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

Abstract

The embodiment of the utility model 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 and the grid electrode of the second source electrode following transistor are connected in a grid electrode sharing mode. The source electrode of the first source electrode following transistor is connected with the source electrode of the second source electrode following transistor. According to the dual-channel single-gate source follower, through the arrangement of dual channels, the pixel circuit can effectively increase the area of an SF transistor, increase the width of a current path and increase the transconductance value on the premise that the area of a photodiode is not reduced and the dynamic range is not affected, and then RTS noise is reduced.

Description

Technical Field

[0001] This application relates to the field of image sensor circuits, and more particularly to a dual-channel single-gate source follower and a pixel circuit with low random telegraph signal noise. Background Technology

[0002] In the design and manufacturing of semiconductor image sensors, pixel signal noise has always been a significant factor affecting image quality. Especially under low-light conditions, random telegraph signal (RTS) noise in pixels can cause them to flicker randomly in the dark, significantly degrading image and video quality. This phenomenon is particularly pronounced in high-sensitivity sensors, such as mobile phone cameras, security monitoring systems, and scientific imaging equipment.

[0003] RTS noise is primarily related to the source follower (SF) transistor in the pixel. A traditional solution to reduce RTS noise is to increase the gate area by increasing the width of the SF transistor, which improves transistor stability and increases the transconductance (gm) of the SF transistor. However, in pixel-limited designs, especially for small pixels (e.g., 1.0 micrometers and below), increasing the width of the SF transistor reduces the area of ​​the photodiode (PD), thereby decreasing photon absorption and full-well capacity, ultimately affecting the overall photoelectric conversion efficiency of the pixel.

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

[0005] In view of this, this 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 a first aspect, embodiments of this application provide 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 drains of the first source follower transistor and the second source follower transistor are both connected to a power supply voltage VDD; the gates of the first source follower transistor and the second source follower transistor are connected by sharing a gate; and the sources of the first source follower transistor and the second source follower transistor are connected.

[0007] To reduce RTS noise, it is necessary to increase the current path width, i.e., 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 reduce the usable area of ​​the photodiode, thereby reducing photon absorption capacity and full-well capacity, ultimately affecting the photoelectric conversion efficiency of the entire pixel. The embodiments of this application use two parallel source follower transistors to form two parallel current signal channels. Without reducing the photodiode area or affecting the dynamic range, the transconductance of the source follower is increased, resulting in better noise suppression without significantly increasing pixel power consumption.

[0008] A shared gate refers to two source follower transistors sharing the same gate. In the embodiments of this application, the polysilicon gates of the two source follower transistors are connected by sharing a gate, eliminating the need for extra metal traces to connect the gates of the two original separate source follower transistors. This reduces the complexity of metal wiring and can also improve the capacitance gain of the overall system.

[0009] In one possible implementation, the first source follower transistor and the second source follower transistor are metal-oxide-semiconductor field-effect transistors (MOSFETs).

[0010] In the embodiments of this application, since MOSFETs have characteristics such as high input impedance, fast switching speed and no secondary breakdown, the source follower can exhibit more stable characteristics.

[0011] In one possible implementation, the gate of the first source follower transistor and the gate of the second source follower transistor are connected to the photodiode (PPD) and the reset circuit of 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.

[0012] In one 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, or 5 / 29 to 11 / 50.

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

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

[0015] In one 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, or 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 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 select transistor of the pixel circuit.

[0017] Secondly, embodiments of this application provide 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 thereof; a photodiode, the floating diffusion node of the photodiode being connected to the gate of the dual-channel single-gate source follower; a reset circuit, the reset circuit being connected to the floating diffusion node of the photodiode; and a selection circuit, the selection circuit being connected to the source of the dual-channel single-gate source follower.

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

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

[0020] The dual-channel single-gate source follower and corresponding pixel circuit provided in this application have a wider current path and simpler layout and wiring, which can effectively reduce RTS noise in the pixel circuit and improve the imaging quality of the image sensor. Attached Figure Description

[0021] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments of the present application. 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 relevant drawings can be obtained based on these drawings without creative work.

[0022] Figure 1 This is a schematic diagram of the RTS noise principle.

