Liquid crystal display device

Abstract

A liquid crystal display device is provided that can timely shift the threshold voltage of a TFT, the threshold voltage of which has been negatively shifted, to a positive voltage. [Solution] The liquid crystal display device includes a liquid crystal panel, a backlight device, and a control unit. The liquid crystal panel includes TFTs with semiconductor layers containing oxide semiconductors. The control unit executes a write process for writing an image based on a video signal to the liquid crystal panel (11) by applying a positive gate voltage to the TFTs, a maintenance process for maintaining the written image with a negative gate voltage applied to the TFTs, a specific process for continuously applying a positive gate voltage to the TFTs at a predetermined timing and then terminating the application, and a specific lighting drive process for lighting the backlight device for at least a predetermined period while the positive gate voltage is being continuously applied to the TFTs during the specific process.

Description

[Technical Field]

[0001] The present disclosure relates to a liquid crystal display device. [Background technology]

[0002] A liquid crystal display device described in Patent Document 1 is known as an example of a liquid crystal display device having transistors using oxide semiconductors. This liquid crystal display device uses an In-Ga-Zn-based oxide semiconductor (IGZO) or the like as the semiconductor material for the thin film transistors (hereinafter referred to as TFTs) that drive each pixel of the liquid crystal display. It is known that the inclusion of hydrogen and moisture in the oxide semiconductor constituting the TFT causes the TFT's threshold voltage to shift in the negative direction (a negative threshold shift occurs). The liquid crystal display device prevents moisture from penetrating the oxide semiconductor by disposing the contact area between the first and third insulating films constituting the TFT between the TFT and a sealing member that seals the liquid crystal. This reportedly prevents a negative threshold shift in a TFT that uses an In-Ga-Zn-based oxide semiconductor (IGZO) or the like as the oxide semiconductor. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2018-185516 Summary of the Invention [Problem to be solved by the invention]

[0004] Here, light from a backlight device included in a liquid crystal display device that is not reflected by the gate electrode of the TFT but is repeatedly reflected within the TFT is referred to as stray light. In a standby state, which accounts for most of the time when the liquid crystal display device is turned on, an oxide semiconductor TFT incorporated in the liquid crystal display device is maintained in a state in which a negative gate voltage is applied to the oxide semiconductor TFT and the TFT is continuously exposed to stray light from the backlight device. The stray light generates electron-hole pairs in the oxide semiconductor, and the holes of the generated electron-hole pairs are attracted to the negative potential of the gate electrode. In response, holes are trapped at the interface between the oxide semiconductor and the gate insulating layer, causing a negative shift in the threshold voltage of the TFT (a negative threshold shift). However, even if such a negative threshold shift occurs, it is known that when a positive gate voltage is applied to the oxide semiconductor TFT and the TFT is exposed to stray light, the threshold voltage can fluctuate in a positive direction (a positive threshold shift) toward the initial value before the negative threshold shift occurred. Patent Document 1 does not disclose any consideration of threshold voltage recovery.

[0005] The present disclosure has been completed in light of the above circumstances, and an object of the present disclosure is to provide a liquid crystal display device that can timely positively shift the threshold value of a TFT whose threshold value has been negatively shifted. [Means for solving the problem]

[0006] The liquid crystal display device of the present disclosure includes a liquid crystal panel, an illumination device that irradiates the liquid crystal panel from behind with light, and a control unit that controls the driving of the liquid crystal panel and the driving of the illumination device, wherein the liquid crystal panel includes a TFT having a semiconductor layer containing an oxide semiconductor, and the control unit executes the following operations: a write process that writes an image based on a video signal to the liquid crystal panel by applying a positive gate voltage to the TFT; a maintenance process that maintains the image written to the liquid crystal panel by the write process in a state in which a negative gate voltage is applied to the TFT; a specific process that continuously applies a positive gate voltage to the TFT at a predetermined timing and then terminates the application of the positive gate voltage; and a specific lighting drive process that lights and drives the illumination device at least for a predetermined period while the positive gate voltage is continuously applied to the TFT during the execution of the specific process. [Effects of the Invention]

[0007] According to the present disclosure, the threshold of a TFT whose threshold has been negatively shifted can be positively shifted in a timely manner. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is an exploded perspective view of a liquid crystal display device according to a first embodiment. [Figure 2] FIG. 2 is a circuit diagram showing a pixel arrangement in a display area of ​​an array substrate provided in a liquid crystal panel that constitutes a liquid crystal display device. [Figure 3] FIG. 3 is a cross-sectional view of an LED, an LED substrate, and a reflective sheet provided in a backlight device that constitutes a liquid crystal display device. [Figure 4] FIG. 4 is a cross-sectional view of a TFT provided on the array substrate. [Figure 5] FIG. 5 is an explanatory diagram showing details of the write process and sustain process in low-frequency driving and high-frequency driving of the TFT, and details of the specific process and specific lighting drive process. [Figure 6] FIG. 6 is a block diagram showing the electrical configuration of the liquid crystal display device. [Figure 7] FIG. 7 is a flowchart of the specific process execution determination process. [Figure 8] FIG. 8 is an explanatory diagram illustrating a usage state of a PC equipped with a liquid crystal display device according to the second embodiment. [Figure 9] FIG. 9 is a block diagram showing how a control unit of the liquid crystal display device receives a usage status signal. [Figure 10] FIG. 10 is a flowchart of the brightness determination process. DETAILED DESCRIPTION OF THE INVENTION

[0009] First, embodiments of the present disclosure will be listed and described. (1) A liquid crystal display device of the present disclosure includes a liquid crystal panel, an illumination device that irradiates the liquid crystal panel from behind, and a control unit that controls driving of the liquid crystal panel and the illumination device, wherein the liquid crystal panel includes a TFT having a semiconductor layer including an oxide semiconductor, and the control unit executes the following operations: a write process that writes an image based on a video signal to the liquid crystal panel by applying a positive gate voltage to the TFT; a maintain process that maintains the image written to the liquid crystal panel by the write process in a state where a negative gate voltage is applied to the TFT; a specific process that continuously applies a positive gate voltage to the TFT at a predetermined timing and then terminates the application of the positive gate voltage; and a specific lighting drive process that lights and drives the illumination device at least for a predetermined period while the positive gate voltage is continuously applied to the TFT during the specific process.

[0010] The control unit of the liquid crystal display device according to the present disclosure can maintain the image written to the liquid crystal panel by executing a sustain process after the write process. This allows the liquid crystal display device to reduce the frequency of the write process and reduce power consumption when displaying a still image, etc. Here, when a negative gate voltage is applied to the TFT and the TFT is continuously exposed to stray light from a lighting device, a problem occurs in that the threshold voltage of the TFT shifts in the negative direction, i.e., a negative threshold shift occurs. However, even if such a negative threshold shift occurs, it is known that when a positive gate voltage is applied to the TFT and the TFT is exposed to stray light, the threshold voltage shifts in the positive direction (positive direction) toward the initial value before the negative threshold shift (positive threshold shift). In the liquid crystal display device according to the present disclosure, during execution of a specific process, the lighting device is driven to light by a specific lighting drive process, at least for a predetermined period while a positive gate voltage is continuously applied to the TFT. Therefore, when a TFT to which a positive gate voltage is applied is irradiated with stray light, the threshold value of the TFT may be shifted positively. Therefore, the liquid crystal display device according to the present disclosure can timely shift the threshold value of a TFT whose threshold value has been shifted negatively to positive.

[0011] (2) In the liquid crystal display device described in (1), the control unit may execute the specific process and the specific lighting drive process when turning off the power supply of the liquid crystal display device.

