, and the other is It is sufficient to connect the switching element 127 to the driving circuit section 121. Alternatively, it may be formed on the same substrate as the pixel section 122, or may be formed on a different substrate. It's okay to have one.

[0096] By using a transistor with a low off-state current as the switching element 127, This can prevent the voltage applied to both terminals of the resistor 215 from decreasing over time.

[0097] The common connection portion is a terminal connected to the source electrode or the drain electrode of the switching element 127. The electrodes are electrically connected to the common electrode 128.

[0098] The source of the switching element 127 is a transistor, which is one type of switching element. One of the electrode and drain electrode of the capacitor 21 is not connected to the transistor 214. 0 and the other electrode of the liquid crystal element 215, and The other of the source electrode and the drain electrode of the switch is connected to the terminal 126B. The gate electrode of switching element 127 is connected to terminal 126A.

[0099] Next, the state of signals supplied to pixels will be explained with reference to the equivalent circuit diagram of the liquid crystal display device shown in FIG. 3 and the equivalent circuit diagram of the liquid crystal display device shown in FIG. The timing chart will be used for explanation.

[0100] FIG. 4 shows the clock signal GCK supplied from the display control circuit 113 to the gate line driving circuit 121A. , and a start pulse GSP. The display control circuit 113 also indicates the source line driver circuit 12 1B. The clock signal SCK and the start pulse SSP are supplied to the To explain the timing of the signal output, Figure 4 shows the waveform of the clock signal as a simple square wave. Shown in.

[0101] 4 shows the potential of the source line (Data line) 125, the potential of the pixel electrode, the potential of the terminal 12 6A, the potential of terminal 126B, and the potential of the common electrode.

[0102] In FIG. 4, a period 1401 corresponds to a period for writing an image signal for displaying a moving image. During the period 1401, an image signal and a common potential are supplied to each pixel and common electrode of the pixel portion 122. It works like this.

[0103] The period 1402 corresponds to a period in which a still image is displayed. The image signal to each pixel of 22 and the common potential to the common electrode are stopped. In a period 1402, signals are supplied to stop the operation of the driving circuit unit. However, depending on the length of the period 1402 and the refresh rate, the image signal is periodically written. It is preferable to incorporate such a configuration to prevent deterioration of the still image quality.

[0104] First, the timing chart for the period 1401 will be described. A clock signal is always supplied as a clock signal GCK, and a vertical start pulse GSP is supplied as a A pulse corresponding to the synchronization frequency is supplied. Also, during period 1401, the clock signal SCK As a result, a clock signal is always supplied, and as a start pulse SSP, one gate selection period A pulse corresponding to the

[0105] In addition, an image signal Data is supplied to the pixels of each row via a source line 125, and In response to the potential of the source line 125, the pixel electrode is supplied with the potential of the source line 125.

[0106] In addition, the display control circuit 113 connects the terminal 126A of the switching element 127 to the switching element 127 is supplied with a potential that makes it conductive, and a common potential is supplied to the common electrode via terminal 126B. do.

[0107] On the other hand, a period 1402 is a period in which a still image is displayed. In period 1402, the clock signal GCK and the start pulse G SP, the clock signal SCK, and the start pulse SSP all stop. At 02, the image signal Data supplied to the source line 125 is stopped. During a period 1402 when both the signal GCK and the start pulse GSP are stopped, the transistor 21 4 is in a non-conductive state, and the potential of the pixel electrode is in a floating state.

[0108] In addition, the display control circuit 113 connects the terminal 126A of the switching element 127 to the switching element A potential is supplied to make the common electrode 127 non-conductive, and the potential of the common electrode is set to a floating state.

[0109] In the period 1402, the potentials of the electrodes at both ends of the liquid crystal element 215, that is, the pixel electrode and the common electrode, are floated. By setting the display to the free state, a still image can be displayed without supplying any new potential.

[0110] Also, the clock signal supplied to the gate line driving circuit 121A and the source line driving circuit 121B is By stopping the signal and the start pulse, it is possible to reduce power consumption.

[0111] In particular, the transistor 214 and the switching element 127 are transistors with low off-state current. By using this, the phenomenon that the voltage applied to both terminals of the liquid crystal element 215 decreases over time is suppressed. can.

[0112] Next, the period when the video is switched to a still image (period 1403 in FIG. 4) and the period when the still image is switched to the video are considered. The operation of the display control circuit during the period (period 1404 in FIG. 4) when the display is switched to the 5A and 5B show the high power supply potential output from the display control circuit. Vdd, clock signal (here GCK), start pulse signal (here GSP), and and the potential of terminal 126A.

[0113] The operation of the display control circuit during the period 1403 when a moving image is switched to a still image is shown in FIG. The display control circuit stops the start pulse GSP (E1 in FIG. 5(A), first step). Next, after the start pulse signal GSP stops, the pulse output reaches the final stage of the shift register. After reaching the threshold, the clock signals GCK are stopped (E2 in FIG. 5A, second step Next, the high power supply potential Vdd of the power supply is changed to the low power supply potential Vss (E3 in FIG. 5(A)). (Third step) Next, the potential of the terminal 126A is set to a value that the switching element 127 is in a non-conducting state. The potential is set to a value that puts the electrode into the positive electrode state (E4 in FIG. 5(A), the fourth step).

[0114] By the above procedure, the drive circuit unit 121 can be operated without causing a malfunction. The signal supplied to 1 can be stopped. The malfunction when switching from video to still image causes noise. Since noise is retained as a still image, it is an LCD display equipped with a display control circuit that is less likely to malfunction. The display device can display still images with little image degradation.

[0115] Next, the operation of the display control circuit during the period 1404 when the still image is switched to the moving image is shown in FIG. 5(B). The display control circuit sets the potential of the terminal 126A to a potential at which the switching element 127 is in a conducting state. (S1 in FIG. 5B, first step). Next, the power supply voltage is set to Vss. Then, the clock signal is turned on to the high power supply potential Vdd (S2 in FIG. 5B, second step). First, a high potential is applied to the clock signal GCK, and then multiple clock signals GCK are supplied (see FIG. 5( S3, the third step of FIG. 5(B)). Then, a start pulse signal GSP is supplied (FIG. 5(B) ) S4, fourth step).

[0116] By the above procedure, the drive circuit unit 121 can be turned on without causing a malfunction of the drive circuit unit 121. By returning the potential of each wiring to the time when the video was displayed in the appropriate order, the supply of the drive signal can be resumed. The driving circuit unit can be driven without any operation.

[0117] 6, a period 601 in which a moving image is displayed or a period 602 in which a still image is displayed is shown. , which shows the frequency of writing image signals for each frame period. "H" indicates the period during which the image signal is held. In addition, in FIG. 6, the period 603 represents one frame period, but it may also represent another period. may be.

[0118] In this way, in the configuration of the liquid crystal display device of this embodiment, the still image displayed in the period 602 The image signal of the image is written in the period 604, and the image signal written in the period 604 is written in the period 6 02 other periods are held.

[0119] The liquid crystal display device exemplified in this embodiment writes an image signal during a period in which a still image is displayed. As a result, it is possible to reduce the power consumption when displaying still images. do.

[0120] Also, when displaying a still image by rewriting the same image multiple times, the image change is not visible. If the image quality is too high, the human eye may feel tired. Since the frequency of signal writing is reduced, it also has the effect of reducing eye fatigue.

