A pixel unit and timing control method thereof

Abstract

The present invention discloses a pixel unit and a timing control method thereof, wherein the pixel unit comprises: a cathode of a photosensitive diode PD is connected to one end of a transmission tube Mtx, an anode of the photosensitive diode PD is grounded, and the photosensitive diode PD is used to generate a charge in response to a light signal; the other end of the transmission tube Mtx is connected to a floating node, and the transmission tube Mtx is used to transmit the charge of the photosensitive diode PD to the floating node; one end of a reset tube Mrx is connected to the floating node, and the other end of the reset tube Mrx is connected to a row selection signal SEL, and the reset tube Mrx is used to reset the pixel unit, and the row selection signal SEL provides a power input for the reset tube Mrx; the gate of an amplifier tube Msf is connected to the floating node, one end of the amplifier tube Msf is connected to a power supply voltage, and the other end of the amplifier tube Msf is connected to a pixel unit output bus, and the amplifier tube Msf is used to amplify a voltage and output the amplified voltage. The pixel unit is used to improve the photosensitivity efficiency of the photosensitive diode, and to improve the conversion gain and luminous efficiency.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor image sensing technology, and in particular to a pixel unit and a timing control method thereof. Background Art

[0002] At present, CMOS image sensors (CIS) have been widely used in imaging fields such as video, monitoring, industrial manufacturing, automobiles, and home appliances. With the continuous improvement of CIS application requirements, CIS must not only have high image quality and stability, but also have a certain cost-effectiveness in order to reduce the overall solution cost. The most commonly used structure is the four-tube pixel unit, which is easy to implement and can provide good photosensitivity. However, the structure of the four-tube pixel unit includes four transistors, so it will occupy a certain area, and due to the limitations of layout and routing, it is difficult to further improve the luminous efficiency of the photosensitive diode. Although the luminous efficiency and sensitivity can be improved by improving the process, this often leads to an increase in the processing cost.

[0003] Therefore, it is urgent to provide a new pixel unit to improve the above problems. Summary of the invention

[0004] The embodiment of the present invention provides a pixel unit and a timing control method thereof, which are used to improve the photosensitivity efficiency of a photodiode, and enhance the conversion gain and the luminous efficiency.

[0005] In a first aspect, the present invention provides a pixel unit, which includes: a cathode of a photosensitive diode PD is connected to one end of a transmission tube Mtx, an anode of the photosensitive diode PD is grounded, and the photosensitive diode PD is used to generate charges in response to a light signal; the other end of the transmission tube Mtx is connected to a floating node, and the transmission tube Mtx is used to transfer the charge of the photosensitive diode PD to the floating node; one end of a reset tube Mrx is connected to the floating node, and the other end of the reset tube Mrx is connected to a row selection signal SEL, and the reset tube Mrx is used to reset the pixel unit, and the row selection signal SEL provides a power input for the reset tube Mrx; a gate of an amplifier tube Msf is connected to the floating node, one end of the amplifier tube Msf is connected to a power supply voltage, and the other end of the amplifier tube Msf is connected to a pixel unit output bus, and the amplifier tube Msf is used to amplify a voltage and output the amplified voltage.

[0006] The beneficial effect of the pixel unit provided by the present invention is that the selection tube in the traditional four-tube pixel unit is removed and simplified to a three-tube pixel unit, so that the area of ​​the photosensitive diode is expanded to a certain extent, which is beneficial to improving the conversion gain, and the photosensitivity efficiency and conversion gain are improved without changing the process conditions.

[0007] Optionally, the amplifier tube Msf is arranged spatially adjacent to the transmission tube Mtx.

[0008] Optionally, the pixel unit further includes a driving circuit, the driving circuit is connected to both the gate of the transmission tube Mtx and the gate of the reset tube Mrx, and the driving circuit is used to drive the transmission tube Mtx and the reset tube Mrx.

[0009] Optionally, the amplifier tube Msf is a source follower tube.