[0012] In this case, the control unit executes a specific process and a specific lighting drive process when the power supply to the liquid crystal display device is turned off, thereby restoring the threshold value of the TFT whose threshold value has shifted negatively without interfering with the user's use of the liquid crystal display device.

[0013] (3) In the liquid crystal display device described in (1) or (2), the control unit can perform both high-frequency driving, which repeats the write process and the maintenance process frequently, and low-frequency driving, which executes the write process less frequently than the high-frequency driving, and can execute the specific process and the specific lighting drive process when the period during which the low-frequency driving is continued is longer than a predetermined time.

[0014] When the execution of low-frequency driving has continued for a predetermined time or longer, a negative shift in the threshold value of the TFT is likely to occur. Furthermore, when the execution of low-frequency driving has continued for a predetermined time or longer, it is considered that the user is not actively using the liquid crystal display device. In such a case, the control unit can timely shift the threshold value of the TFT whose threshold value has shifted negatively to a positive value by executing the specific process and the specific lighting drive process.

[0015] (4) In the liquid crystal display device described in any one of (1) to (3), the control unit may lower the brightness of the lighting device during the specific lighting drive process below the brightness of the lighting device when executing the writing process and the maintenance process.

[0016] When the write process and sustain process are executed, an image based on a video signal is written to the liquid crystal panel. In such a case, the control unit drives the lighting device at a predetermined luminance required to maintain the visibility of the image. On the other hand, in the specific lighting drive process, it is sufficient to drive the lighting device at a luminance required to cause a positive shift in the threshold of a TFT to which a positive gate voltage is applied. Therefore, the control unit may lower the luminance of the lighting device in the specific lighting drive process compared to the luminance when the write process and sustain process are executed. In this case, the power consumption required to positively shift the threshold of a TFT whose threshold has been negatively shifted can be reduced. Furthermore, driving the lighting device by the specific lighting drive process can avoid causing discomfort to the user.

[0017] (5) In the liquid crystal display device described in any one of (1) to (4), the control unit may determine the brightness of the lighting device in the specific lighting drive process according to the result of determining the usage status of the liquid crystal display device.

[0018] The liquid crystal display device is used in a variety of situations. The control unit determines the brightness of the illumination device in the specific lighting drive process according to the various usage situations of the liquid crystal display device, and executes the specific lighting drive process at the determined brightness. Therefore, the liquid crystal display device can avoid causing discomfort to the user when the illumination device is driven to light by the specific lighting drive process.

[0019] (6) In the liquid crystal display device according to any one of (1) to (5), the semiconductor layer of the TFT may be configured to include an In-Ga-Zn-based oxide semiconductor.

[0020] When the semiconductor layer of a TFT is composed of an In-Ga-Zn-based oxide semiconductor, low-frequency driving, which performs the write process less frequently than high-frequency driving, which repeats the write process and the sustain process more frequently, tends to be more dominant. This makes it more likely that the TFT's threshold value will be negatively shifted. When such a TFT is used, the control unit of the liquid crystal display device executes the specific process and the specific lighting drive process, thereby appropriately shifting the threshold value of the TFT whose threshold value has been negatively shifted to a positive value.

[0021] <Details of the first embodiment of the present disclosure> A first embodiment of the present disclosure will be described with reference to FIGS. 1 to 7. In this specification, a liquid crystal display device 10 is illustrated. Note that X-axis, Y-axis, and Z-axis are shown in some of the drawings, and each axis direction is depicted as being in the direction shown in each drawing. Also, the upper side of FIGS. 1, 3, and 4 is the front side, and the lower side of the drawings is the back side.

[0022] 1, a liquid crystal display device 10 includes a liquid crystal panel 11, a backlight device 12 disposed on the rear side (back surface side) of the liquid crystal panel 11, a pair of polarizing plates 13 and 14 disposed with the liquid crystal panel 11 sandwiched therebetween, and an optical member 15 disposed on the front side (front surface side) of the backlight device 12. The backlight device 12 is an example of an illumination device. The liquid crystal panel 11 is capable of displaying a video (image) using light irradiated from the backlight device 12.

[0023] The configurations of the liquid crystal panel 11 and the backlight device 12 will be described. First, as shown in FIG. 1, the liquid crystal panel 11 has at least a pair of substrates 20 and 21. The front side of the pair of substrates 20 and 21 is a counter substrate 20, and the back side is an array substrate 21. A liquid crystal layer is interposed between the pair of substrates 20 and 21. Both the counter substrate 20 and the array substrate 21 are formed by laminating various films on the inner surface of a glass substrate. The counter substrate 20 is provided with color filters exhibiting R (red), G (green), B (blue), etc., and a light-shielding portion (black matrix) separating adjacent color filters. The liquid crystal layer contains liquid crystal molecules, which are a substance whose optical properties change when an electric field is applied. An alignment film for aligning the liquid crystal molecules contained in the liquid crystal layer is provided on the innermost surface of the counter substrate 20. A seal portion for sealing the liquid crystal layer is provided between the outer peripheral edges of the pair of substrates 20 and 21.

[0024] As shown in Fig. 2, at least TFTs (thin film transistors, switching elements) 23 and pixel electrodes 24 are provided on the inner surface of the array substrate 21. The TFTs 23 and pixel electrodes 24 are arranged in a matrix (rows and columns) with multiple TFTs 23 and multiple pixel electrodes 24 spaced apart along the X-axis and Y-axis directions. Gate wiring (scanning wiring) 25 and source wiring (image wiring, signal wiring) 26 are arranged around these TFTs 23 and pixel electrodes 24, intersecting each other at right angles. The gate wiring 25 extends along the X-axis direction, with multiple wirings spaced apart along the Y-axis. The source wiring 26 extends along the Y-axis direction, with multiple wirings spaced apart along the X-axis.

[0025] The TFT 23 includes a gate electrode 23A connected to the gate line 25, a source electrode 23B connected to the source line 26, a drain electrode 23C connected to the pixel electrode 24, and a semiconductor layer 23D made of a semiconductor material and connected to the source electrode 23B and the drain electrode 23C. The TFT 23 is an active element that applies a voltage to the gate electrode 23A to control the current flowing through the semiconductor layer 23D and switch the current between the source electrode 23B and the drain electrode 23C. Specifically, the TFT 23 is driven based on a scanning signal supplied to the gate electrode 23A by the gate line 25. Then, a potential related to an image signal (data signal) supplied to the source electrode 23B by the source line 26 is supplied to the drain electrode 23C via the semiconductor layer 23D. As a result, the pixel electrode 24 is charged to the potential related to the image signal. The pixel electrode 24 is disposed in a region surrounded by the gate line 25 and the source line 26 and has a planar shape of, for example, a substantially rectangular shape.

[0026] Next, we will explain the configuration of the backlight device 12. As shown in Fig. 3, the backlight device 12 at least includes a plurality of LEDs 40 that are light sources, an LED substrate 43 on which the plurality of LEDs 40 are provided, and a reflective sheet 44. This backlight device 12 is a so-called direct type.

[0027] The LED 40 is configured by sealing an LED chip with a sealing material on a substrate portion mounted on an LED substrate 43. The LED 40 is, for example, a blue LED that emits monochromatic blue light. The sealing material provided in the LED 40 contains a dispersed phosphor. The phosphor contained in the sealing material includes yellow phosphor, green phosphor, red phosphor, etc. The LED 40 equipped with such an LED chip and sealing material emits white light as a whole. The LED 40 is a so-called top-emitting type in which the light-emitting surface 40A faces the front side, which is opposite the LED substrate 43 side.