[0121] In particular, in the liquid crystal display device of this embodiment, a transistor with low off-state current is used in each pixel and a common By applying this to the switching element of the electrode, the period (time) during which the voltage can be maintained by the storage capacitor can be increased. As a result, the frequency of writing image signals can be dramatically reduced. This has a significant effect on reducing power consumption when displaying still images and reducing eye fatigue. It has fruit.

[0122] In addition, in the liquid crystal display device 200 of this embodiment, when the stop means 117 is selected, a stop signal is A fixed potential is written to the capacitance elements 210 of all pixels. By writing the potential, the potential difference between the electrodes of the capacitor element 210 is eliminated, and the capacitor element 210 is in a responsive state. The liquid crystal is set to the initial state of non-response. Therefore, the initial state liquid crystal is displayed on the display screen. The initial status image is displayed.

[0123] After the initial state image is displayed, the power supply 116 is stopped and the supply of the power supply potential to the display panel 120 is stopped. Therefore, the liquid crystal is not required in the off state. The necessary electric field is not continuously applied, and the initial state can be kept stable.

[0124] As described above, when the liquid crystal display device is turned on and power is supplied, Depending on the image signal of the succeeding frame, the video display mode or still image display mode is selected appropriately. The power consumption is reduced and a fixed voltage is applied to the liquid crystal element before the device is turned off. By writing the initial position and displaying the initialization image, deterioration of the liquid crystal element is prevented, and good image display is possible for a long time. This allows functionality to be maintained while also improving security.

[0125] Therefore, a liquid crystal display device with higher reliability and lower power consumption is achieved, and It is possible to provide a driving method.

[0126] (Embodiment 3) In this embodiment, an example of a transistor that can be applied to the liquid crystal display device disclosed in this specification will be described. The structure of the transistor that can be applied to the liquid crystal display device disclosed in this specification is not particularly limited. For example, a staggered type or a planar type with a top gate structure or a bottom gate structure is used. In addition, the transistor can be a single gate transistor in which one channel forming region is formed. In the gate structure, two gates are formed in a double gate structure, and three gates are formed in a triple gate structure. Alternatively, two gate insulating layers may be disposed above and below the channel region. The transistor may be a dual gate type having a gate electrode layer. 7A to 7D show an example of a cross-sectional structure of the transistor. The advantage of using an oxide semiconductor is that it is relatively The main advantage is that high mobility and low off-state current can be obtained through a relatively simple and low-temperature process. However, other semiconductors may also be used.

[0127] The transistor 410 shown in FIG. 7A is a thin film transistor with a bottom gate structure. It is also called an inverted staggered thin film transistor.

[0128] The transistor 410 includes a gate electrode layer 401, a gate electrode layer 402, a gate electrode layer 403, a gate electrode layer 404, a gate electrode layer 405, a gate electrode layer 406, a gate electrode layer 407, a gate electrode layer 408, a gate electrode layer 409 ...10, a gate electrode layer 411, a gate electrode layer 412, a gate electrode The insulating layer 402, the oxide semiconductor layer 403, the source electrode layer 405a, and the drain electrode layer 40 5b. In addition, an insulating film that covers the transistor 410 and is stacked on the oxide semiconductor layer 403 A protective insulating layer 409 is further formed on the insulating film 407. .

[0129] The transistor 420 illustrated in FIG. 7B is a channel-protective transistor (also called a channel-stop transistor). ) and is also called an inverted staggered thin film transistor.

[0130] The transistor 420 includes a gate electrode layer 401, a gate The insulating layer 402, the oxide semiconductor layer 403, and the insulating layer 402 are formed on the oxide semiconductor layer 403. The insulating layer 427 serving as a channel protective layer, the source electrode layer 405a, and the drain electrode A protective insulating layer 409 is formed to cover the transistor 420. .

[0131] The transistor 430 shown in FIG. 7C is a bottom-gate thin film transistor. A gate electrode layer 401, a gate insulating layer 402, a silicon substrate 404, a silicon nitride film 406, a silicon nitride film 408, a silicon nitride film 409, a silicon nitride film 410, a silicon nitride film 411, a silicon nitride film 412, a silicon nitride film 413, a silicon nitride film 414, a silicon nitride film 415, a silicon nitride film 416, a silicon nitride film 417, a silicon nitride film 418, a silicon nitride film 419, a silicon nitride film 420, a silicon nitride film 421, a silicon nitride film 422, a silicon nitride film 423, a silicon nitride film 424, a silicon nitride film 425, a silicon nitride film 426, a silicon nitride film 427, a silicon nitride film 428, a silicon nitride film 429, a silicon nitride film 430, a silicon nitride film 431, a silicon nitride film The semiconductor layer 403 includes a source electrode layer 405a, a drain electrode layer 405b, and an oxide semiconductor layer 403. An insulating film 407 is provided to cover the transistor 430 and to be in contact with the oxide semiconductor layer 403. A protective insulating layer 409 is further formed on the insulating film 407.

[0132] In transistor 430, gate insulating layer 402 is formed between substrate 400 and gate electrode layer 400. 1, a source electrode layer 405a and a drain electrode layer 405b are provided on the gate insulating layer 402. The gate insulating layer 402 and the source electrode layer 405b are provided in contact with each other. 5a, an oxide semiconductor layer 403 is provided over a drain electrode layer 405b.

[0133] The transistor 440 shown in FIG. 7D is a thin film transistor with a top gate structure. The transistor 440 is formed on a substrate 400 having an insulating surface, an insulating layer 437, an oxide The semiconductor layer 403, the source electrode layer 405a, the drain electrode layer 405b, and the gate insulating layer 4 02, the gate electrode layer 401, the source electrode layer 405a, the drain electrode layer 405b, and The wiring layers 436a and 436b are provided adjacent to each other and electrically connected to each other.

[0134] In this embodiment, as described above, the oxide semiconductor layer 403 is used as the semiconductor layer. The oxide semiconductor used for the compound semiconductor layer 403 is a quaternary metal oxide, In-Sn- Ga-Zn-O system, ternary metal oxides In-Ga-Zn-O system, In-Sn-Z nO system, In-Al-Zn-O system, Sn-Ga-Zn-O system, Al-Ga-Zn-O system , Sn-Al-Zn-O system, binary metal oxides In-Zn-O system, Sn-Zn- O-based, Al-Zn-O-based, Zn-Mg-O-based, Sn-Mg-O-based, In-Mg-O-based, In-O, Sn-O, Zn-O, etc. can be used. The body may contain SiO2. For example, the In-Ga-Zn-O based oxide semiconductor The oxide is an oxide containing at least In, Ga, and Zn, and there is no particular limitation on the composition ratio. Furthermore, elements other than In, Ga, and Zn may be contained.

[0135] The oxide semiconductor layer 403 is represented by the chemical formula InMO3(ZnO)m (m>0). A thin film can be used, where M is one selected from Ga, Al, Mn, and Co. For example, M may be Ga, Ga and Al, Ga and Mn, or or Ga and Co.

[0136] The transistors 410, 420, 430, and 440 including the oxide semiconductor layer 403 are in an off state. Therefore, the current value (off-state current value) in the OFF state can be reduced. The signal retention time can be extended, and the write interval can also be set longer when the power is on. Therefore, the frequency of refresh operations can be reduced, which is effective in reducing power consumption. To bear fruit.