[0010] In a second aspect, the present invention further provides an image sensor, comprising a pixel array composed of any possible pixel unit of the first aspect, and its beneficial effects can be seen from the above description.

[0011] In a third aspect, the present invention provides a timing control method for a pixel unit, the method comprising: at a first moment, turning on a transmission tube Mtx, a reset tube Mrx and an amplifier tube Msf to reset the pixel unit; then at a second moment, turning off the transmission tube Mtx, the reset tube Mrx and the amplifier tube Msf and exposing, the photosensitive diode PD senses light and accumulates charge;

[0012] At the third moment, the reset tube Mrx and the amplifier tube Msf are turned on to reset the pixel unit;

[0013] At the fourth moment, the reset tube Mrx and the amplifier tube Msf are turned off, and the reset potential Vrst of the floating node FD is read;

[0014] At the fifth moment, the transmission tube Mtx is turned on, and then at the sixth moment, the transmission tube Mtx is turned off, and the integrated potential Vsig of the floating node FD is read;

[0015] At the seventh moment, the transmission tube Mtx, the reset tube Mrx and the amplifier tube Msf are turned on to reset the pixel unit.

[0016] Optionally, the method further includes: the voltage DOUT output by the pixel unit output bus = (Vrst-Vsig) × 2 N / Vref, where N is the bit width of the analog-to-digital converter and Vref is the reference voltage range.

[0017] The timing control method of the pixel unit provided by the present invention can effectively collect the charges overflowing from the photoelectric conversion part when using the three-tube pixel unit, thereby collecting the photogenerated electrons in all exposure time periods. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0019] Figure 1 A schematic diagram of a circuit structure of a pixel unit provided by the prior art;

[0020] Figure 2 A schematic flow chart of a timing control method for a pixel unit provided in the prior art;

[0021] Figure 3 A schematic diagram of the circuit layout design of a unit pixel provided by the prior art;

[0022] Figure 4 A schematic diagram of a circuit structure of a pixel unit provided by an embodiment of the present invention;

[0023] Figure 5 A schematic flow chart of a timing control method for a pixel unit provided by an embodiment of the present invention;

[0024] Figure 6 A schematic diagram of a signal waveform under an operation sequence provided by an embodiment of the present invention;

[0025] Figure 7 A schematic diagram of a circuit layout design of a unit pixel provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0026] The circuit structure of the current CIS standard four-pixel unit is as follows Figure 1 As shown, this structure is commonly used in row exposure CIS, which consists of a photodiode (PD), a transmission tube Mtx, a reset tube Mrx, an amplifier tube Msf, and a row selection tube Msel. The photodiode is sensitive to light and generates photoelectrons proportional to the light intensity. The function of Mtx is to transfer the photoelectrons in the PD. When the transmission signal TX is a high voltage, Mtx is turned on and the photoelectrons in the PD are transferred to the floating node FD. The function of the reset tube Mrx is to reset the pixel unit when the reset signal RX is a high potential. When the selection signal SEL is a high potential and Msel is turned on, the amplifier tube Msf and the row selection tube Msel form a path with the current source to the ground. At this time, the amplifier tube Msf is essentially a source follower, which follows the change of the FD potential and is finally output by the pixel unit output bus PIX_OUT.

[0027] Figure 2The operation timing of the four-pixel unit is divided into the reset (Rst) stage, the exposure (Exp) stage, and the signal reading (Read) stage.

[0028] When the pixel unit is in the reset stage, the transmission signal TX and the reset signal RX are both high, the transmission tube Mtx and the reset tube Mrx are both turned on, the FD is reset and its potential is pulled up to the power supply voltage VDD. After that, the reset signal RX and the transmission signal TX are switched to a low level, the pixel unit enters the exposure stage, and the PD is sensitive to light and accumulates electrons.