[0028] As shown in Fig. 3, the LED substrate 43 is disposed with its mounting surface on which the LEDs 40 are mounted facing the front. The LEDs 40 are arranged in a planar manner at intervals on the mounting surface of the LED substrate 43. The reflective sheet 44 is made of a white or silver synthetic resin whose surface has excellent light reflectivity. The reflective sheet 44 is laminated so as to cover almost the entire mounting surface of the LED substrate 43 from the front side, and is formed with a plurality of LED insertion holes 44A at positions overlapping the LEDs 40 to allow the LEDs 40 to pass through individually. The reflective sheet 44 can reflect light emitted from the LEDs 40 toward the front side.

[0029] As shown in Fig. 1, the optical member 15 is plate- or sheet-shaped with a main surface parallel to each main surface of the liquid crystal panel 11 and the LED substrate 43 of the backlight device 12. The optical member 15 is made up of one or more plate- or sheet-shaped members. The optical member 15 is disposed between the liquid crystal panel 11 and the backlight device 12 in the Z-axis direction, and has the function of imparting a predetermined optical effect to the light emitted from the backlight device 12 while emitting the light toward the liquid crystal panel 11. The optical member 15 includes a diffusion plate, a prism sheet, a diffusion sheet, a wavelength conversion sheet, etc.

[0030] The detailed configuration of the TFT 23 will be described. As shown in FIG. 4, the TFT 23 includes a gate electrode 23A, a gate insulating film 27, a semiconductor layer 23D, a source electrode 23B, and a drain electrode 23C. The gate electrode 23A is formed on a glass substrate 211 that constitutes the array substrate 21. The gate insulating film 27 covers the gate electrode 23A. The semiconductor layer 23D is formed on the gate insulating film 27. The semiconductor layer 23D has a channel region 23D1 and a source contact region 23D2 and a drain contact region 23D3 disposed on both sides of the channel region 23D1. The channel region 23D1 overlaps the gate electrode 23A via the gate insulating film 27. The source contact region 23D2 is a region connected to the source electrode 23B, and the source electrode 23B is disposed on the source contact region 23D2. The drain contact region 23D3 is a region connected to the drain electrode 23C, and the drain electrode 23C is disposed on the drain contact region 23D3. On the TFT 23, a passivation film 23E and an organic insulating film 23F are further provided to prevent influence from the external environment and adhesion of contaminants.

[0031] In this embodiment, the semiconductor layer 23D is mainly composed of an oxide semiconductor. The term "mainly composed" refers to the component that is contained in the largest amount among the components that make up the semiconductor layer 23D. The oxide semiconductor may be either amorphous or crystalline, but an amorphous oxide semiconductor is preferably used. This is because a semiconductor film made of an oxide semiconductor has much higher charge mobility than a semiconductor film made of amorphous silicon (amorphous silicon), and can be driven at a lower voltage.

[0032] The semiconductor layer 23D of this embodiment is primarily composed of an In-Ga-Zn-based oxide semiconductor (IGZO) containing In (indium), Ga (gallium), and Zn (zinc). A particularly preferred composition of the In-Ga-Zn-based oxide semiconductor is InGaZnO4, where In:Ga:Zn=1:1:1. This is because oxide semiconductors with this composition are characterized by a tendency for electron mobility to increase as electrical conductivity increases. However, the proportions of In, Ga, and Zn in IGZO can be selected as appropriate.

[0033] The semiconductor layer 23D may be formed using other oxide semiconductors instead of IGZO, such as indium oxide, zinc oxide, tin oxide, In-Zn based oxide (IZO), Zn-Ti based oxide (ZTO), Cd-Ge based oxide, and Cd-Pb based oxide.

[0034] The TFT 23 shown in this example has a bottom-gate structure in which the gate electrode 23A is disposed below the semiconductor layer 23D. The TFT 23 may have a top-gate structure in which the gate electrode 23A is disposed above. The TFT 23 shown in this example has a top-contact structure in which the source electrode 23B and the drain electrode 23C are disposed above the semiconductor layer 23D. The TFT 23 may have a bottom-contact structure in which the source electrode 23B and the drain electrode 23C are disposed below the semiconductor layer 23D.

[0035] When the semiconductor layer 23D is mainly made of IGZO, which has excellent electron mobility, the TFT 23 can be driven by applying a positive gate voltage to the gate electrode 23A, and a write process can be performed frequently to write an image based on a video signal to the liquid crystal panel 11. The more frequently the write process is performed, the more frames of the image are displayed on the liquid crystal panel 11 per unit time, and the smoother the image is displayed. The number of times the write process is performed per second is expressed in Hz. A drive mode of the TFT 23 in which the write process is performed relatively frequently is referred to as "high-frequency drive" below.

[0036] When the semiconductor layer 23D is mainly made of IGZO, the resistance value of the semiconductor layer 23D becomes higher when no positive gate voltage is applied to the gate electrode 23A, and the leakage current flowing through the channel region 23D1 becomes very small, compared to when the semiconductor layer 23D is mainly made of amorphous silicon.

[0037] Specifically, when the semiconductor layer 23D is primarily made of amorphous silicon, if a positive gate voltage is not applied to the gate electrode 23A for a certain period of time, a leakage current flows through the channel region 23D1. This causes a gradual decrease in the pixel potential applied to the pixel electrode 24. To avoid this, even when a still image is displayed on the liquid crystal panel 11, a positive gate voltage must be applied to the gate electrode 23A dozens of times per second to maintain the brightness of the image on the liquid crystal panel 11. In contrast, when the semiconductor layer 23D is primarily made of IGZO, the leakage current flowing through the channel region 23D1 is significantly smaller than when the semiconductor layer 23D is primarily made of amorphous silicon, making it difficult for the pixel potential applied to the pixel electrode 24 to decrease. Therefore, when the semiconductor layer 23D is primarily made of IGZO, an image written on the liquid crystal panel 11 is retained for a certain period of time even if a positive gate voltage is not applied to the gate electrode 23A for a certain period of time, in other words, even if a negative gate voltage is applied. Such a process of maintaining the state in which a negative gate voltage is applied to the gate electrode 23A is called a sustain process.

[0038] The aforementioned high-frequency driving refers to a driving mode of the TFT 23 in which the write process and the sustain process are repeated frequently. A driving mode of the TFT 23 for displaying a still image, in which the write process is performed less frequently than in the high-frequency driving mode, is hereinafter referred to as "low-frequency driving." Low-frequency driving of the TFT 23 is particularly possible when the semiconductor layer 23D is primarily composed of IGZO. In this embodiment, low-frequency driving refers to a state in which the write process is performed at a frequency of 1 Hz to less than 60 Hz, and high-frequency driving refers to a state in which the write process is performed at 60 Hz or more. The control unit 50 of the liquid crystal display device 10, which will be described later, determines whether to drive the TFT 23 at low frequency or high frequency depending on the situation in which an image is written to the liquid crystal panel 11, and drives the TFT 23 at low frequency or high frequency depending on the determination.

[0039] The high-frequency control and low-frequency control of the TFT 23 will be described in detail with reference to FIG. 5. In FIG. 5, the positive gate voltage applied to the gate electrode 23A for the write process is represented as VgH, and the negative gate voltage applied to the gate electrode 23A for the sustain process is represented as VgL. The ground potential (0 V) is represented as GND. VgH is set to +20 V, for example. VgL is set to a voltage lower than GND, for example, −10 V. A power supply circuit 52 included in a control unit 50 (described later) of the liquid crystal display device 10 generates a positive gate voltage for the write process and a negative gate voltage for the sustain process based on an externally applied power supply.

[0040] First, an example of 120 Hz display will be described, in which the write process is performed 120 times per second. In response to the write process, a positive gate voltage is applied to the gate electrode 23A of the TFT 23. Then, until the next write process is performed, a negative gate voltage is applied to the gate electrode 23A in response to the sustain process. In the example of 120 Hz display, the write process and sustain process are repeatedly performed 120 times per second.