[0137] The transistors 410, 420, 430, and 440 including the oxide semiconductor layer 403 are Since a relatively high field effect mobility can be obtained, high speed driving is possible. By using such a transistor in a pixel portion of a device, a high-quality image can be provided. In addition, the transistors can be separately formed in a driver circuit portion and a pixel portion on the same substrate. This allows the number of components in the liquid crystal display device to be reduced.

[0138] There is no significant limitation on the substrate that can be used for the substrate 400 having an insulating surface. A glass substrate such as borosilicate glass or aluminoborosilicate glass is used.

[0139] In the bottom-gate transistors 410, 420, and 430, an insulating film serving as a base film The base film may be provided between the substrate and the gate electrode layer. It has a function of blocking the light and is made of silicon nitride film, silicon oxide film, silicon nitride oxide film, or oxynitride film. The insulating film may be formed by a laminate structure of one or more films selected from silicon dioxide films.

[0140] The material of the gate electrode layer 401 is molybdenum, titanium, chromium, tantalum, tungsten, Metallic materials such as aluminum, copper, neodymium, scandium, etc., or alloys containing these as their main components The gold material can be used to form a single layer or a multilayer structure.

[0141] The gate insulating layer 402 is formed by depositing a silicon oxide layer, a silicon dioxide ... Silicon nitride layer, silicon oxynitride layer, silicon nitride oxide layer, aluminum oxide layer, nitride An aluminum layer, an aluminum oxynitride layer, an aluminum nitride oxide layer, or a hafnium oxide layer For example, the first gate insulating layer may be formed of a polyimide film. A silicon nitride layer (SiNy(y)) with a thickness of 50 nm to 200 nm was formed by plasma CVD. >0)) and forming a second gate insulating layer on the first gate insulating layer with a thickness of 5 nm or more. A silicon oxide layer (SiOx (x>0)) of 00 nm or less is laminated to a total thickness of 200 nm. The gate insulating layer is

[0142] The conductive film used for the source electrode layer 405a and the drain electrode layer 405b is, for example, Al , Cr, Cu, Ta, Ti, Mo, W, or a material containing the above elements as a component. The alloy film may be a combination of the above elements. A high melting point metal layer such as Ti, Mo, W, etc. is formed on either or both the upper and lower sides of a metal layer such as u. It may also be a laminated structure. It also prevents the occurrence of hillocks and whiskers in the Al film. By using Al material with added elements (Si, Nd, Sc, etc.), heat resistance is improved. It is possible to raise it.

[0143] The wiring layer 436a connected to the source electrode layer 405a and the drain electrode layer 405b, The conductive film such as 6b is also made of the same material as the source electrode layer 405a and the drain electrode layer 405b. It can be used.

[0144] In addition, the source electrode layer 405a, the drain electrode layer 405b (wiring formed in the same layer as this) The conductive film (including the layer) may be formed of a conductive metal oxide. The oxides include indium oxide (In2O3), tin oxide (SnO2), zinc oxide (ZnO ), indium oxide tin oxide alloy (In2O3-SnO2, abbreviated as ITO), Indium zinc oxide alloy (In2O3-ZnO) or these metal oxide materials with silicon oxide It is possible to use a material containing kon.

[0145] The insulating films 407, 427, and 437 are typically silicon oxide films, silicon oxynitride films, or oxide films. An inorganic insulating film such as an aluminum nitride film or an aluminum oxynitride film can be used. do.

[0146] The protective insulating layer 409 is made of a silicon nitride film, an aluminum nitride film, a silicon nitride oxide film, a nitride An inorganic insulating film such as an aluminum oxide film can be used.

[0147] In addition, a planarizing insulating film is formed on the protective insulating layer 409 to reduce surface irregularities caused by the transistor. The planarization insulating film may be formed using a material such as polyimide, acrylic, or benzocyclobutene. In addition to the above organic materials, low dielectric constant materials (low In addition, multiple insulating films made of these materials can be stacked. A planarization insulating film may be formed by performing the above-mentioned process.

[0148] As described above, in this embodiment, a transistor including an oxide semiconductor layer having a low off-state current value is By using the capacitor, it is possible to provide a liquid crystal display device with low power consumption.

[0149] (Fourth embodiment) In this embodiment, an example of a transistor including an oxide semiconductor layer and a manufacturing method thereof will be described with reference to FIGS. The same parts as those in the above embodiment or parts and steps having similar functions are described below. This can be done in the same way as in the above embodiment, and the repeated explanation will be omitted. Detailed explanation will be omitted.

[0150] 8A to 8E show examples of cross-sectional structures of transistors. The transistor 510 shown in FIG. 7A is a bottom gate transistor similar to the transistor 410 shown in FIG. It is an inverted staggered thin film transistor.

[0151] The oxide semiconductor used in the semiconductor layer of this embodiment is an oxide semiconductor that does not contain hydrogen as an n-type impurity. and purify the oxide semiconductor to minimize the amount of impurities other than the main component. This results in an i-type (intrinsic) oxide semiconductor or an oxide semiconductor that is as close to i-type (intrinsic) as possible. That is, instead of adding impurities to make it i-type, impurities such as hydrogen and water are By removing as much as possible, it is possible to obtain a highly purified i-type (intrinsic semiconductor) or something close to it. Therefore, the oxide semiconductor layer included in the transistor 510 is highly purified and and an oxide semiconductor layer that has been made electrically i-type (intrinsic).

[0152] In addition, there are very few carriers (close to zero) in highly purified oxide semiconductors. The arsenic concentration is less than 1×1014 / cm3, preferably less than 1×1012 / cm3, and more preferably Preferably, it is less than 1×10 11 / cm 3 .

[0153] Since there are very few carriers in the oxide semiconductor, the off-state current of the transistor can be reduced. The smaller the off-state current, the better.

[0154] Specifically, the thin film transistor having the above-mentioned oxide semiconductor layer has a channel width of 1 μm. The off-state current density per unit area is 10 aA / μm (1 × 10-17 A / μm) or less at room temperature. Furthermore, it should be 1aA / μm (1×10-18A / μm) or less, and even 10zA / μm (1×10-20 A / μm) or less.

[0155] A transistor having an extremely small current value in an off state (off current value) is used as the image display device of the first embodiment. By using it as a transistor in the element part, refresh operation in the still image area This allows the image data to be written a small number of times.

[0156] In addition, the transistor 510 including the above-described oxide semiconductor layer has little temperature dependence of on-state current. The off-state current remains very small.

[0157] Hereinafter, a process of manufacturing a transistor 510 on a substrate 505 will be described with reference to FIGS. Explain.

[0158] First, a conductive film is formed on a substrate 505 having an insulating surface, and then a first photolithography is performed. A gate electrode layer 511 is formed by a process. If the resist mask is formed by the inkjet method, a photomask can be used. Therefore, manufacturing costs can be reduced.

[0159] The substrate 505 having an insulating surface is the same as the substrate 400 shown in Embodiment 3. In this embodiment mode, a glass substrate is used as the substrate 505.

[0160] An insulating film serving as a base film may be provided between the substrate 505 and the gate electrode layer 511. , which has the function of preventing the diffusion of impurity elements from the substrate 505, and The insulating film is made of one or more films selected from a silicon film, a silicon nitride oxide film, and a silicon oxynitride film. The insulating film can be formed by a laminated structure.

[0161] The material of the gate electrode layer 511 is molybdenum, titanium, tantalum, tungsten, or aluminum. Metallic materials such as aluminum, copper, neodymium, scandium, etc., or alloy materials containing these as the main components The insulating film can be formed as a single layer or a laminate using a material.