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

[0024] Figure 3 This illustration shows a schematic diagram of a dual-channel single-gate source follower provided in an embodiment of this application;

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

[0026] Figure 5 This illustration shows a layout and wiring diagram of a dual-channel single-gate source follower provided in an embodiment of this application. Detailed Implementation

[0027] The technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings.

[0028] In the description of this application, it should be noted that the terms "inner" and "outer" and the like indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, or the orientations or positional relationships in which the product of this application is typically placed when in use. These terms are intended solely to facilitate the description of this application and simplify the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, be constructed, or operate in a specific orientation. Therefore, they should not be construed as limitations on this application. Furthermore, the terms "first" and "second" and the like are used solely for distinction and should not be construed as indicating or implying relative importance.

[0029] It should also be noted that, unless otherwise explicitly specified and limited, the terms "setup" and "connection" should be interpreted broadly. For example, they can refer to a direct connection, an indirect connection through an intermediate medium, or a connection within two components. Those skilled in the art can understand the specific meaning of these terms in this application based on the specific circumstances.

[0030] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

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

[0032] In the design and manufacturing of semiconductor image sensors, pixel signal noise has always been a significant factor affecting image quality. Especially under low-light conditions, random telegraph signal (RTS) noise in pixels can cause them to flicker randomly in dark environments, significantly degrading image and video quality. This phenomenon is particularly pronounced in high-sensitivity sensors, such as mobile phone cameras, security monitoring systems, and scientific imaging equipment.

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

[0034]

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

[0036] Figure 2 The diagram illustrates the schematic of a traditional pixel. In a conventional pixel design, each pixel contains only one source follower transistor, and its width is limited by the pixel size. 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 transistor's stability and transconductance gm. However, when pixel size is limited, especially in the design of small pixels (e.g., 1.0 micrometers and below), increasing the width of the SF transistor reduces the area of ​​the photodiode PD, thereby reducing photon absorption and full-well capacity, ultimately affecting the overall photoelectric conversion efficiency of the pixel.

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

[0038] Therefore, this 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, by dividing the current path of the SF transistor into two parallel channels, significantly improves the transconductance (gm) of the transistor without reducing the photodiode area or affecting the dynamic range, and provides better noise suppression without significantly increasing pixel power consumption.

[0039] Please see Figure 3 , Figure 3 A schematic diagram of a dual-channel single-gate DCSG source follower is shown in an embodiment of this application.

[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 a second source follower transistor. The drains of both the first and second source follower transistors are connected to the power supply voltage VDD. The gates of both transistors are connected and also connected to the photodiode PPD and the reset circuit in the pixel circuit. The sources of both transistors are connected to the selection circuit of the pixel circuit.

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

[0042] Thus, by using parallel source follower transistors, the unused space in the pixel is effectively utilized without affecting the area of ​​the photodiode (PD). By using two parallel SF transistors, approximately 4 to 5 times the SF width gain can be provided within the same pixel area, thereby significantly reducing RTS noise.

[0043] Those skilled in the art will readily understand that, depending on the spatial configuration of the specific pixels, multiple SF transistors can be connected in parallel to form a multi-channel single-gate source follower, and this implementation is still within the scope of protection of this application.

[0044] In some embodiments, the first source follower transistor and the second source follower transistor are metal-oxide-semiconductor field-effect transistors (MOSFETs). Those skilled in the art will readily understand that MOSFETs, due to their high input impedance, fast switching speed, and lack of secondary breakdown, enable source followers to 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; this application does not limit this to specific embodiments.

[0045] Please see Figure 4 , Figure 4 Two connection methods for 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 by metal wiring, such as... Figure 4 As shown on the left, the gates of the two SF transistors are connected together by 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 a 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, it is no longer necessary to use extra metal traces to connect the gates of the two separate SF transistors. This design reduces the complexity of metal wiring and improves the overall system capacitance gain (CG).