[0041] In the example of 60 Hz display, where the write process is performed 60 times per second, the frequency of the write process is halved compared to the example of 120 Hz display. Therefore, as shown in FIG. 5, in the case of 60 Hz display, the period during which the sustain process is performed per unit time is longer than in the case of 120 Hz display. That is, the lower the frequency of the write process, the greater the proportion of the state in which a negative gate voltage is applied to the gate electrode 23A. The examples of 120 Hz display and 60 Hz display, in which the write process and the sustain process are performed several tens of times per second, are examples of high-frequency driving. Although not shown, high-frequency driving may also include a case in which the write process is performed more frequently than in the case of 120 Hz display, such as 240 Hz display.

[0042] FIG. 5 shows an example of low-frequency driving, in which the writing process is performed once per second, at 1 Hz. When low-frequency driving is performed, the frequency of the writing process is significantly lower than when high-frequency driving is performed, and the sustaining process is performed for most of the time. That is, as shown in FIG. 5, when low-frequency driving is performed, the negative gate voltage applied to the gate electrode 23A accounts for most of the time. When a still image or the like is displayed on the liquid crystal panel 11, or when there is a continuous period in which the image displayed on the liquid crystal panel 11 is not updated, the writing process may be performed less frequently, and low-frequency driving is performed. With such low-frequency driving, the frequency of the writing process decreases, thereby reducing the power consumption of the liquid crystal display device 10. Furthermore, the longer the period during which low-frequency driving is performed, the more the power consumption of the liquid crystal display device 10 is reduced.

[0043] Here, the influence of stray light on the TFT 23 will be described with reference to FIG. 4. Although not shown in FIG. 4, the backlight device 12 described above is disposed on the rear surface of the glass substrate 211 (the surface opposite to the surface on which the TFT 23 is disposed). Most of the light L1 emitted from the backlight device 12 and traveling inside the glass substrate 211 toward the TFT 23 is reflected by the gate electrode 23A, which is a metal layer, and does not enter the semiconductor layer 23D. On the other hand, some of the light L2 that enters the TFT 23 without being reflected by the gate electrode 23A is repeatedly reflected by the surfaces of the metal layers, such as the source electrode 23B, the drain electrode 23C, and the gate electrode 23A, and may enter the channel region 23D1 of the semiconductor layer 23D. Light that is repeatedly reflected inside the TFT 23, such as the light L2, is hereinafter referred to as "stray light."

[0044] When a negative gate voltage is applied to the gate electrode 23A while the channel region 23D1 of the semiconductor layer 23D is irradiated with light, the threshold voltage, which is the gate voltage at which the TFT 23 switches from an off state to an on state, shifts in the negative direction. This negative shift of the threshold voltage is called a negative threshold shift. The mechanism by which this negative threshold shift occurs is explained below. When the channel region 23D1 of the semiconductor layer 23D is irradiated with light, electron-hole pairs are generated in the channel region 23D1. When a negative gate voltage is applied to the gate electrode 23A, the holes of the electron-hole pairs generated in the channel region 23D1 are attracted to the negative potential of the gate electrode 23A. The attracted holes are trapped at the interface between the semiconductor layer 23D and the gate insulating film 27 or in a level within the gate insulating film 27. The trapped holes behave like positive fixed charges, and the positive electric field from the holes is thought to make the semiconductor layer 23D more likely to turn on, i.e., a negative threshold shift occurs. When low-frequency driving is performed, a state in which a negative gate voltage is applied to the gate electrode 23 A is dominant. Therefore, in this state, if light due to stray light is irradiated onto the channel region 23D1 of the semiconductor layer 23D, a negative shift in the threshold voltage is likely to occur.

[0045] Another effect of stray light on the TFT 23 will now be described. When a negative shift in the threshold voltage occurs, as described above, holes are trapped at the interface between the semiconductor layer 23D and the gate insulating film 27 or at levels within the gate insulating film 27. Under these conditions, when a positive gate voltage is applied to the gate electrode 23A, the holes trapped at the interface between the semiconductor layer 23D and the gate insulating film 27 or at levels within the gate insulating film 27 are pushed by the positive potential of the gate electrode 23A and move toward the semiconductor layer 23D. At this time, light is irradiated onto the channel region 23D1 of the semiconductor layer 23D, generating electron-hole pairs in the channel region 23D1. Then, the holes trapped at the interface between the semiconductor layer 23D and the gate insulating film 27 or at levels within the gate insulating film 27 are attracted to the free electrons of the electron-hole pairs generated in the channel region 23D1. That is, when a positive gate voltage is applied to the gate electrode 23A and light is irradiated onto the channel region 23D1 of the semiconductor layer 23D, holes trapped at the interface between the semiconductor layer 23D and the gate insulating film 27 or at levels in the gate insulating film 27 are encouraged to move toward the channel region 23D1 of the semiconductor layer 23D. The holes that have moved to the channel region 23D1 combine with free electrons in the channel region 23D1 and lose their charge. This causes the threshold voltage to shift (recover) in the positive direction toward the initial value before the threshold voltage shifted negatively. This recovery of the threshold voltage is called a positive shift of the threshold voltage.

[0046] The inventors of the present application have focused on this positive threshold voltage shift and intentionally created a situation in which stray light is incident on the channel region 23D1 of the semiconductor layer 23D when a positive gate voltage is applied to the gate electrode 23A. By creating this situation at the appropriate time, a positive threshold voltage shift occurs in the TFT 23, which has experienced a negative threshold voltage shift due to low-frequency driving, thereby improving the characteristics of the TFT 23. In other words, the TFT 23 can be driven at low frequency for a longer period of time, and the frequency of writing operations during low-frequency driving can be further reduced.

[0047] 5, the specific processing and the specific lighting drive processing executed by the control unit 50 of the liquid crystal display device 10 will be described. The specific processing execution processing is executed when its execution is determined by a specific processing execution determination processing, which will be described later, executed by the control unit 50. The specific lighting drive processing is executed in synchronization with the specific processing.

[0048] As shown in FIG. 5, the specific process is a process in which a positive gate voltage is continuously applied to the gate electrode 23A of the TFT 23, and then the application of the positive gate voltage is terminated. In this embodiment, the time during which the positive gate voltage is continuously applied to the gate electrode 23A as the specific process is approximately 1 / 60 seconds to 1 second. After the positive gate voltage has been continuously applied to the gate electrode 23A for a predetermined time, the application of the positive gate voltage is terminated. As a result, the potential at the gate electrode 23A then transitions to the ground potential.

[0049] 5, the specific lighting drive process is a process for driving the backlight device 12 to light up during a predetermined period while a positive gate voltage is continuously applied to at least the gate electrode 23A of the TFT 23 during execution of the specific process. As the specific lighting drive process, the control unit 50, which will be described later, transmits an instruction to the backlight control circuit 54 included in the control unit 50 to light up the LEDs 40. Upon receiving this instruction, the backlight control circuit 54 lights up the LEDs 40. This causes the backlight device 12 to light up.

[0050] By performing the specific lighting drive process in synchronization with the specific process, stray light due to light emitted from the backlight device 12 is likely to be generated during the period when a positive gate voltage is continuously applied to the gate electrode 23A. Therefore, when a positive gate voltage is applied to the gate electrode 23A, light due to stray light is likely to be incident on the channel region 23D1 of the semiconductor layer 23D, which makes it easier for a positive shift in the threshold voltage to occur. Therefore, the threshold voltage of the TFT 23 is likely to be restored.

