Dual-channel single-gate source follower and low random telegraph signal noise pixel circuit

Abstract

The embodiment of the invention provides a dual-channel single-gate source follower which is used for a pixel circuit with low random telegraph signal noise. The dual-channel single-gate source follower comprises a first source follower transistor and a second source follower transistor, the first source electrode following transistor and the second source electrode following transistor are connected in parallel; the drain electrode of the first source electrode following transistor and the drain electrode of the second source electrode following transistor are both connected to a power supply voltage VDD; the grid electrode of the first source electrode following transistor is connected with the grid electrode of the second source electrode following transistor, and is connected to a photodiode PPD and a reset circuit in the pixel circuit; the source electrode of the first source electrode following transistor and the source electrode of the second source electrode following transistor are connected with a bit line selection transistor of the pixel circuit. According to the dual-channel single-gate source follower, the area of an SF transistor can be effectively increased and the width of a current path can be improved on the premise that the area of a photodiode is not reduced in a dual-channel mode, so that RTS noise is reduced.

Description

Technical Field

[0001] The present application relates to the field of image sensor circuits, and in particular to a dual-channel single-gate source follower and a low random telegraph signal noise pixel circuit. Background Art

[0002] In the design and manufacture of semiconductor image sensors, the signal noise problem of pixels has always been one of the important factors affecting image quality. Especially under low-light conditions, the random telegraph signal (RTS) noise in pixels can cause pixels to flicker randomly in the dark, significantly reducing the quality of images and videos. This phenomenon is particularly evident in highly sensitive sensors, such as mobile phone cameras, security monitoring systems, and scientific imaging equipment.

[0003] The generation of RTS noise is mainly related to the source follower (SF) transistor in the pixel. In order to reduce RTS noise, the traditional solution is to increase the gate area by increasing the width of the SF transistor, which can improve the stability of the transistor and increase the transconductance gm of the SF transistor. However, when the pixel size is limited, especially in the design of small pixels (such as 1.0 microns and below), increasing the width of the SF transistor will reduce the area of ​​the photodiode (PD), thereby reducing the photon absorption capacity and full well capacity, and ultimately affecting the photoelectric conversion efficiency of the entire pixel.

[0004] How to effectively increase the area of ​​the SF transistor without affecting the photodiode area, especially to increase the current path width and thus reduce the RTS noise, has become an important challenge in small pixel design. Summary of the invention

[0005] In view of this, the present application provides a dual-channel single-gate source follower and a pixel circuit using the source follower to reduce RTS noise in the pixel circuit and improve the image quality of the image sensor.

[0006] In the first aspect, an embodiment of the present application provides a dual-channel single-gate source follower for a pixel circuit with low random telegraph signal noise, comprising: a first source follower transistor and a second source follower transistor; the first source follower transistor and the second source follower transistor are connected in parallel; the drain of the first source follower transistor and the drain of the second source follower transistor are both connected to a power supply voltage VDD; the gate of the first source follower transistor is connected to the gate of the second source follower transistor, and is connected to a photodiode PPD and a reset circuit in the pixel circuit; the source of the first source follower transistor and the source of the second source follower transistor are both connected to a bit line selection transistor of the pixel circuit.

[0007] In order to reduce RTS noise, it is necessary to increase the current path width, that is, to increase the transconductance of the source follower. In traditional pixel circuits, increasing the transconductance of the source follower is usually achieved by increasing the gate area of ​​the source follower. However, in small pixel circuits, the pixel size is limited, and increasing the gate area of ​​the source follower will cause the available area of ​​the photodiode to become smaller, thereby reducing the photon absorption capacity and full well capacity, and ultimately affecting the photoelectric conversion efficiency of the entire pixel. The embodiment of the present application sets two parallel source follower transistors to form two parallel current signal channels, so that the transconductance of the source follower is improved, and a better noise suppression effect is improved without significantly increasing the pixel power consumption.

[0008] In a possible implementation, the first source follower transistor and the second source follower transistor are metal-oxide-semiconductor field effect transistors (MOSFET).

[0009] In the embodiment of the present application, since MOSFET has the characteristics of high input impedance, fast switching speed and no secondary breakdown, the source follower can exhibit more stable characteristics.