[0162] Next, a gate insulating layer 507 is formed on the gate electrode layer 511. The gate insulating layer 507 is , a silicon oxide layer, a silicon nitride layer, an oxide layer, etc. are formed by using a plasma CVD method or a sputtering method. Silicon nitride layer, silicon oxynitride layer, aluminum oxide layer, aluminum nitride layer, oxide Aluminum nitride layer, aluminum nitride oxide layer, or hafnium oxide layer, either as a single layer or as a stack It can be formed as follows.

[0163] The oxide semiconductor of this embodiment is an oxide semiconductor that has been made i-type or substantially i-type by removing impurities. Such highly purified oxide semiconductors have low resistance to interface states and interface charges. Since the interface between the oxide semiconductor layer and the gate insulating layer is extremely sensitive to the temperature, the interface between the oxide semiconductor layer and the gate insulating layer is important. Therefore, the gate insulating layer in contact with the highly purified oxide semiconductor is required to have high quality.

[0164] For example, the high-density plasma CVD method using microwaves (for example, a frequency of 2.45 GHz) This is preferable because it allows the formation of a high-quality insulating layer with high dielectric strength. The close contact between the body and the high-quality gate insulating layer reduces the interface state and improves the interface characteristics. Because it can be made into a thing.

[0165] Of course, if a good insulating layer can be formed as a gate insulating layer, sputtering or plating is also possible. Other film formation methods such as plasma CVD can be applied. The insulating layer may be one in which the film quality of the gate insulating layer and the interface characteristics with the oxide semiconductor are improved. In any case, it is necessary to have good film quality as a gate insulating layer, and also to have oxide semiconductor Any material may be used as long as it can reduce the interface state density with the body and form a good interface.

[0166] In addition, the gate insulating layer 507 and the oxide semiconductor film 530 contain hydrogen, a hydroxyl group, and moisture as much as possible. In order to prevent the oxide semiconductor film 530 from being broken down, sputtering was performed as pretreatment before the formation of the oxide semiconductor film 530. The substrate 505 on which the gate electrode layer 511 is formed or the gate insulating layer 5 The substrate 505 on which the above-mentioned steps 107 are formed is preheated to remove hydrogen, moisture, etc. adsorbed on the substrate 505. It is preferable to desorb and exhaust the impurities. A pump is preferable. However, this preheating process can be omitted. Before the insulating layer 516 is formed, the source electrode layer 515a and the drain electrode layer 515b are heated. The same process may be carried out on the formed substrate 505 .

[0167] Next, a film having a thickness of 2 nm to 200 nm, preferably 5 nm or more, is formed on the gate insulating layer 507. An oxide semiconductor film 530 having a thickness of 30 nm or less is formed (see FIG. 8A).

[0168] Note that before the oxide semiconductor film 530 was formed by a sputtering method, argon gas was introduced. The reverse sputtering that generates plasma is carried out to remove the powder adhering to the surface of the gate insulating layer 507. It is preferable to remove the substance (also called particles or dust). No voltage was applied to the target side, and voltage was applied to the substrate side using an RF power supply in an argon atmosphere. This method generates plasma near the substrate and modifies the surface. Alternatively, nitrogen, helium, oxygen, etc. may be used.

[0169] The oxide semiconductor used for the oxide semiconductor film 530 is the quaternary metal oxide semiconductor described in Embodiment 3. ternary metal oxides, binary metal oxides, In-O, Sn-O, Zn-O, etc. Any oxide semiconductor can be used. In addition, the oxide semiconductor may contain SiO2. In this embodiment, the oxide semiconductor film 530 is an In—Ga—Zn—O-based oxide film. The cross section at this stage corresponds to Figure 8(A). The oxide semiconductor film 530 is heated under a rare gas (typically, argon) atmosphere or an oxygen atmosphere. It can be formed by sputtering under atmospheric pressure or under a mixed atmosphere of rare gas and oxygen. .

[0170] The oxide semiconductor film 530 is formed by sputtering using a target having a composition of, for example, The ratio can be In2O3:Ga2O3:ZnO=1:1:1 [molar ratio]. In addition, In2O3:Ga2O3:ZnO=1:1:2 [molar ratio], Or a ternary tantalum having a composition ratio of In2O3:Ga2O3:ZnO=1:1:4 [molar ratio] A fill rate of the oxide target is preferably 90% or more and 100% or less. The filling rate is between 95% and 99.9%. As a result, the formed oxide semiconductor film becomes a dense film.

[0171] The oxide semiconductor film 530 is formed using a sputtering gas containing hydrogen, water, a hydroxyl group, or hydrogen. It is preferable to use a high-purity gas from which impurities such as oxides have been removed.

[0172] The substrate is held in a film-forming chamber maintained in a reduced pressure state, and the substrate temperature is preferably set to 100°C or more and 600°C or less. The temperature is preferably 200°C or higher and 400°C or lower. The concentration of impurities contained in the sputtered oxide semiconductor film can be reduced. Damage caused by coating is reduced. The removed sputtering gas is introduced, and an oxide semiconductor is deposited on the substrate 505 using the target. To remove residual moisture in the deposition chamber, an adsorption type vacuum pump, e.g. For example, it is preferable to use a cryopump, an ion pump, or a titanium sublimation pump. In addition, the exhaust means is a turbo molecular pump with a cold trap added. The deposition chamber evacuated using a cryopump may contain, for example, hydrogen atoms, water (H2 O) and other compounds (more preferably compounds containing carbon atoms) are exhausted. The concentration of impurities contained in the oxide semiconductor film formed in the deposition chamber can be reduced.

[0173] As an example of the film formation conditions, the distance between the substrate and the target is 100 mm, and the pressure is 0.6 Pa. The conditions were: DC power 0.5kW, oxygen (oxygen flow rate 100%) atmosphere. In addition, when a pulsed DC power supply is used, powdery substances (particles, etc.) generated during film formation are This is preferable because it can reduce the thickness (also called "slippage") and make the film thickness distribution uniform.

[0174] Next, the oxide semiconductor film 530 is subjected to a second photolithography process to form an island-shaped oxide semiconductor film. In addition, a resist mask for forming an island-shaped oxide semiconductor layer is applied to the substrate. If the resist mask is formed by the ink jet method, the photomask Since no disks are used, manufacturing costs can be reduced.

[0175] In addition, when a contact hole is formed in the gate insulating layer 507, the process is performed using an oxide semiconductor This can be done simultaneously with the processing of the film 530 .

[0176] The etching of the oxide semiconductor film 530 here can be performed by dry etching or wet etching. For example, wet etching of the oxide semiconductor film 530 may be used. The etching solution used for etching is a mixture of phosphoric acid, acetic acid, and nitric acid. Alternatively, ITO07N (manufactured by Kanto Chemical Co., Ltd.) may be used.

[0177] Next, the oxide semiconductor layer is subjected to first heat treatment. The conductor layer can be dehydrated or dehydrogenated. The temperature of the first heat treatment is 400°C. The temperature is set to 750°C or higher, or 400°C or higher but lower than the distortion point of the substrate. The substrate was placed in an electric furnace, which is one of the facilities, and the oxide semiconductor layer was heated to 450°C in a nitrogen atmosphere. After the heat treatment for 1 hour, the oxide semiconductor layer was cooled to room temperature and then cooled to room temperature without being exposed to the air. The recontamination of elements is prevented, and an oxide semiconductor layer 531 is obtained (see FIG. 8B).