[0051] 5 illustrates an example in which the backlight device 12 is turned on while a positive gate voltage is continuously applied to the gate electrode 23A of the TFT 23, and then turned off. Alternatively, the backlight device 12 may be turned on by a specific lighting drive process during at least a period while a positive gate voltage is continuously applied to the gate electrode 23A of the TFT 23 during execution of the specific process. The backlight device 12 may be turned on by a specific lighting drive process during both a period while a positive gate voltage is continuously applied to the gate electrode 23A of the TFT 23 and after the application of the positive gate voltage has ended during execution of the specific process. That is, it is sufficient that the backlight device 12 is turned on during a predetermined period while a positive gate voltage is continuously applied to at least the gate electrode 23A of the TFT 23 during execution of the specific process.

[0052] Next, a circuit configuration for controlling the driving of the above-mentioned liquid crystal panel 11 and backlight device 12 will be described with reference to FIG. 6. As shown in FIG. 6, the liquid crystal display device 10 has a control unit 50 that controls the driving of the liquid crystal panel 11 and the backlight device 12. The control unit 50 has a CPU that is responsible for overall control of the liquid crystal display device 10. The control unit 50 also has a storage unit (not shown). The storage unit includes a ROM, RAM, flash memory, etc. that store various parameters required when the control unit 50 executes various programs. The storage unit stores a program for causing the control unit 50 to execute a specific process execution determination process, which will be described later with reference to FIG. 7. The storage unit functions as an example of a processor that executes the specific process execution determination process by expanding the program stored in the storage unit.

[0053] The control unit 50 includes electrical circuits such as a video signal processing circuit 51, a panel control circuit 53, and a backlight control circuit 54 that transmit drive signals (e.g., drive currents) to the liquid crystal panel 11 and the backlight device 12 in response to instructions from the CPU. The video signal processing circuit 51 processes a video signal supplied from an external host system and outputs the processed video signal. The panel control circuit 53 writes an image based on the processed video signal output from the video signal processing circuit 51 to the liquid crystal panel 11. If the liquid crystal display device 10 includes a gate driver and a source driver for writing an image to the liquid crystal panel 11, the panel control circuit 53 controls the driving of the gate driver and the source driver. The backlight control circuit 54 controls the LEDs 40 to adjust the light emission amount of the LEDs 40, for example. The backlight control circuit 54 can perform PWM (Pulse Width Modulation) dimming or the like when controlling the LEDs 40. If the liquid crystal display device 10 includes an LED driver for driving the LEDs 40, the backlight control circuit 54 controls the driving of the LED driver. The control unit 50 also includes a power supply circuit 52 that generates and supplies voltages necessary for driving the liquid crystal panel 11 and the backlight device 12 based on an externally supplied power supply voltage. The power supply circuit 52 generates, in particular, a positive gate voltage (VgH) and a negative gate voltage (VgL) to be applied to the TFT 23. The generated VgH and VgL are supplied appropriately to the gate electrode 23A of the TFT 23 of the liquid crystal panel 11 depending on the frequency at which images are written to the liquid crystal panel 11.

[0054] With reference to Figure 7, the specific process execution decision process executed by the control unit 50 of the liquid crystal display device 10 will be described. An interrupt signal generating circuit (not shown) is connected to the control unit 50. The interrupt signal generating circuit generates an interrupt signal every time a clock signal is input from a clock circuit (not shown) that outputs a clock signal of a constant frequency. The control unit 50 executes the specific process execution decision process every time an interrupt signal is input from the interrupt signal circuit. Hereinafter, each step of the process will be abbreviated as "S".

[0055] When the specific process execution determination process is started, the control unit 50 determines whether the liquid crystal display device 10 has been powered off (S11). This determination is made, for example, based on the detection result of a detection unit that is provided in the control unit 50 and that can detect the power supply state (power on / off state). If the control unit 50 determines that the liquid crystal display device 10 has been powered off (S11: YES), the control unit 50 instructs the panel control circuit 53 to execute the specific process shown in FIG. 5, and also instructs the backlight control circuit 54 to execute the specific lighting drive process shown in FIG. 5 (S18). Thereafter, the control unit 50 ends the specific process execution determination process.

[0056] As described above, in this embodiment, the control unit 50 executes a specific process and a specific lighting drive process when the liquid crystal display device 10 is powered off. In the liquid crystal display device 10, even when the user turns off the power, the display may not immediately clear, and an image similar to an afterimage may remain on the liquid crystal panel 11. This condition is called "burn-in." When the liquid crystal display device 10 is powered off, the discharge path of the charge held in the pixel electrodes 24 of the array substrate 21 may be interrupted, causing residual charge to accumulate in the pixel electrodes 24. This is one of the causes of burn-in. Therefore, when powering off the liquid crystal display device 10, a common practice is to apply a positive gate voltage to all gate electrodes 23A, and then terminate the application of the positive gate voltage to discharge the charge in the pixel electrodes 24 and equalize the potential of the pixel electrodes 24 to ground potential. The specific process corresponds to applying a positive gate voltage to all gate electrodes 23A.

[0057] In this embodiment, the control unit 50 executes the specific process when the power is turned off, and also executes the specific lighting drive process. As a result, when a positive gate voltage is applied to the gate electrode 23A, stray light generated based on light emitted from the backlight device 12 is more likely to be incident on the channel region 23D1 of the semiconductor layer 23D of the gate electrode 23A. This makes it easier for a positive shift in the threshold voltage to occur, and the threshold voltage of the TFT 23 is restored.

[0058] By executing the specific process and the specific lighting drive process when the liquid crystal display device 10 is powered off, the control unit 50 can restore the threshold voltage of the TFT 23 without interfering with the user's use of the liquid crystal display device 10. The specific process can be executed not only to restore the threshold voltage of the TFT 23 but also to prevent image burn-in. Therefore, the control unit 50 applies the process conventionally performed when the power is turned off to prevent image burn-in as the specific process, and by executing the specific lighting drive process in addition to the specific process, the threshold voltage of the TFT 23 can be immediately restored. Therefore, the liquid crystal display device 10 can appropriately provide an opportunity for the threshold voltage of the TFT 23 to be restored without requiring major design changes.

[0059] Here, the luminance of the backlight device 12 that is driven to light by the specific lighting drive process will be described. Generally, the luminance of the backlight device 12 in a state where an image written to the liquid crystal panel 11 is displayed by low-frequency driving or high-frequency driving of the TFT 23 (a state where an image is displayed on the liquid crystal panel 11, that is, a so-called image display mode) is 350 cd / m 2 to 1000 cd / m 2 In this embodiment, when the liquid crystal display device 10 is used indoors, the luminance of the backlight device 12 in the image display mode is set to about 300 cd / m 2 to 700 cd / m 2In addition, when the liquid crystal display device 10 is used outdoors, the luminance of the backlight device 12 in the image display mode is set to about 1200 cd / m 2 It is set above.

[0060] In this embodiment, the luminance of the backlight device 12 that is driven to light by the specific lighting drive process is set lower than the luminance in a state where an image written to the liquid crystal panel 11 is displayed by performing low frequency driving or high frequency driving in which the writing process and the sustaining process are repeatedly performed (image display mode state). Specifically, the control unit 50 sets the luminance of the backlight device 12 that is driven to light by the specific lighting drive process to 1 cd / m 2 to 200 cd / m 2 This is to prevent the user of the liquid crystal display device 10 from feeling uncomfortable, as if there is a malfunction such as a failure in the liquid crystal display device 10 or the backlight device 12, due to the backlight device 12 being driven to light at high brightness when the power is turned off. By setting the brightness of the backlight device 12, which is driven to light by the specific lighting drive process, to a relatively low brightness, the liquid crystal display device 10 can reduce the possibility of the user feeling uncomfortable, even when the specific lighting drive process is executed, and can restore the threshold voltage of the TFT 23.