[0029] After that, it enters the signal reading stage, the row selection signal SEL is high, the reset signal RX is first high to reset the pixel unit, and then the reset signal RX switches to a low level, and the transmission signal TX remains at a low level. At this time, the amplifier Msf is controlled by the FD potential and outputs the reset potential "Vrst" through the pixel unit output bus PIX_OUT.

[0030] Afterwards, the transmission signal TX switches to a high level, transferring the electrons on PD to FD. At this time, the amplifier Msf is controlled by the FD potential and outputs the signal potential "Vsig" through PIX_OUT.

[0031] Figure 3 This is a schematic diagram of the layout of a four-tube pixel unit. Since there are four transistors in the unit, in order to save area and signal wiring, the remaining transistors except the transmission tube Mtx need to be arranged on the right side of the photosensitive diode PD. The floating node FD is connected to the drain end of the transmission tube Mtx and the gate of the amplifier tube Msf through a metal connection, but the distance is relatively large, which affects the luminous efficiency of the photosensitive diode.

[0032] To this end, the present invention removes the selection tube in the traditional four-tube pixel unit and simplifies it into a three-tube pixel unit, so that the area of ​​the photosensitive diode is expanded to a certain extent, and the amplifier tube Msf can be arranged closer to the floating node in the layout, which is beneficial to improving the conversion gain. Without changing the process conditions, the photosensitivity efficiency and conversion gain are improved.

[0033] To make the content of the present invention more clear and understandable, the content of the present invention is further described below in conjunction with the accompanying drawings of the specification. Of course, the present invention is not limited to this specific embodiment, and general replacements known to those skilled in the art are also covered within the protection scope of the present invention.

[0034] It should be noted that in the following specific embodiments, when describing the embodiments of the present invention in detail, in order to clearly represent the structure of the present invention for the convenience of explanation, the structures in the accompanying drawings are not drawn according to general proportions, and are partially enlarged, deformed and simplified. Therefore, it should be avoided to understand this as a limitation of the present invention.

[0035] An embodiment of the present invention provides an image sensor, comprising a pixel array composed of at least one pixel unit. Figure 4 The pixel unit includes a photodiode (PD), a transmission tube Mtx, a reset tube Mrx and an amplifier tube Msf.

[0036] The cathode of the photosensitive diode PD is connected to one end of the transmission tube Mtx, the anode of the photosensitive diode PD is grounded, and the photosensitive diode PD is used to generate charges in response to a light signal.

[0037] The other end of the transmission tube Mtx is connected to the floating node FD, and the transmission tube Mtx is used to transmit the charge of the photodiode PD to the floating node FD.

[0038] One end of the reset tube Mrx is connected to the floating node FD, and the other end of the reset tube Mrx is connected to the row selection signal SEL. The reset tube Mrx is used to reset the pixel unit, and the row selection signal SEL provides power input for the reset tube Mrx.

[0039] The gate of the amplifier tube Msf is connected to the floating node, one end of the amplifier tube Msf is connected to the power supply voltage, the other end of the amplifier tube Msf is connected to the pixel unit output bus, and the amplifier tube Msf is used to amplify the voltage and output the amplified voltage. Optionally, the amplifier tube is a source follower tube (Source Follower, SF).

[0040] It can be seen that the three-tube pixel unit circuit proposed in this embodiment is characterized in that the gate tube in the traditional structure is removed, and the drain end of the reset tube Mrx is no longer connected to the voltage VDD, but is directly connected to the row selection signal SEL. The row selection signal SEL serves as both the gate tube and the power input of the reset tube. The source end of the amplifier tube Msf is directly connected to the pixel unit output PIX_OUT bus. When the pixel is not selected, since the row selection signal SEL is at a low level, the amplifier tube Msf can be turned off, thereby not affecting the pixel unit output PIX_OUT.

[0041] In a possible embodiment, the pixel unit further includes a driving circuit, the driving circuit is connected to the gate of the transmission tube Mtx and the gate of the reset tube Mrx, and the driving circuit is used to drive the transmission tube Mtx and the reset tube Mrx.