[0178] The heat treatment device is not limited to an electric furnace, and may be a heat treatment device using heat conduction or heat from a heat source such as a resistance heating element. A device that heats the object to be treated by radiation may be used. For example, a GRTA (Gas Reactor Tank Apparatus) apid Thermal Anneal) equipment, LRTA (Lamp Rapid T RTA (Rapid Thermal Anneal) equipment, etc. The LRTA device can be used with halogen lamps, metal halide lamps, etc. lamp, xenon arc lamp, carbon arc lamp, high-pressure sodium lamp, high-pressure mercury lamp It is a device that heats the object to be treated by radiating light (electromagnetic waves) emitted from a lamp or other lamp. The GRTA device is a device that uses high-temperature gas to perform heat treatment. Inert gases such as argon or nitrogen that do not react with the material to be treated by heat treatment An active gas is used.

[0179] For example, the first heat treatment is performed by heating an inert gas to a high temperature of 650° C. or more and 700° C. or less. The substrate is moved into the inert gas, heated for a few minutes, and then moved to a high-temperature inert gas. GRTA can be performed by releasing the gas.

[0180] In the first heat treatment, nitrogen or a rare gas such as helium, neon, or argon is used. It is preferable that the nitrogen introduced into the heat treatment device does not contain water, hydrogen, etc. Or the purity of rare gases such as helium, neon, and argon must be 6N (99.9999%) or higher. Preferably, the impurity concentration is 7N (99.99999%) or more (i.e., 1 ppm or less, It is preferable that the concentration is 0.1 ppm or less.

[0181] After the oxide semiconductor layer is heated by the first heat treatment, high-purity oxygen gas, high-purity SiO 2 gas, and Introduce high-temperature N2O gas or ultra-dry air (dew point below -40°C, preferably below -60°C). It is preferable that the oxygen gas or N2O gas does not contain water, hydrogen, etc. Alternatively, the purity of the oxygen gas or N2O gas introduced into the heat treatment device is preferably 6N or more. or 7N or more (i.e., the impurity concentration in oxygen gas or N2O gas is 1 ppm or less, preferably It is preferable to set the concentration of the oxygen gas or N2O gas to 0.1 ppm or less. Oxygen that is simultaneously reduced by the process of removing impurities through dehydration or dehydrogenation treatment By supplying the oxide semiconductor layer, the oxide semiconductor layer is highly purified and made electrically i-type (intrinsic).

[0182] In addition, the first heat treatment of the oxide semiconductor layer is performed on the oxide semiconductor layer before it is processed into the island-shaped oxide semiconductor layer. The semiconductor film 530 can also be subjected to the first heat treatment. In that case, after the first heat treatment, The substrate is removed and subjected to a photolithography process.

[0183] In addition to the above, the first heat treatment may be performed after the oxide semiconductor layer is formed. After stacking the source electrode layer and the drain electrode layer on the insulating layer, or This may be done either after forming an insulating layer on the drain electrode layer or after forming an insulating layer on the drain electrode layer.

[0184] In addition, when a contact hole is formed in the gate insulating layer 507, the process is performed using an oxide semiconductor This may be done before or after the film 530 is subjected to the first heat treatment.

[0185] In addition, the oxide semiconductor layer is formed in two separate steps and heat-treated in two separate steps. Regardless of the material of the component, such as oxide, nitride, or metal, the film thickness is thick and the crystalline region (single crystal region) In other words, even if an oxide semiconductor layer having a crystal region with a c-axis aligned perpendicular to the film surface is formed, For example, a first oxide semiconductor film having a thickness of 3 nm to 15 nm is formed, and nitrogen, oxygen, In a rare gas or dry air atmosphere, the temperature is 450°C or higher and 850°C or lower, preferably 550°C or higher. The first heat treatment is performed at 750°C or less, and a crystalline region (including plate-like crystals) is formed in the region including the surface. Then, a second oxide semiconductor film having a thickness larger than that of the first oxide semiconductor film is formed. 2, and the oxide semiconductor film is formed at 450° C. or higher and 850° C. or lower, preferably 600° C. or higher and 70° C. or lower. Second heat treatment is performed at 0° C. or lower, and the first oxide semiconductor film is used as a seed for crystal growth. Crystal growth is performed to crystallize the entire second oxide semiconductor film, resulting in a thick crystalline region. Alternatively, an oxide semiconductor layer having a region may be formed.

[0186] Next, a source electrode layer and a drain electrode layer are formed on the gate insulating layer 507 and the oxide semiconductor layer 531. A conductive film is formed to become the source electrode layer (including wiring formed in the same layer). The conductive film used for the source electrode layer 4 shown in Embodiment 3 can be used as the conductive film for the drain electrode layer. The materials used for the drain electrode layer 405a and the drain electrode layer 405b can be used.

[0187] A resist mask is formed on the conductive film by a third photolithography process, and selective etching is performed. After forming the source electrode layer 515a and the drain electrode layer 515b by etching, The mask is removed (see FIG. 8(C)).

[0188] The third photolithography process involves exposure to ultraviolet light or KrF laser light when forming a resist mask. The source electrodes adjacent to each other on the oxide semiconductor layer 531 may be formed by using a laser beam or an ArF laser beam. The width of the gap between the bottom end of the drain electrode layer and the bottom end of the drain electrode layer determines the width of the gap between the bottom end of the drain electrode layer and the bottom end of the transistor to be formed later. The channel length L is determined. When performing exposure with a channel length L of less than 25 nm, Extreme ultraviolet rays have extremely short wavelengths, ranging from several nanometers to several tens of nanometers. The exposure during the resist mask formation in the third photolithography process is performed using a Extreme ultraviolet light exposure has high resolution and a large depth of focus. It is also possible to set the channel length L of the transistor to 10 nm or more and 1000 nm or less. This allows the operating speed of the circuit to be increased.

[0189] In addition, in order to reduce the number of photomasks and steps used in the photolithography process, The resist mask is formed by a multi-tone mask, which is an exposure mask that allows the incident light to have multiple intensities. The etching process may be performed using a resist mask formed using a multi-tone mask. The mask has a shape with multiple film thicknesses, and the shape can be further deformed by etching. Therefore, it can be used in multiple etching processes to process different patterns. Therefore, one multi-tone mask can handle at least two different patterns. Therefore, the number of exposure masks can be reduced. Since the corresponding photolithography process can also be eliminated, the process can be simplified.

[0190] Note that when the conductive film is etched, the oxide semiconductor layer 531 is etched and divided. However, it is desirable to optimize the etching conditions so that the conductive film alone does not It is difficult to achieve a condition in which the oxide semiconductor layer 531 is etched while the oxide semiconductor layer 532 is not etched at all. When the conductive film is etched, only a part of the oxide semiconductor layer 531 is etched, and the groove In some cases, the oxide semiconductor layer may have a recess (concave portion).

[0191] In this embodiment, a Ti film is used as the conductive film, and an In—Ga— Since a Zn-O-based oxide semiconductor was used, ammonia hydrogen peroxide (AHPO) was used as an etchant for the conductive film. A mixture of ammonia, water, and hydrogen peroxide is used.