[0061] Returning to the explanation of FIG. 7 , if the control unit 50 determines that the liquid crystal display device 10 is not powered off (S11: NO), it determines whether low-frequency driving has started (S12). The control unit 50 is assumed to be able to distinguish whether low-frequency driving or high-frequency driving is being performed. The control unit 50 is also assumed to be equipped with a well-known timer counter. The timer counter is assumed to be able to measure the elapsed time from a predetermined point in time by counting up clock signals input from the clock circuit described above. The control unit 50 is assumed to be able to measure the duration during which low-frequency driving is being performed using this timer counter. The control unit 50 is assumed to determine that low-frequency driving has started if, at the time of determination in S12, the duration has not been measured using the timer counter and low-frequency driving is being performed.

[0062] When the control unit 50 determines that the low frequency driving has started (S12: YES), the control unit 50 starts measuring the duration using the timer counter (S14). After that, the control unit 50 ends the specific process execution determination process.

[0063] On the other hand, if the control unit 50 does not determine that low-frequency driving has started (S12: NO), it determines whether low-frequency driving has ended and transitioned to high-frequency driving (S15). The control unit 50 determines that low-frequency driving has ended if the control unit 50 is measuring the duration and high-frequency driving is being performed at the time of determination in S15. If the control unit 50 determines that low-frequency driving has ended and transitioned to high-frequency driving (S15: YES), it ends measurement of the duration in the timer counter and clears the duration stored in the timer counter to 0 (S16). Thereafter, the control unit 50 ends the specific process execution determination process.

[0064] On the other hand, if the control unit 50 is not measuring the duration and high-frequency driving is being performed, or if the control unit 50 is measuring the duration and low-frequency driving is being performed, the control unit 50 determines that low-frequency driving has not ended and that the transition to high-frequency driving has not occurred (S15: NO). In this case, the control unit 50 refers to the timer counter and determines whether the measured duration is equal to or longer than a predetermined time (S17). If the measured duration is less than the predetermined time or if high-frequency driving is being performed, the control unit 50 determines that the measured duration is not equal to or longer than the predetermined time (S17: NO), and ends the specific process execution determination process.

[0065] On the other hand, if the measured duration is equal to or longer than the predetermined time (S17: YES), the control unit 50 instructs the panel control circuit 53 to execute the specific process shown in Fig. 5, and also instructs the backlight control circuit 54 to execute the specific lighting drive process shown in Fig. 5 (S18). Thereafter, the control unit 50 ends the specific process execution determination process.

[0066] Thus, in this embodiment, the control unit 50 executes the specific process and the specific lighting drive process when the period during which low-frequency driving continues is equal to or longer than the predetermined time. In this embodiment, the predetermined time is, for example, one hour. This predetermined time can be set arbitrarily by the designer of the liquid crystal display device 10 within a range appropriate for restoring the threshold value of the TFT 23.

[0067] When the execution of low-frequency driving continues for a predetermined time or longer, a negative shift in the threshold of the TFT 23 is likely to occur. Furthermore, when the execution of low-frequency driving continues for a predetermined time or longer, it is likely that the user is not actively using the liquid crystal display device 10, for example, when a period of time has continued during which no external operation has been performed on the liquid crystal display device 10. In such a case, the control unit 50 executes the specific process and the specific lighting drive process, thereby enabling the threshold of the TFT 23, whose threshold has shifted negatively due to the continuation of low-frequency driving, to be shifted positively in a timely manner, without interfering with the user's use of the liquid crystal display device 10 as much as possible.

[0068] As described above, the liquid crystal display device 10 includes a liquid crystal panel 11, a backlight device 12 that irradiates the liquid crystal panel 11 with light from behind, and a control unit 50 that controls driving of the liquid crystal panel 11 and the backlight device 12. The liquid crystal panel 11 includes a TFT 23 that has a semiconductor layer 23D containing an oxide semiconductor. The control unit 50 executes the following processes: a write process that writes an image based on a video signal to the liquid crystal panel 11 by applying a positive gate voltage to the TFT 23; a maintenance process that maintains the image written to the liquid crystal panel 11 by the write process in a state in which a negative gate voltage is applied to the TFT 23; a specific process that continuously applies a positive gate voltage to the TFT 23 at a predetermined timing and then terminates the application of the positive gate voltage; and a specific lighting drive process that lights and drives the backlight device 12 for at least a predetermined period while the positive gate voltage is continuously applied to the TFT during the specific process.

[0069] The control unit 50 of the liquid crystal display device 10 can maintain the image written to the liquid crystal panel 11 by the writing process by executing a maintenance process. This allows the liquid crystal display device 10 to reduce the frequency of the writing process and reduce its power consumption when displaying a still image, etc. Here, if a negative gate voltage is applied to the TFT 23 and the TFT 23 is continuously exposed to stray light from the backlight device 12, the threshold voltage of the TFT 23 may shift in the negative direction, resulting in a negative threshold shift. However, even if such a negative threshold shift occurs, it is known that the threshold voltage may shift in the positive direction (positive direction) toward the initial value before the negative threshold shift occurred (positive threshold shift) by applying a positive gate voltage to the TFT 23 and exposing the TFT 23 to stray light. During the execution of the specific process, the liquid crystal display device 10 drives the backlight device 12 to light by a specific lighting drive process, at least for a predetermined period while a positive gate voltage is continuously applied to the TFT 23. Therefore, when the TFT 23 to which a positive gate voltage is applied is irradiated with stray light, a positive shift in the threshold value of the TFT 23 may occur. Therefore, the liquid crystal display device 10 can timely positively shift the threshold value of the TFT 23 whose threshold value has been negatively shifted.

[0070] The control unit 50 executes the specific process and the specific lighting drive process when turning off the power supply of the liquid crystal display device 10 (S11: NO, S18).

[0071] In this case, the control unit 50 executes a specific process and a specific lighting drive process when the power supply to the liquid crystal display device 10 is turned off, so that the threshold value of the TFT 23 whose threshold value has shifted negatively can be restored without interfering with the user's use of the liquid crystal display device 10.

[0072] The control unit 50 can perform both high-frequency driving, which repeats the write process and the maintain process frequently, and low-frequency driving, which performs the write process less frequently than the high-frequency driving, and when the period during which the low-frequency driving is continued is longer than a predetermined time, it performs the specific process and the specific lighting drive process (S17: YES, S18).

[0073] When the execution of low-frequency driving has continued for a predetermined time or longer, a negative shift in the threshold of the TFT 23 is likely to occur. Furthermore, when the execution of low-frequency driving has continued for a predetermined time or longer, it is considered that the user is not actively using the liquid crystal display device 10. In such a case, the control unit 50 executes the specific process and the specific lighting drive process, thereby enabling the threshold of the TFT 23, whose threshold has shifted negatively, to be shifted positively in a timely manner.

[0074] The control unit 50 sets the luminance of the illumination device in the specific lighting drive process to be lower than the luminance of the backlight device 12 when the writing process and the sustaining process are executed (in the image display mode).

[0075] When the write process and sustain process are executed (in the image display mode), an image based on a video signal is written to the liquid crystal panel 11. In this case, the control unit 50 drives the backlight device 12 to light at a predetermined brightness necessary to maintain the visibility of the image. On the other hand, in the specific lighting drive process, it is sufficient to drive the backlight device 12 to light at a brightness necessary to cause a positive shift in the threshold of the TFT 23 to which a positive gate voltage is applied. Therefore, the control unit 50 may lower the brightness of the lighting device in the specific lighting drive process compared to the brightness when the write process and sustain process are executed. In this case, the power consumption required to positively shift the threshold of the TFT 23 whose threshold has been negatively shifted can be reduced. Furthermore, driving the backlight device 12 to light in the specific lighting drive process can avoid causing discomfort to the user.