[0010] In a possible implementation, the first source follower transistor and the second source follower transistor are gate-connected by sharing a gate.

[0011] Shared gate may refer to two source follower transistors sharing the same gate. In the embodiment of the present application, the polysilicon gates of the two source follower transistors are connected by sharing the gate, and no redundant metal wiring is required to connect the gates of the two original separate source follower transistors, which reduces the complexity of metal wiring and can also improve the capacitance gain of the overall system.

[0012] In a possible implementation, the channel width-to-length ratio of the first source-follower transistor and the second source-follower transistor ranges from 0.2 / 1.16 to 0.22 / 1, that is, 5 / 29 to 11 / 50.

[0013] In a possible implementation, the channel width-to-length ratio of the first source follower transistor is the same as the channel width-to-length ratio of the second source follower transistor.

[0014] In the embodiment of the present application, by setting the sizes of the two source follower transistors to be exactly the same, double transconductance can be provided while reducing the mismatch problem in process size.

[0015] In a possible implementation, the channel width-to-length ratio of the first source-follower transistor and the channel width-to-length ratio of the second source-follower transistor are both 0.22 / 1.16, that is, 11 / 58.

[0016] In one possible implementation, the gate of the first source follower transistor is connected to the gate of the second source follower transistor and is connected to the floating diffusion node of the photodiode PPD in the pixel circuit; the source of the first source follower transistor and the source of the second source follower transistor are both connected to the drain of the bit line selection transistor of the pixel circuit.

[0017] In the second aspect, an embodiment of the present application provides a low random telegraph signal RTS noise pixel circuit, the pixel circuit comprising: a dual-channel single-gate source follower as provided in the first aspect or any possible implementation of the first aspect; a photodiode, a floating diffusion node of the photodiode is connected to the gate of the dual-channel single-gate source follower; a reset circuit, the reset circuit is connected to the floating diffusion node of the photodiode; and a selection circuit, the selection circuit is connected to the source of the dual-channel single-gate source follower.

[0018] In a possible implementation, the reset circuit includes a reset transistor, a source of the reset transistor is connected to the floating diffusion node of the photodiode, a gate of the reset transistor is connected to a reset signal RST, and a drain of the reset transistor is connected to a power supply VDD.

[0019] In one possible implementation, the selection circuit includes a bit line selection transistor, the drain of the bit line selection transistor is connected to the source of the dual-channel single-gate source follower, the gate of the bit line selection transistor is connected to the selection signal SEL, and the source of the bit line selection transistor is connected to the column signal transmission line COL BUS.

[0020] The dual-channel single-gate source follower and the corresponding pixel circuit provided in the present application have a wider current path and simple layout and wiring, which can effectively reduce the RTS noise in the pixel circuit and improve the imaging quality of the image sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the embodiments of the present application will be briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without paying creative work.

[0022] Figure 1 This is the RTS noise schematic diagram;

[0023] Figure 2 is a schematic diagram of a traditional pixel;

[0024] Figure 3 A schematic diagram of a dual-channel single-gate source follower provided in an embodiment of the present application is shown;

[0025] Figure 4 A schematic diagram of a pixel circuit with low RTS noise provided by an embodiment of the present application is shown;

[0026] Figure 5 A schematic diagram of the layout and wiring of a dual-channel single-gate source follower provided in an embodiment of the present application is shown. DETAILED DESCRIPTION

[0027] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application.

[0028] In the description of this application, it should be noted that the terms "inside", "outside", etc. indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, or the orientation or positional relationship in which the product of the application is usually placed when in use, which is only for the convenience of describing this application and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on this application. In addition, the terms "first", "second", etc. are only used to distinguish the description, and cannot be understood as indicating or implying relative importance.

[0029] It should also be noted that, unless otherwise clearly specified and limited, the terms "disposed" and "connected" should be understood in a broad sense, for example, it can be directly connected, or indirectly connected through an intermediate medium, or it can be the internal connection of two components. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0030] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0031] First, in order to facilitate understanding of the embodiments of the present application, the technical problems that need to be solved by the embodiments of the present application are analyzed in detail below.