[0192] Next, plasma treatment is performed using gases such as N2O, N2, or Ar to remove the exposed The plasma treatment may be performed to remove adsorbed water or the like attached to the surface of the oxide semiconductor layer. In this case, the insulating layer 5, which serves as a protective insulating film in contact with a part of the oxide semiconductor layer, is formed without being exposed to the air. Form 16.

[0193] The insulating layer 516 has a thickness of at least 1 nm, and is formed by a method such as sputtering. The insulating layer 516 can be formed by appropriately using a method that does not mix impurities such as hydrogen. When hydrogen is contained in the oxide semiconductor layer, the hydrogen penetrates into the oxide semiconductor layer, or the oxide semiconductor layer is deformed by the hydrogen. The oxygen in the layer is extracted, and the back channel of the oxide semiconductor layer becomes low resistance (n-type). Therefore, the insulating layer 516 should be as thin as possible. It is important that the deposition process does not use hydrogen, resulting in a hydrogen-free film.

[0194] In this embodiment, a silicon oxide film having a thickness of 200 nm is formed as the insulating layer 516 by sputtering. The substrate temperature during film formation may be set to a temperature between room temperature and 300° C. The temperature is set to 100°C. The silicon oxide film is formed by sputtering using a rare gas (typically This should be done under an atmosphere of argon, oxygen, or a mixture of rare gas and oxygen. In addition, a silicon oxide target or a silicon target can be used as the target. For example, a silicon target can be used in an atmosphere containing oxygen. Silicon oxide can be formed in contact with the oxide semiconductor layer by sputtering. The insulating layer 516 does not contain impurities such as moisture, hydrogen ions, and OH-, and these impurities are not absorbed from the outside. An inorganic insulating film is used to block the penetration of silicon dioxide and silicon nitride. A silicon film, an aluminum oxide film, an aluminum oxynitride film, or the like is used.

[0195] As in the case of forming the oxide semiconductor film 530, residual moisture in the deposition chamber for the insulating layer 516 is removed. To achieve this, it is preferable to use an adsorption type vacuum pump (such as a cryopump). The concentration of impurities contained in the insulating layer 516 formed in a deposition chamber evacuated using an opto-pump was reduced. In addition, the following exhaust means can be used to remove residual moisture in the deposition chamber for the insulating layer 516: A turbomolecular pump with a cold trap added may also be used.

[0196] The insulating layer 516 is formed using a sputtering gas such as hydrogen, water, a hydroxyl group, or a hydride. It is preferable to use a high-purity gas from which impurities have been removed.

[0197] Next, a second heat treatment (preferably 2 For example, the temperature is increased by heating in a nitrogen atmosphere. The second heat treatment is carried out at 250°C for 1 hour under atmospheric pressure. A part of the body layer (channel forming region) is heated while being in contact with the insulating layer 516 .

[0198] Through the above steps, the oxide semiconductor film is subjected to the first heat treatment to remove hydrogen and Impurities such as moisture, a hydroxyl group, or hydrides (also called hydrogen compounds) are intentionally removed from the oxide semiconductor layer. The oxide semiconductor is formed by eliminating impurities and reducing the impurity concentration. Therefore, the oxide semiconductor layer can be supplied with oxygen, which is one of the main components of the oxide semiconductor layer. It is purified and electrically made i-type (intrinsic).

[0199] Through the above steps, a transistor 510 is formed (see FIG. 8D).

[0200] Furthermore, when a silicon oxide layer containing many defects is used as the insulating layer 516, after the silicon oxide layer is formed, The heat treatment reduces hydrogen, moisture, a hydroxyl group, hydride, or the like contained in the oxide semiconductor layer. Impurities are diffused into the oxide insulating layer, and the impurities contained in the oxide semiconductor layer are further reduced. This has the effect of

[0201] A protective insulating layer 506 may be further formed on the insulating layer 516. For example, the protective insulating layer 506 may be formed by RF sputtering. The RF sputtering method is suitable for mass production, so it is used to form a silicon nitride film. This is a preferable film formation method. The protective insulating layer does not contain impurities such as moisture, and these impurities are easily absorbed from the outside. Inorganic insulating films such as silicon nitride and aluminum nitride are used to block the penetration of In this embodiment, the protective insulating layer 506 is formed using a silicon nitride film. (See Figure 8(E)).

[0202] In this embodiment, the substrate 505 on which the insulating layer 516 is formed is used as the protective insulating layer 506. It contains high-purity nitrogen that has been heated to a temperature between 100°C and 400°C and has had hydrogen and moisture removed. A sputtering gas is introduced and a silicon nitride film is formed using a silicon semiconductor target. In this case, similar to the insulating layer 516, the protective insulating layer 516 is formed while removing the residual moisture in the processing chamber. Preferably, layer 506 is deposited.

[0203] After the protective insulating layer 506 is formed, the heating is continued in the atmosphere at 100° C. or more and 200° C. or less for 1 hour or more and 30 The heating treatment may be carried out for a period of time not exceeding 1 hour. Alternatively, the temperature may be increased from room temperature to a heating temperature of 100°C or more and 200°C or less, and then the heating temperature may be increased to 100°C or more and 200°C or less. The temperature may be lowered to room temperature several times.

[0204] In this manner, the transistor including the highly purified oxide semiconductor layer manufactured according to this embodiment By using a transistor, the current value in the off state (off current value) can be reduced. Therefore, the retention time of electrical signals such as image signals can be extended, and writing The interval can also be set longer, so the frequency of refresh operations can be reduced. Therefore, the effect of suppressing power consumption can be enhanced.

[0205] Furthermore, a transistor including a highly purified oxide semiconductor layer can have high field-effect mobility. Therefore, when this transistor is used in a pixel portion of a liquid crystal display device, high-speed operation is possible. By using this transistor, it is possible to provide high quality images. Since the driver circuit section and pixel section can be separately manufactured on one substrate, This allows for a reduction in the number of parts required for the device.

[0206] This embodiment mode can be implemented by being appropriately combined with the configurations described in other embodiments. is.

[0207] (Embodiment 5) The liquid crystal display device disclosed in this specification can be applied to various electronic devices (including gaming machines). The electronic device can be, for example, a television device (television or television (also called digital receivers), computer monitors, digital cameras, digital video cameras cameras such as digital cameras, digital photo frames, mobile phones (also called mobile phones or mobile phone devices) (c), portable game machines, personal digital assistants, audio playback devices, large game machines such as pachinko machines, etc. In this embodiment, the liquid crystal display device described in the above embodiment is Examples of the electronic devices that can be used will be described below.

[0208] FIG. 9A shows an electronic book (also called an E-book), which includes a housing 9630, a display portion 9631, and a , an operation key 9632, a solar cell 9633, and a charge / discharge control circuit 9634. The electronic book shown in Figure 9(A) displays various information (still images, videos, text images, etc.). Functions that display calendars, dates, or times on the display, functions that display The ability to manipulate or edit information, control processing through various software (programs), 9A shows an example of a charge and discharge control circuit 9634. The battery 9635 and the DC-DC converter (hereafter referred to as the converter) 9636 are used. The liquid crystal display device shown in any one of Embodiments 1 to 4 is By applying it to the display part 9631, it is possible to maintain good image display function for a longer period of time and improve security. This makes it possible to create an e-book reader that is highly portable and consumes low power.