[0076] The semiconductor layer 23D of the TFT 23 is configured to include an In-Ga-Zn based oxide semiconductor.

[0077] When the semiconductor layer 23D of the TFT 23 includes an In-Ga-Zn oxide semiconductor, low-frequency driving, which performs the write process less frequently than high-frequency driving, which repeats the write process and the sustain process more frequently, tends to be more effective. This makes it more likely that the threshold value of the TFT 23 will shift negatively. When using such a TFT 23, the control unit 50 of the liquid crystal display device 10 executes the specific process and the specific lighting drive process, thereby appropriately shifting the threshold value of the TFT 23, which has shifted negatively, to a positive value.

[0078] <Details of the second embodiment of the present disclosure> A second embodiment of the present disclosure will be described with reference to Figures 8 to 10. In the second embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, and redundant descriptions of the structure, operation, and effects will be omitted.

[0079] An example of how the liquid crystal display device 10 is used will be described with reference to FIG. 8. As shown in FIG. 8, the liquid crystal display device 10 may be installed in a personal computer (PC) 100 and used as its display unit. In the second embodiment, the PC 100 is a well-known notebook computer (laptop personal computer) that integrates a display unit, a keyboard, and an operation unit such as a touch panel, and that can be folded in half with the display unit and operation unit facing inward. In the following description, as shown in the upper part of FIG. 8, the state in which the PC 100 is not folded in half and the display unit of the PC 100, which is made up of the liquid crystal display device 10, can be viewed by the user is referred to as an "open state." As shown in the lower part of FIG. 8, the state in which the PC 100 is folded in half and the display unit of the PC 100, which is made up of the liquid crystal display device 10, cannot be viewed by the user as is is referred to as a "closed state." The situations in which the PC 100 is used include at least both the open state and the closed state.

[0080] As shown in FIG. 8 , the PC 100 includes an open / close sensor 101 that can detect whether the PC 100 is in an open state or a closed state. The open / close sensor 101 is, for example, a well-known magnetic sensor. The open / close sensor 101 may also be a non-contact sensor such as an optical sensor. The open / close sensor 101 does not necessarily have to be built into the inside of the PC 100, but may also be provided on the outside of the PC 100. For example, the open / close sensor 101 may be a sensor that physically detects a protrusion provided at an arbitrary position on the inside of the PC 100 when the PC 100 is folded in half. The open / close sensor 101 may be any type of sensor that can detect whether the PC 100 is in an open state or a closed state. The open / close sensor 101 is configured to output information indicating whether the PC 100 is in an open state or a closed state as a signal. Hereinafter, the signal output by the open / close sensor 101 will be referred to as a usage status signal.

[0081] In the second embodiment, the control unit 50 is configured to receive a usage status signal output by the open / close sensor 101, as shown in Fig. 9. This allows the control unit 50 to determine whether the PC 100 is in an open state or a closed state. The storage unit of the control unit 50 stores a program for causing the control unit 50 to execute a brightness determination process, which will be described later with reference to Fig. 10. The storage unit functions as an example of a processor that executes the brightness determination process by expanding the program stored in the storage unit.

[0082] The luminance determination process executed by the control unit 50 will be described with reference to Fig. 10. The control unit 50 executes the luminance determination process when the process of S18 in the specific process execution determination process described above with reference to Fig. 7 is executed.

[0083] When the brightness determination process is started, the control unit 50 refers to the usage status information indicated by the usage status signal transmitted from the open / close sensor 101 (S21). The control unit 50 determines whether the referred usage status information indicates the open state (S22). When the control unit 50 determines that the usage status information indicates the open state (S22: YES), it determines the brightness when the backlight device 12 is driven to light by the specific lighting drive process to be the first brightness (S23). The first brightness is the same as the brightness when the backlight device 12 is driven to light by the specific lighting drive process in the first embodiment. In other words, the first brightness is lower than the brightness when the write process and the sustain process are executed (in the image display mode). Specifically, the first brightness is 1 cd / m 2 to 200 cd / m 2 The control unit 50 proceeds to step S25.

[0084] On the other hand, when the control unit 50 determines that the usage status information indicates the closed state and not the open state (S22: NO), it determines the luminance when the backlight device 12 is driven to light by the specific lighting drive process to be the second luminance (S24). The control unit 50 proceeds to S25. In the second embodiment, the second luminance is set to be higher than the first luminance. That is, the second luminance is set to be 1 cd / m 2 to 200 cd / m 2 In this embodiment, the second luminance is 300 cd / m 2 The second luminance may be set to about 100%. The second luminance may be set to be higher than the first luminance. The second luminance may be set to be higher or lower than the luminance set when the writing process and the sustaining process are executed (in the image display mode).

[0085] In the open state, the liquid crystal display device 10 is visible to the user of the PC 100. For example, when the PC 100 is in the open state, the backlight device 12 of the liquid crystal display device 10 is driven to light at high brightness through the specific lighting drive process as the PC 100 is powered off. In this case, the user of the PC 100 may feel uneasy, wondering if there is a problem with the backlight device 12, the liquid crystal display device 10, or the PC 100. Therefore, when the PC 100 is in the open state, the control unit 50 determines, through the process of S23, the luminance when the backlight device 12 is driven to light through the specific lighting drive process to be a first luminance that is unlikely to cause discomfort to the user. Because the first luminance is relatively low, the liquid crystal display device 10 can restore the threshold voltage of the TFT 23 without causing discomfort to the user.

[0086] On the other hand, in the closed state, the liquid crystal display device 10 is not visible to the user in the PC 100. Therefore, it is thought that the user is unlikely to feel uncomfortable even if the backlight device 12 is driven to light at high brightness by the specific lighting drive process. The PC 100 may be closed when the power to the PC 100 is turned off or when the PC 100 has not been used for a long period of time. Therefore, when the PC 100 is closed, the control unit 50 determines, in the process of S24, the brightness when the backlight device 12 is driven to light by the specific lighting drive process to be the second brightness, which is higher than the first brightness. This allows the liquid crystal display device 10 to actively generate stray light and effectively achieve a positive shift in the threshold value of the TFT 23.

[0087] Thereafter, the control unit 50 instructs the backlight control circuit 54 to drive the backlight device 12 to light in the specific lighting drive process at the luminance determined in S23 or S24 (S25). The backlight control circuit 54 drives the backlight device 12 to light in the specific lighting drive process at the first luminance or second luminance determined in the process of S25. The control unit 50 ends the luminance determination process.

[0088] As described above, the control unit 50 determines the luminance of the backlight device 12 in the specific lighting drive process in accordance with the result of determining the usage status of the liquid crystal display device 10.

[0089] The liquid crystal display device 10 is used in a variety of situations. The control unit 50 determines the luminance of the backlight device 12 in the specific lighting drive process in accordance with the various usage situations of the liquid crystal display device 10, and executes the specific lighting drive process at the determined luminance. Therefore, the liquid crystal display device 10 can avoid causing discomfort to the user when the backlight device 12 is driven to light in the specific lighting drive process.

[0090] <Other embodiments> The present disclosure is not limited to the embodiments described above and in the drawings, and for example, the following embodiments are also included within the technical scope of the present disclosure. Furthermore, various modifications other than those described below can be made without departing from the spirit of the present disclosure.

[0091] (1) In the above embodiment, the control unit 50 executes the specific process and the specific lighting drive process as processes when the supply of external power is interrupted and as processes when low-frequency driving continues for a predetermined time or more. Here, the display mode of the liquid crystal display device 10 may be an image display mode in which an image written to the liquid crystal panel 11 is displayed by low-frequency driving or high-frequency driving, and a sleep mode in which the image display on the liquid crystal panel 11 is turned off to save power in the liquid crystal display device 10. The control unit 50 may execute the specific process and the specific lighting drive process as processes when the liquid crystal display device 10 transitions from the image display mode to the sleep mode, for example.