[0042] It is worth noting that the transmission tube Mtx, the reset tube Mrx and the amplifier tube Msf can be NMOS tubes or PMOS tubes. In this application, one end of any transistor is the source and the other end is the drain, or one end of any transistor is the drain and the other end is the source.

[0043] Based on the above pixel unit, the following is further combined Figure 5 A schematic flow chart of a timing control method for a pixel unit is shown, which includes the following steps.

[0044] S501, at the first moment, turn on the transmission tube Mtx, the reset tube Mrx and the amplifier tube Msf to reset the pixel unit; then at the second moment, turn off the transmission tube Mtx, the reset tube Mrx and the amplifier tube Msf, and expose, the photosensitive diode PD senses light and accumulates charge.

[0045] S502, at the third moment, turn on the reset tube Mrx and the amplifier tube Msf to reset the pixel unit.

[0046] S503, at the fourth moment, the reset tube Mrx and the amplifier tube Msf are turned off, and the reset potential Vrst of the floating node FD is read.

[0047] S504 , at the fifth moment, the transmission transistor Mtx is turned on, and then at the sixth moment, the transmission transistor Mtx is turned off, and the integrated potential Vsig of the floating node FD is read.

[0048] S505, at the seventh moment, turn on the transmission tube Mtx, the reset tube Mrx and the amplifier tube Msf to reset the pixel unit.

[0049] For example, in combination Figure 6 For example, the exposure stage is entered at time T1, and the exposure stage ends at time T3. At time TI, the reset signal RX, the row selection signal SEL, and the transmission signal TX become high level, that is, the transmission tube Mtx, the reset tube Mrx and the amplifier tube Msf are turned on to reset the pixel unit once. Then, at time T2, the reset signal RX, the row selection signal SEL, and the transmission signal TX are all switched to low level, that is, the transmission tube Mtx, the reset tube Mrx and the amplifier tube Msf are turned off, and the photosensitive diode PD begins to expose and accumulate charge; at time T3, the signal reading stage is entered, and the signal reading ends at time T7. At time T3, the reset signal RX and the row selection signal SEL are first switched from low level to high level to reset the pixel unit, that is, the reset tube Mrx and the amplifier tube Msf are turned on to reset the pixel unit. At T4, the reset signal RX and the row selection signal SEL are switched from high level to low level, that is, the reset tube Mrx and the amplifier tube Msf are turned off and then the reset potential "Vrst" is read out. Thereafter, at T5, the transmission signal TX is high level, that is, the transmission tube Mtx is turned on. At T6, the transmission signal TX is switched to low level again, and then the integration potential "Vsig" is read out. Finally, at T7, the reset phase is entered, and the reset phase ends at T8. At T7, the reset signal RX, the row selection signal SEL, and the transmission signal TX are all switched from low level to high level, that is, the transmission tube Mtx, the reset tube Mrx and the amplifier tube Msf are turned on, and the pixel unit is reset.

[0050] Afterwards, the "Vrst" and "Vsig" potentials are converted into digital quantities by the subsequent analog-to-digital converter (ADC) circuit and subtracted to obtain the digital quantity actually corresponding to the photoelectrons on the photodiode, that is, the voltage DOUT output by the pixel unit output bus = (Vrst-Vsig) × 2 N / Vref, where N is the bit width of the analog-to-digital converter and Vref is the reference voltage range. If the analog-to-digital converter (ADC) is 12 bits and the analog-to-digital converter (ADC) reference voltage range is Vref, then the final pixel unit output bus PIX_OUT output DOUT = (Vrst-Vsig) × 2 12 / Vref.