[0048] In some embodiments, the first source follower transistor has the same size as the second source follower transistor. This dual-channel configuration provides approximately 4 to 5 times the SF width gain, thereby significantly reducing RTS noise. It should be noted that in some embodiments, "same size" means the two transistors are identical; in other embodiments, "same size" may also mean the width and length of the transistor gates are the same; and in still other embodiments, "same size" may also mean the transistors have the same aspect ratio. This application does not limit these embodiments. By setting the two SF transistors to have the same size, mismatch problems in integrated circuit manufacturing process design can be reduced.

[0049] The aspect ratio of the SF transistor can be selected according to the needs of the actual pixel circuit. In some embodiments, the channel aspect 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 aspect ratio of both the first source follower transistor and the second source follower transistor is 0.22 / 1.16, i.e., 11 / 58.

[0050] Those skilled in the art will readily understand that, in some embodiments, the length of the SF transistor can be flexibly adjusted according to the specific application requirements to achieve the optimal balance between transconductance gm and capacitance gain CG. For example, in low-light applications, a longer transistor length is used, as the 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 used, as the shorter transistor can reduce capacitive load, thereby improving response speed.

[0051] The dual-channel single-gate source follower provided in this application embodiment is applied to pixel circuits, especially in small pixel circuits, where it can better reduce RTS noise and improve image quality. Therefore, those skilled in the art will readily 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. See also... Figure 3 and Figure 5 , Figure 3 and Figure 5 A schematic diagram of the RTS noise pixel circuit in an embodiment of this application is shown, which includes: a dual-channel single-gate source follower as described in any embodiment of this application; a pinned photodiode (PPD), the floating diffusion node (FD) of the photodiode being connected to the gate of the dual-channel single-gate source follower; a reset circuit, the reset circuit being connected to the floating diffusion node of the photodiode; and a selection circuit, the selection circuit being connected to the source of the dual-channel single-gate source follower.

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

[0054] In some embodiments, as 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 a dual-channel single-gate source follower. The gate of the bit line selection transistor is connected to the selection signal SEL. The source of the bit line selection transistor is connected to the column signal transmission line COL BUS.

[0055] In some embodiments, as Figure 5 As shown, the photodiode PPD converts light into electrons and stores them. When reading, the 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 a voltage signal. Then, they are read out by the two parallel channels of the SF, and after being selected by the bit line selection transistor, they are converted into a digital signal for output, thus completing the scanning of one pixel.

[0056] The dual-channel single-gate source follower and low-RTS pixel circuit provided in this application significantly reduce RTS noise through a dual-channel single-gate design, particularly in low-light applications, resulting in a significant improvement in pixel image quality. Despite increasing the width of the SF transistor, this design does not reduce the area of ​​the photodiode, ensuring that the pixel's photoelectric conversion efficiency remains unaffected. Embodiments of this application allow for flexible adjustment of the SF transistor length to achieve an optimal balance between transconductance and capacitance gain, thereby meeting performance requirements in various scenarios.

[0057] In the above embodiments, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions in other embodiments.

[0058] In the several embodiments provided in this application, it should be understood that the disclosed apparatus can be implemented in other ways. For example, the apparatus embodiments described above are merely illustrative; for instance, the division of the units described above is only a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units or components may be combined or integrated into another system, or some features may be ignored or not executed. Furthermore, the coupling or direct coupling or communication connection shown or discussed may be through some interfaces; the indirect coupling or communication connection between devices or units may be electrical or other forms.

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

[0060] Furthermore, the functional units in the various embodiments of this application can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit. The integrated unit can be implemented in hardware or as a software functional unit.

[0061] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A dual-channel single-gate source follower for pixel circuits with low random telegraph signal noise, characterized in that, The dual-channel single-gate source follower includes: First source follower transistor and 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 the power supply voltage VDD. The gate of the first source follower transistor and the gate of the second source follower transistor are connected in a shared gate manner; The source of the first source follower transistor is connected to the source of the second source follower transistor.

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

3. The dual-channel single-gate source follower as described in claim 2, characterized in that: The gate of the first source follower transistor and the gate of the second source follower transistor are connected to the photodiode PPD and the reset circuit of 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.

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

6. The dual-channel single-gate source follower as described 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 described 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 the floating diffusion node of the photodiode PPD in the pixel circuit.

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

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

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

Images