[0092] (2) The control unit 50 may execute the specific process and the specific lighting drive process at a timing arbitrarily selected by the user. For example, the liquid crystal display device 10 may be provided with a predetermined operation unit that can be operated by the user, and the control unit 50 may execute the specific process and the specific lighting drive process when a predetermined operation is performed on the operation unit.

[0093] (3) The orientation of the liquid crystal display device 10 may be taken into consideration as a usage situation of the liquid crystal display device 10. Specifically, when the liquid crystal display device 10 is used as a display unit of a smartphone or a tablet terminal, the liquid crystal display device 10 may include a sensor, such as an acceleration sensor connected to the control unit 50, that can detect the movement or tilt of the liquid crystal display device 10. For example, when the liquid crystal display device 10 is in a state where its front side faces downward, even if the backlight device 12 is driven to light at high brightness by the specific lighting drive process, the lighting drive is less noticeable and the user is less likely to feel uncomfortable. For this reason, when the front side of the liquid crystal display device 10 faces downward, the brightness of the lighting drive of the backlight device 12 in the specific lighting drive process may be determined to be high. Furthermore, when the liquid crystal display device 10 is in a state where its front side faces upward, if the backlight device 12 is driven to light at high brightness by the specific lighting drive process, the lighting drive is more likely to be visible to the user. Therefore, when the front side of the liquid crystal display device 10 faces upward, the luminance of the backlight device 12 in the specific lighting drive process may be determined to be low. Note that the sensor capable of detecting the movement or tilt of the liquid crystal display device 10 may be externally connected to the liquid crystal display device 10. In this case, the control unit 50 may receive information indicating the attitude of the liquid crystal display device 10 from the sensor, and may determine the luminance of the backlight device 12 in the lighting drive process according to the received information.

[0094] (4) The brightness of the surroundings of the liquid crystal display device 10 may be taken into consideration as a usage condition of the liquid crystal display device 10. Specifically, the liquid crystal display device 10 may include an illuminance sensor connected to the control unit 50. Here, the illuminance sensor is a well-known sensor capable of detecting the brightness of the surroundings. The control unit 50 may then determine the luminance of the backlight device 12 in the specific lighting drive process according to information indicating the brightness of the surroundings of the liquid crystal display device 10 indicated by the illuminance sensor. For example, when the brightness of the surroundings of the liquid crystal display device 10 is relatively high, even if the backlight device 12 is driven to light at high brightness by the specific lighting drive process, the lighting drive is unlikely to be noticeable and the user is unlikely to feel uncomfortable. Therefore, when the brightness of the surroundings of the liquid crystal display device 10 is relatively high, the luminance of the backlight device 12 in the specific lighting drive process may be determined to be high. Furthermore, when the brightness of the surroundings of the liquid crystal display device 10 is relatively low, when the backlight device 12 is driven to light at high brightness by the specific lighting drive process, the lighting drive is likely to be visible to the user. Therefore, when the brightness of the surroundings of the liquid crystal display device 10 is relatively low, the brightness of the lighting drive of the backlight device 12 in the specific lighting drive process may be determined to be low. The illuminance sensor is not limited to being built into the liquid crystal display device 10, but may be one externally connected to the liquid crystal display device 10. In this case, the control unit 50 may receive information indicating the brightness of the surroundings of the liquid crystal display device 10 from the illuminance sensor, and determine the brightness of the lighting drive of the backlight device 12 in the lighting drive process according to the received information.

[0095] (5) The liquid crystal display device 10 may be configured so that the brightness of the lighting drive of the backlight device 12 in the specific lighting drive process can be adjusted in response to a predetermined operation by the user.

[0096] (6) In the above embodiment, the backlight device 12 is a direct type in which a light source is disposed on the rear side of the liquid crystal panel 11. However, the backlight device 12 may be an edge-light type with a single-side light entrance, in which a light source is disposed at one end of the rear side of the liquid crystal panel 11. The backlight device 12 may also be an edge-light type with a double-side light entrance, in which a light source is disposed at both ends of the rear side of the liquid crystal panel 11.

[0097] (7) In addition to blue LEDs, red LEDs that emit red light, green LEDs that emit green light, etc. may be used as the light source of the backlight device 12. Furthermore, light sources other than LEDs (laser light sources, organic EL (Electro Luminescence), etc.) may be used as the light source of the backlight device 12.

[0098] (8) The steps of the specific process execution determination process and the brightness determination process of the liquid crystal display device 10 are not limited to being executed by the CPU of the control unit 50 of the liquid crystal display device 10. Some or all of the steps of the specific process execution determination process and the brightness determination process may be executed by another electronic device (e.g., an ASIC, etc.) or the CPU of a personal computer, which is an external device. The steps of the specific process execution determination process and the brightness determination process may be distributed among multiple electronic devices (e.g., multiple CPUs). The order of the steps of the specific process execution determination process and the brightness determination process may be changed, and steps may be omitted or added as necessary. The scope of the present invention also includes an embodiment in which an operating system (OS) running on the liquid crystal display device 10 executes some or all of the specific process execution determination process and the brightness determination process based on instructions from the control unit 50.

[0099] (9) The control unit 50 may be configured to be able to wirelessly communicate with an external information device. The programs for executing the specific process execution determination process and the brightness determination process may be downloaded via wireless communication from a server connected to a network (not shown), i.e., transmitted as a transmission signal, and stored in the storage unit of the control unit 50. In this case, the programs for executing the specific process execution determination process and the brightness determination process may be stored in a non-transitory storage medium such as an HDD provided in the server. [Explanation of symbols]

[0100] 10: Liquid crystal display device 11: Liquid crystal panel 12: Backlight device 23: TFT 23D: Semiconductor layer 40: LED 50: Control unit 101: Open / close sensor

Claims

1. An LCD panel, an illumination device that irradiates the liquid crystal panel with light from the back side; a control unit that controls driving of the liquid crystal panel and driving of the lighting device, the liquid crystal panel includes a TFT having a semiconductor layer including an oxide semiconductor; The control unit a writing process for writing an image based on a video signal onto the liquid crystal panel by applying a positive gate voltage to the TFT; a sustaining process for sustaining the image written to the liquid crystal panel by the writing process in a state in which a negative gate voltage is applied to the TFT; a specific process of continuously applying a positive gate voltage to the TFT at a predetermined timing and then terminating the application of the positive gate voltage; a specific lighting driving process for lighting the illumination device for a predetermined period during which a positive gate voltage is continuously applied to the TFT during the specific process; A liquid crystal display device that performs the above.

2. The liquid crystal display device according to claim 1 , wherein the control unit executes the specific process and the specific lighting drive process when the power supply of the liquid crystal display device is turned off.

3. The control unit It is possible to perform both high-frequency driving, in which the write process and the sustain process are repeated at a high frequency, and low-frequency driving, in which the write process is performed less frequently than the high-frequency driving, The liquid crystal display device according to claim 1 , wherein the specific process and the specific lighting drive process are executed when the period during which the low frequency drive is continued is equal to or longer than a predetermined time.

4. The liquid crystal display device according to claim 1 , wherein the control unit sets the luminance of the illumination device in the specific lighting drive process to be lower than the luminance of the illumination device when the writing process and the sustaining process are executed.

5. The liquid crystal display device according to claim 1 , wherein the control unit determines the luminance of the illumination device in the specific lighting drive process in accordance with a result of determining a usage state of the liquid crystal display device.

6. 2. The liquid crystal display device according to claim 1, wherein the semiconductor layer of the TFT includes an In--Ga--Zn-based oxide semiconductor.

Images