[0209] By using the structure shown in FIG. 9A, the display portion 9631 can be a semi-transmissive or reflective type. When using a liquid crystal display, it is expected that it will be used in relatively bright conditions, and solar cell 9633 This is preferable because it allows for efficient power generation by the solar cell and charging of the battery 9635. The solar cell 9633 may be appropriately installed in the empty space (front or back) of the housing 9630. This is preferable because it allows for efficient charging of the battery 9635. In addition, if a lithium-ion battery is used as the battery 9635, it can be made smaller. There are other advantages, such as:

[0210] The configuration and operation of the charge / discharge control circuit 9634 shown in FIG. 9A are shown in FIG. 9B. A block diagram is shown and explained. In FIG. 9(B), a solar cell 9633, a battery 9635, Inverter 9636, converter 9637, switches SW1 to SW3, display unit 9631 The figure shows the battery 9635, converter 9636, converter 9637, Switches SW1 to SW3 correspond to the charge / discharge control circuit 9634.

[0211] First, an example of operation when power is generated by the solar cell 9633 using external light will be described. The power generated by the solar cell is converted to a voltage for charging the Battery 9635. The voltage is increased or decreased by the photovoltaic cell 9636. When power from the 633 is used, switch SW1 is turned on and the converter 9637 The voltage is increased or decreased to the voltage required for the display unit 9631. When not using the display, turn SW1 off and SW2 on to power the battery 9635 The charging may be performed in the above manner.

[0212] Next, an example of operation when external light does not generate electricity by the solar cell 9633 will be described. The power stored in Battery 9635 is converted by turning on switch SW3. The voltage is increased or decreased by the power supply 9637. Power will be drawn from Lee 9635.

[0213] Although the solar cell 9633 is shown as an example of a charging means, it may be possible to charge the battery by other means. It may be configured to charge the Terry 9635. It may also be configured to charge the Terry 9635 in combination with other charging means. This may also be configured as follows.

[0214] FIG. 10A shows a notebook personal computer, which includes a main body 3001 and a housing 300 2, a display unit 3003, a keyboard 3004, etc. By applying the liquid crystal display device shown in any one of the above to 4 to the display portion 3003, A notebook PC that maintains good image display capabilities, is highly secure, and consumes low power. It can be a global computer.

[0215] FIG. 10B shows a personal digital assistant (PDA), which has a main body 3021 including a display unit 3023 and a An external interface 3025 and operation buttons 3024 are provided. The LCD panel shown in any one of the first to fourth embodiments has a stylus 3022 as an accessory. By applying a display device to the display unit 3023, it is possible to provide a more convenient and secure display. It can be used as a power-efficient personal digital assistant (PDA).

[0216] FIG. 10C shows an example of an electronic book. For example, an electronic book 2700 is housed in a housing 27 It consists of two housings, housing 2701 and housing 2703. 3 is integrated with a shaft portion 2711, and performs opening and closing operations around the shaft portion 2711. With this configuration, it is possible to operate like a paper book.

[0217] A display unit 2705 is incorporated in the housing 2701, and a display unit 2707 is incorporated in the housing 2703. The display unit 2705 and the display unit 2707 are configured to display a continuous screen. Alternatively, a different screen may be displayed. For example, a sentence is displayed on the right display unit (display unit 2705 in FIG. 10C), and An image can be displayed on the display portion (the display portion 2707 in FIG. 10C). The liquid crystal display device shown in any one of 1 to 4 is applied to the display portion 2705 and the display portion 2707. This allows for better image display for longer periods, high security, and low power consumption. It can be a powerful e-book 2700.

[0218] FIG. 10C shows an example in which an operation unit and the like are provided in the housing 2701. For example, The housing 2701 includes a power supply 2721, operation keys 2723, a speaker 2725, etc. The operation keys 2723 can be used to turn pages. The surface may be provided with a keyboard, a pointing device, etc. On the back and sides, there are external connection terminals (earphone terminal, USB terminal, etc.), storage media insertion port, etc. Furthermore, the electronic book 2700 may have a function as an electronic dictionary. A similar configuration may also be used.

[0219] The electronic book 2700 may also be configured to be able to send and receive information wirelessly. The desired book data can be purchased and downloaded from the e-book server. is also possible.

[0220] FIG. 10(D) shows a mobile phone, which is composed of two housings, a housing 2800 and a housing 2801. The housing 2801 contains a display panel 2802, a speaker 2803, a microphone, and 2804, pointing device 2806, camera lens 2807, external connection terminal The housing 2800 also includes a solar panel for charging the portable information terminal. It is equipped with a battery cell 2810, an external memory slot 2811, etc. The antenna is also attached to the case. The liquid crystal display device shown in any one of the first to fourth embodiments is built in the body 2801. By applying this to the display panel 2802, it is possible to maintain good image display function for a long time and This results in a mobile phone with high security and low power consumption.

[0221] The display panel 2802 is equipped with a touch panel, and the image displayed in FIG. The multiple operation keys 2805 are indicated by dotted lines. It also has a boost circuit to boost the voltage required for each circuit.

[0222] The display direction of the display panel 2802 changes appropriately depending on the usage mode. The camera lens 2807 is located on the same surface as the camera lens 2802, so video calls are possible. The speaker 2803 and microphone 2804 are not limited to voice calls, but also to video calls, Recording and playback are possible. Furthermore, the housing 2800 and the housing 2801 can be slid apart. It can be folded from the unfolded state shown in 10(D) to the overlapped state, making it suitable for carrying. This makes it possible to miniaturize the device.

[0223] The external connection terminal 2808 can be connected to various cables such as AC adapters and USB cables. It is possible to charge the battery and to communicate data with a personal computer, etc. By inserting a recording medium into the memory slot 2811, it is possible to store and transfer a larger amount of data. do.

[0224] In addition to the above functions, even if the device has infrared communication function, TV reception function, etc. good.

[0225] FIG. 10(E) shows a digital video camera, which includes a main body 3051, a display unit (A) 3057, Eyepiece 3053, operation switch 3054, display unit (B) 3055, battery 3056, etc. The liquid crystal display device according to any one of the first to fourth embodiments is configured by the following. By applying it to the part (A) 3057 and the display part (B) 3055, a better image display can be achieved for a longer period of time. It is a digital video camera that maintains display functionality, has high security, and consumes low power. This can be done.

[0226] FIG. 10(F) shows an example of a television device. The television device 9600 includes: A display unit 9603 is incorporated in a housing 9601. The display unit 9603 displays images. In this case, the housing 9601 is supported by a stand 9605. The liquid crystal display device shown in any one of the first to fourth embodiments has a display unit 96 By applying it to 03, it will maintain good image display function for a longer period of time and also have high security. This makes it possible to provide a television device with low power consumption.

[0227] The television device 9600 can be operated using an operation switch on the housing 9601 or a separate remote control. This can be done by a remote control operator. A display unit for displaying the output information may be provided.

[0228] The television device 9600 is configured to include a receiver, a modem, and the like. It can receive more general TV broadcasts and can also receive them via wired or wireless modems. By connecting to a communication network, it can be one-way (sender to receiver) or two-way. It is also possible to communicate information (between a sender and a receiver, or between receivers).

[0229] This embodiment mode can be implemented by being appropriately combined with the configurations described in other embodiments. is. [Example]

[0230] In this example, a liquid crystal display device that displayed an initialization image before being turned off was used. Comparison of the display state of LCD devices that were turned off while still displaying the image before the off state Shows.