[0032] In the design and manufacture of semiconductor image sensors, the signal noise problem of pixels has always been one of the important factors affecting image quality. Especially under low-light conditions, random telegraph signal (RTS) noise in pixels can cause pixels to flicker randomly in the dark, significantly reducing the quality of images and videos. This phenomenon is particularly evident in high-sensitivity sensors, such as mobile phone cameras, security monitoring systems, and scientific imaging equipment.

[0033] The generation of RTS noise is mainly related to the source follower (SF) transistor in the pixel. RTS noise is also called 1 / f noise. Its change trend and calculation method are as follows Figure 1 As shown:

[0034]

[0035] Among them, K f is a constant related to the process, C ox represents the gate capacitance per unit area, and W and L represent the width and length of the gate of the SF transistor, respectively.

[0036] Figure 2 The schematic diagram of a conventional pixel is shown. In the conventional pixel design, each pixel contains only one source follower transistor, and its width is limited by the size of the pixel. In order to reduce RTS noise, the traditional solution is to increase the gate area by increasing the width of the SF transistor, which can improve the stability and transconductance gm of the transistor. However, when the pixel size is limited, especially in the design of small pixels (e.g., 1.0 microns and below), increasing the width of the SF transistor will reduce the area of ​​the photodiode PD, thereby reducing the photon absorption capacity and full well capacity, and ultimately affecting the photoelectric conversion efficiency of the entire pixel.

[0037] Therefore, how to effectively increase the area of ​​the SF transistor, especially to increase the current path width and thus reduce the RTS noise without reducing the photodiode area, becomes an important challenge in small pixel design.

[0038] Therefore, the embodiment of the present application provides an innovative dual channel single gate (DCSG) source follower design, the purpose of which is to increase the width of the SF transistor in the pixel, thereby effectively reducing the random telegraph signal RTS noise while keeping the area of ​​the photodiode PD unchanged. This design greatly improves the transconductance gm of the transistor by dividing the current path of the SF transistor into two parallel channels, providing better noise suppression effect without significantly increasing the power consumption of the pixel.

[0039] See also Figure 3 , Figure 3 A schematic diagram of a dual-channel single-gate DCSG source follower in an embodiment of the present application is shown.

[0040] like Figure 3 As shown in the dashed box, in some embodiments, the dual-channel single-gate DCSG source follower includes two parallel first source follower transistors and second source follower transistors. The drain of the first source follower transistor and the drain of the second source follower transistor are both connected to the power supply voltage VDD, the gate of the first source follower transistor is connected to the gate of the second source follower transistor, and both are connected to the photodiode PPD and the reset circuit in the pixel circuit; the source of the first source follower transistor and the source of the second source follower transistor are both connected to the selection circuit of the pixel circuit.

[0041] Current flows from the power supply voltage VDD through the two channels Ch1 and Ch2, and finally converges at the selection circuit SEL and is output.

[0042] In this way, by setting up parallel source follower transistors, the idle space in the pixel is effectively utilized without affecting the photodiode PD area. By connecting two SF transistors in parallel, it is possible to provide a SF width gain of about 4 to 5 times at the same pixel area, thereby significantly reducing RTS noise.

[0043] It is not difficult for those skilled in the art to understand that, according to the spatial conditions of a specific pixel, multiple SF transistors connected in parallel may be provided to form a multi-channel single-gate source follower, and this implementation scheme is still within the protection scope of the present application.

[0044] In some embodiments, the first source follower transistor and the second source follower transistor are metal-oxide-semiconductor field effect transistors MOSFET. It is not difficult for those skilled in the art to understand that since MOSFET has the characteristics of high input impedance, fast switching speed and no secondary breakdown, the source follower can exhibit more stable characteristics. Optionally, in other embodiments, the first source follower transistor and the second source follower transistor can be complementary metal oxide semiconductor (CMOS) transistors, which is not limited in the embodiments of the present application.

[0045] See also Figure 4 , Figure 4 Two ways of connecting the gates of two or more source follower transistors are shown.

[0046] In some embodiments, the polysilicon gates of the first source follower transistor and the second source follower transistor are connected via metal wiring, such as Figure 4 As shown on the left, the gates of the upper and lower SF transistors are connected together through metal wiring.