[0051] Figure 6 The schematic diagram of the proposed three-tube pixel unit layout. Since the gate tube Msel is removed, the amplifier tube Msf can be set closer to the transmission tube Mtx, so that the parasitic capacitance of the FD point is very small. The smaller the capacitance, the more charges are converted into electrons, that is, the higher the conversion gain (CG), which improves the photosensitivity of the pixel. Since the total number of transistors is reduced, the internal wiring of the pixel unit is reduced, and the signal line wiring is more flexible, which also increases the effective area of ​​PD, which helps to improve the quantum efficiency (Quantum Efficiency, QE). It can be seen that although the proposed three-tube pixel unit omits the gate tube, it can still realize the function of the four-tube pixel, and has higher photosensitivity and photosensitivity than the four-tube pixel unit.

[0052] In summary, the advantage of the present invention is that it provides a new pixel unit, which includes three transistors, so that the area of ​​the photosensitive diode is expanded to a certain extent, and the layout can arrange the amplifier tube (SF) closer to the floating node, which is beneficial to improving the conversion gain. Without changing the process conditions, the photosensitivity efficiency and conversion gain are improved.

[0053] The above-described embodiments are only preferred embodiments of the present invention, and the embodiments are not intended to limit the patent protection scope of the present invention. Therefore, any equivalent structural changes made using the description and drawings of the present invention should also be included in the protection scope of the present invention.

[0054] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention.

[0055] Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A pixel unit, characterized in that: include: The cathode of the photodiode PD is connected to one end of the transmission tube Mtx, the anode of the photodiode PD is grounded, and the photodiode PD is used to generate charges in response to the light signal; The other end of the transmission tube Mtx is connected to the floating node, and the transmission tube Mtx is used to transmit the charge of the photodiode PD to the floating node; One end of the reset tube Mrx is connected to the floating node, and the other end of the reset tube Mrx is connected to the row selection signal SEL. The reset tube Mrx is used to reset the pixel unit, and the row selection signal SEL provides a power input for the reset tube Mrx. The gate of the amplifier tube Msf is connected to the floating node, one end of the amplifier tube Msf is connected to the power supply voltage, the other end of the amplifier tube Msf is connected to the pixel unit output bus, and the amplifier tube Msf is used to amplify the voltage and output the amplified voltage; The pixel unit does not include a gate tube.

2. The pixel unit according to claim 1, characterized in that: The amplifier tube Msf is arranged spatially adjacent to the transmission tube Mtx.

3. The pixel unit according to claim 1, characterized in that: It also includes a driving circuit, which is connected to the gate of the transmission tube Mtx and the gate of the reset tube Mrx, and is used to drive the transmission tube Mtx and the reset tube Mrx.

4. The pixel unit according to claim 1, characterized in that: The amplifier tube is a source follower tube.

5. An image sensor, characterized in that: A pixel array comprising the pixel units as claimed in any one of claims 1 to 4.

6. A timing control method for a pixel unit, applied to the pixel unit according to any one of claims 1 to 4, characterized in that: The timing control method comprises: At the first moment, the transmission tube Mtx, the reset tube Mrx and the amplifier tube Msf are turned on to reset the pixel unit; then at the second moment, the transmission tube Mtx, the reset tube Mrx and the amplifier tube Msf are turned off and exposed, and the photosensitive diode PD senses light and accumulates charge; At the third moment, the reset tube Mrx and the amplifier tube Msf are turned on to reset the pixel unit; At the fourth moment, the reset tube Mrx and the amplifier tube Msf are turned off, and the reset potential Vrst of the floating node FD is read; At the fifth moment, the transmission tube Mtx is turned on, and then at the sixth moment, the transmission tube Mtx is turned off, and the integrated potential Vsig of the floating node FD is read; At the seventh moment, the transmission tube Mtx, the reset tube Mrx and the amplifier tube Msf are turned on to reset the pixel unit.

7. The method according to claim 6, characterized in that The voltage DOUT output by the pixel unit output bus is (Vrst-Vsig)×2N / Vref, wherein N is the bit width of the analog-to-digital converter, and Vref is the reference voltage range.

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