[0231] In Figures 12(A) and 13(A) (Figures 12(A) and 13(A) are the same photograph), 12(A) and 13(A) show the screens in which the images in the ON state before the OFF state are displayed. The image shown in Figure 1 is a black and white checkerboard pattern, and the oxide film has a low off-current as the pixel switching element. A transistor using a compound semiconductor layer (In-Ga-Zn-O layer) was applied. The liquid crystal display device is a transmissive liquid crystal display device, and light is supplied from a backlight. In this embodiment, the liquid crystal display device is turned off, and the display panel including the driving circuit unit and the pixel unit Even after the power supply voltage to the display is stopped, the backlight remains lit so that the display status can be seen. The liquid crystal display device of this embodiment is a normally white liquid crystal display device, The initial state of the liquid crystal is to transmit light from the backlight and display white.

[0232] As shown in FIG. 12B, before the display device is turned off, a fixed potential is written to the capacitor element to reset the liquid crystal to the initial state. When the power supply potential to the display panel including the driver circuit unit and the pixel unit is stopped after returning to the normal state, This shows the display screen immediately after turning it off. The display screen is the initial state of the LCD, which is a completely white screen. Therefore, the liquid crystal displays a stable initial state when no electric field is applied in the OFF state. It is clear that this is the situation.

[0233] On the other hand, as a comparative example, FIG. 13(B) shows a display image of the grid pattern shown in FIG. 13(A). When the liquid crystal display device is turned off and the supply of the power supply potential to the display panel is stopped, In Fig. 13(B), the display screen is displayed in the ON state just before the power is turned off. A faint checkerboard pattern can be seen, indicating that an electric field continues to be applied to the LCD even after the screen is turned off. Applying an electric field to the liquid crystal for such an unnecessary period of time can lead to deterioration of the liquid crystal, and can cause deterioration of the liquid crystal display device. This can lead to a decrease in image display functionality and reliability.

[0234] As can be seen from the above, a fixed voltage is applied to the liquid crystal element before the OFF state is set. By writing the initial position and displaying the initialization image, deterioration of the liquid crystal element is prevented, and good image display is possible for a long time. This allows functionality to be maintained while also improving security.

[0235] Therefore, a liquid crystal display device with higher reliability and lower power consumption is achieved, and It is possible to provide a driving method. [Explanation of symbols]

[0236] 100 LCD display device 110 Image processing circuit 111 Memory circuit 111b frame memory 112 Comparison circuit 113 Display control circuit 115 Selection circuit 116 Power supply 117 Stopping means 120 Display Panel 121 Drive circuit section 121A Gate line driving circuit 121B Source line driver circuit 122 pixel section 123 pixels 124 gate lines 125 source lines 126 Terminal section 126A terminal 126B terminal 127 Switching element 128 Common electrode 130 Backlight section 131 Backlight control circuit 132 Backlight 200 LCD display device 210 Capacitor element 214 transistor 215 Liquid crystal element 400 boards 401 Gate electrode layer 402 Gate insulating layer 403 Oxide semiconductor layer 405a Source electrode layer 405b Drain electrode layer 407 Insulating Film 409 Protective Insulation Layer 410 Transistor 420 transistors 427 Insulating Layer 430 transistors 436a Wiring layer 436b wiring layer 437 Insulating Layer 440 transistors 505 board 506 Protective insulation layer 507 Gate insulating layer 510 Transistor 511 Gate electrode layer 515a Source electrode layer 515b Drain electrode layer 516 Insulating Layer 530 Oxide semiconductor film 531 Oxide semiconductor layer 601 period 602 period 603 period 604 period 1401 period 1402 period 1403 period 1404 period 2700 e-books 2701 Housing 2703 Housing 2705 ​​Display section 2707 Display section 2711 Shaft 2721 Power supply 2723 Operation Key 2725 Speaker 2800 chassis 2801 Case 2802 Display Panel 2803 Speaker 2804 Microphone 2805 Operation Key 2806 Pointing Device 2807 Camera lenses 2808 External connection terminal 2810 solar cell 2811 external memory slot 3001 main unit 3002 Case 3003 Display section 3004 Keyboard 3021 Main Unit 3022 stylus 3023 Display section 3024 Operation button 3025 External Interface 3051 Main Unit 3053 Eyepiece 3054 Operation switch 3055 Display section (B) 3056 Battery 3057 Display section (A) 9600 Television Equipment 9601 Housing 9603 Display section 9605 Stand 9630 chassis 9631 Display section 9632 Operation Key 9633 Solar Cells 9634 Charge / Discharge Control Circuit 9635 Battery 9636 Converter 9637 Converter

Claims

1. a plurality of pixels in which an image signal is written to a capacitance via a channel formation region of a transistor; a liquid crystal display device in which, in a still image display mode, the image signal is written to the pixel less frequently than in a moving image display mode; The pixel is a first conductive layer; and a first silicon nitride layer having an area above the first conductive layer; a first silicon oxide layer having an area above the first silicon nitride layer; an oxide semiconductor layer having a region above the first silicon oxide layer; a second conductive layer having a region above the oxide semiconductor layer; a third conductive layer having a region above the oxide semiconductor layer; a second silicon oxide layer having a region above the second conductive layer, a region above the third conductive layer, and a region above the oxide semiconductor layer; a second silicon nitride layer having an area above the second silicon oxide layer; the first conductive layer has a region that functions as a gate electrode layer of the transistor, the first silicon nitride layer has a region in contact with an upper surface of the first conductive layer; the first silicon oxide layer has a region in contact with an upper surface of the first silicon nitride layer; the first silicon nitride layer and the first silicon oxide layer have a region that functions as a gate insulating layer of the transistor, the oxide semiconductor layer has a region in contact with an upper surface of the first silicon oxide layer, the oxide semiconductor layer has a region that functions as a channel formation region of the transistor, the second conductive layer has a region in contact with an upper surface of the oxide semiconductor layer, the second conductive layer has a region functioning as one of a source electrode and a drain electrode of the transistor, the third conductive layer has a region in contact with an upper surface of the oxide semiconductor layer, the third conductive layer has a region functioning as the other of the source electrode and the drain electrode of the transistor, the second silicon oxide layer has a region in contact with an upper surface of the second conductive layer, a region in contact with an upper surface of the third conductive layer, and a region in contact with an upper surface of the oxide semiconductor layer; the second silicon nitride layer has a region in contact with an upper surface of the second silicon oxide layer, the oxide semiconductor layer includes a first oxide semiconductor layer and a second oxide semiconductor layer that is provided in contact with an upper surface of the first oxide semiconductor layer and has a thickness larger than that of the first oxide semiconductor layer; the second oxide semiconductor layer has crystals whose c-axes are oriented along a direction perpendicular to a surface of the second oxide semiconductor layer; a proportion of c-axis oriented crystals in the second oxide semiconductor layer is greater than a proportion of c-axis oriented crystals in the first oxide semiconductor layer.

2. In claim 1, the first oxide semiconductor layer has an oxide containing In, Ga, and Zn; The liquid crystal display device, wherein the second oxide semiconductor layer has an oxide containing In, Ga, and Zn.

3. In claim 1, the first oxide semiconductor layer includes an In—Ga—Zn—O-based oxide semiconductor; The second oxide semiconductor layer includes an In—Ga—Zn—O-based oxide semiconductor.

4. In claim 1, the first oxide semiconductor layer includes an In—O-based oxide semiconductor; The second oxide semiconductor layer includes an In—O-based oxide semiconductor.

Images