[0047] In other embodiments, Figure 4 As shown on the right, the first source follower transistor and the second source follower transistor are connected by sharing the gate. By extending the polysilicon gates of the two SF transistors until they are connected, a larger shared gate is formed. Since the two SF gates are connected by polysilicon, there is no need to use extra metal traces to connect the two separate SF gates. This design reduces the complexity of metal wiring and improves the capacitance gain (CG) of the overall system.

[0048] In some embodiments, the size of the first source follower transistor is the same as the size of the second source follower transistor. In this way, through the dual-channel setting, a SF width gain of about 4 to 5 times is provided, thereby significantly reducing the RTS noise. It should be noted that in some embodiments, the same size means that the two transistors are exactly the same; in other embodiments, the same size can also mean that the width and length of the transistor gate are the same; in some other embodiments, the same size can also mean that the width-to-length ratio of the transistor is the same. The embodiments of the present application are not limited to this. By setting the size of the two SF transistors to be the same, the mismatch problem in the integrated circuit manufacturing process design can be reduced.

[0049] The width-to-length ratio of the SF transistor can be selected according to the needs of the actual pixel circuit. In some embodiments, the channel width-to-length ratio of the first source follower transistor and the second source follower transistor ranges from 0.2 / 1.16 to 0.22 / 1, i.e., 5 / 29 to 11 / 50. Preferably, in some embodiments, the channel width-to-length ratio of the first source follower transistor and the second source follower transistor is 0.22 / 1.16, i.e., 11 / 58.

[0050] It is not difficult for those skilled in the art to understand that in some embodiments, the length of the SF transistor can be flexibly adjusted according to the needs of specific applications to achieve an optimal balance between the transconductance gm and the capacitive gain CG. For example, in low-light applications, a longer transistor length is set, and a longer SF transistor can provide a larger channel area, thereby reducing RTS noise and improving image quality; while in high frame rate applications, a shorter transistor length is set, and a shorter transistor can reduce the capacitive load, thereby improving the response speed.

[0051] The dual-channel single-gate source follower provided in the embodiments of the present application is applied to pixel circuits, especially small pixel circuits, to better reduce RTS noise and improve image quality. Therefore, it is not difficult for those skilled in the art to understand that in some embodiments, the gate of the first source follower transistor is connected to the gate of the second source follower transistor, and is connected to the floating diffusion node of the photodiode PPD in the pixel circuit. In some embodiments, the source of the first source follower transistor is connected to the source of the second source follower transistor, and is connected to the selection circuit in the pixel circuit.

[0052] This application also provides a low random telegraph signal RTS noise pixel circuit. Figure 3 and Figure 5 , Figure 3 and Figure 5 A schematic diagram of the RTS noise pixel circuit in an embodiment of the present application is shown, which includes: a dual-channel single-gate source follower as described in any embodiment of the present application; a photodiode (Pinned Photodiode, PPD), a floating diffusion node (floating diffusion, FD) of the photodiode is connected to the gate of the dual-channel single-gate source follower; a reset circuit, the reset circuit is connected to the floating diffusion node of the photodiode; and a selection circuit, the selection circuit is connected to the source of the dual-channel single-gate source follower.

[0053] In some embodiments, Figure 5 As shown, the reset circuit includes a reset transistor, a source of the reset transistor is connected to the floating diffusion node of the photodiode, a gate of the reset transistor is connected to the reset signal, and a drain of the reset transistor is connected to the power supply VDD.

[0054] In some embodiments, Figure 5 As shown, the selection circuit includes a bit line selection transistor, the drain of the bit line selection transistor is connected to the source of the dual-channel single-gate source follower, the gate of the bit line selection transistor is connected to the selection signal SEL, and the source of the bit line selection transistor is connected to the column signal transmission line COL BUS.

[0055] In some embodiments, Figure 5 As shown, the photodiode PPD converts light into electrons and stores them. When reading, TG is turned on and the electrons are transferred to the FD node by the electric field in the channel, where they are converted into voltage signals. They are then read out by the two parallel channels of SF and converted into digital signal output after being selected by the bit line selection transistor, thereby completing the scanning of a pixel.

[0056] The dual-channel single-gate source follower and low RTS pixel circuit provided by the present application can significantly reduce RTS noise through the dual-channel single-gate design, especially in low-light applications, and can significantly improve the image quality of the pixel. Although the width of the SF transistor is increased, the design of the present invention does not reduce the area of ​​the photodiode, ensuring that the photoelectric conversion efficiency of the pixel is not affected. The embodiment of the present application allows the length of the SF transistor to be flexibly adjusted according to different application requirements to achieve the best balance between transconductance and capacitance gain, thereby meeting the performance requirements in different scenarios.

[0057] In the above embodiments, the description of each embodiment has its own emphasis. For parts that are not described in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0058] In the several embodiments provided in the present application, it should be understood that the disclosed device can be implemented in other ways. For example, the device embodiments described above are only schematic, such as the division of the above-mentioned units, which is only a logical function division. There may be other division methods in actual implementation, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be through some interfaces, and the indirect coupling or communication connection of devices or units can be electrical or other forms.

[0059] The units described above as separate components may or may not be physically separated, and the components shown as units may or may not be physical units, that is, they may be located in one place or distributed on multiple network units. Some or all of the units may be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0060] In addition, each functional unit in each embodiment of the present application may be integrated into one processing unit, or each unit may exist physically separately, or two or more units may be integrated into one unit. The above-mentioned integrated unit may be implemented in the form of hardware or in the form of software functional units.

[0061] As described above, the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A dual-channel single-gate source follower for use in a pixel circuit with low random telegraph signal noise, characterized in that: The dual-channel single-gate source follower comprises: a first source follower transistor and a second source follower transistor; The first source follower transistor and the second source follower transistor are connected in parallel; The drain of the first source follower transistor and the drain of the second source follower transistor are both connected to a power supply voltage VDD; The gate of the first source follower transistor is connected to the gate of the second source follower transistor, and is connected to the photodiode PPD and the reset circuit in the pixel circuit; The source of the first source follower transistor and the source of the second source follower transistor are both connected to the selection circuit of the pixel circuit.

2. The dual-channel single-gate source follower as claimed in claim 1, characterized in that: The first source follower transistor and the second source follower transistor are metal-oxide-semiconductor field effect transistors MOSFET.

3. The dual-channel single-gate source follower as claimed in claim 2, characterized in that: The first source follower transistor and the second source follower transistor are gate-connected by sharing a gate.

4. The dual-channel single-gate source follower as claimed in claim 3, characterized in that: The channel width-to-length ratio of the first source follower transistor and the second source follower transistor ranges from 5 / 29 to 11 / 50.

5. The dual-channel single-gate source follower as claimed in any one of claims 1 to 4, characterized in that: A size of the first source follower transistor is the same as a size of the second source follower transistor.

6. The dual-channel single-gate source follower as claimed in claim 5, characterized in that: The channel width-to-length ratio of the first source follower transistor and the channel width-to-length ratio of the second source follower transistor are both 11 / 58.

7. The dual-channel single-gate source follower as claimed in claim 1, characterized in that: The gate of the first source follower transistor is connected to the gate of the second source follower transistor and is connected to a floating diffusion node of a photodiode PPD in the pixel circuit.

8. A low random telegraph signal RTS noise pixel circuit, characterized in that: include: The dual-channel single-gate source follower as described in any one of claims 1 to 7; a photodiode, wherein a floating diffusion node of the photodiode is connected to a gate of the dual-channel single-gate source follower; a reset circuit connected to a floating diffusion node of the photodiode; A selection circuit is connected to the source of the dual-channel single-gate source follower.

9. The low random telegraph signal RTS noise pixel circuit according to claim 8, characterized in that: The reset circuit includes a reset transistor, a source of the reset transistor is connected to a floating diffusion node of the photodiode, a gate of the reset transistor is connected to a reset signal RST, and a drain of the reset transistor is connected to a power supply VDD.

10. The low random telegraph signal RTS noise pixel circuit according to any one of claims 8 to 9, characterized in that: The selection circuit includes a bit line selection transistor, the drain of the bit line selection transistor is connected to the source of the dual-channel single-gate source follower, the gate of the bit line selection transistor is connected to the selection signal SEL, and the source of the bit line selection transistor is connected to the column signal transmission line COL BUS.

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