A dual-port CMOS image sensor pixel structure and 5T pixel structure

Abstract

The present application relates to a pixel structure of a CMOS image sensor, and particularly relates to a dual-port CMOS image sensor pixel structure and a 5T pixel structure, which solves the technical problem of low time resolution capability of the existing CMOS image sensor. The pixel structure provided by the present application is an axisymmetric structure, comprising a photodiode, two charge transfer transistors and two storage nodes. The photodiode is an axisymmetric polygonal structure, which is composed of multiple trapezoidal shapes and a conical shape; the main body structure of the gate of the charge transfer transistor is a slanted rectangle, one side of which is in contact with two waist lines in the conical shape respectively; and the storage node is a right triangle structure, the hypotenuse of which is in contact with the other side of the gate of the two charge transfer transistors respectively. The present application can generate a charge transfer electric field inside the photodiode, so that both ports can achieve a very high charge transfer speed, and the time resolution capability of the pixel structure is enhanced, which can be applied to a pixel structure with dual charge transfer ports.

Description

Technical Field

[0001] This invention relates to the pixel structure of a CMOS image sensor, specifically to a dual-port CMOS image sensor pixel structure and a 5T pixel structure. Background Technology

[0002] CMOS image sensors, with their advantages of low power consumption, low cost, and high integration, have become the mainstream image sensors in industries such as manufacturing, smartphones, and scientific research. With the continuous advancement of CMOS image sensor design and manufacturing technologies, higher demands are being placed on the performance of CMOS image sensors, including frame rate, resolution, and noise. Temporal resolution is a crucial performance indicator for CMOS image sensors, often a key focus in high-speed imaging applications such as transient physics research and high-speed motion analysis. In high-speed imaging, insufficient temporal resolution of the CMOS image sensor can cause image ghosting, severely impacting image quality. The temporal resolution of a CMOS image sensor is determined by the minimum time required for the photogenerated charge signal generated by the photodiode in the pixel device to transfer to the floating diffusion node.

[0003] CMOS image sensors can generally be classified into 3T pixel structure, 4T pixel structure, and 5T pixel structure, depending on the number of integrated transistors. Among them, 4T pixel structure and 5T pixel structure are the most common. Figure 1 , Figure 2 The circuit diagrams show the schematics for 4T and 5T pixel structures, respectively. Both structures include a photodiode V1, a transfer transistor V2, a floating diffusion node reset transistor V3, a source follower transistor V4, and a row selection transistor V5. In the 4T structure, photodiode V1 has only one charge transfer port connected to the transfer transistor V2, while in the 5T structure, photodiode V1 requires two charge transfer ports, connected to the transfer transistor V2 and the photodiode reset transistor V6, respectively. Compared to the 4T pixel structure, the 5T pixel structure allows for faster global exposure and is the most widely used pixel structure type for dual-port pixel applications.

[0004] Figure 3This is a simplified top view of a traditional dual-port pixel structure. The photodiode 101 of a traditional dual-port pixel often adopts a rectangular structure, with the first charge transfer transistor 102, the first storage node 103, and the second charge transfer transistor 104 and the second storage node 105 located on opposite sides of the rectangular region. Because the photodiode 101 uses a rectangular region, according to relevant theories of semiconductor device physics, its internal potential is relatively flat. The charge generated inside the photodiode 101 is mainly transferred to the floating diffusion node through diffusion, which is relatively slow, resulting in lower time resolution.

[0005] To improve the charge transfer rate within a pixel structure, an effective current solution is to generate an electric field within the photodiode 101 through pixel structure and fabrication design. This electric field makes drift motion the primary driving force for charge transfer. Reported methods include shape design, gradient doping design, and applying an external bias voltage. However, since the electric field generated within the photodiode 101 is directional, these methods are mainly effective for pixel structures with a single charge transfer port, such as 4T pixels. They are not applicable to dual-port or multi-port pixel structures, such as 5T pixels. Summary of the Invention

[0006] The purpose of this invention is to solve the technical problem of low temporal resolution of existing pixel structures, and to provide a dual-port CMOS image sensor pixel structure and a 5T pixel structure based thereon.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A dual-port CMOS image sensor pixel structure is characterized by including a photodiode 101, a first charge transfer transistor 102, a first storage node 103, a second charge transfer transistor 104, and a second storage node 105.

[0009] The photodiode 101 is used to convert the collected light pixel signal into a photogenerated charge signal. Its anode is grounded, and its cathode is coupled to the source of the first charge transfer transistor 102 and the source of the second charge transfer transistor 104. The photodiode 101 is an axisymmetric special polygonal structure composed of m trapezoidal structures 106 and a conical structure 107, where m ≥ 1 and m is an integer.

[0010] The trapezoidal structures 106 generate a charge transfer electric field from their short base to their long base. Their long base contacts the long base of the conical structure 107, transferring the photogenerated charge signal to the conical structure 107. The conical structure 107 is coupled to the source of the first charge transfer transistor 102 and the source of the second charge transfer transistor 104, respectively. Its two waistlines are in contact with the gates of the first charge transfer transistor 102 and the second charge transfer transistor 104, respectively. It generates a charge transfer electric field inside, transferring the photogenerated charge signal towards its central region. When the first charge transfer transistor 102 or the second charge transfer transistor 104 is turned on, the photogenerated charge signal is rapidly transferred towards the first charge transfer transistor 102 or the second charge transfer transistor 104.

[0011] The drain of the first charge transfer transistor 102 is coupled to the first storage node 103, and the gate is connected to an external input signal, which is used to transmit the photogenerated charge signal from the photodiode 101 to the first storage node 103 under the action of the external input signal;

[0012] The drain of the second charge transfer transistor 104 is coupled to the second storage node 105, and the gate is connected to an external input signal, which is used to transfer the charge signal from the photodiode 101 to the second storage node 105 under the action of the external input signal;

[0013] The first storage node 103 and the second storage node 105 are used to collect the transferred photogenerated charge signals and convert them into voltage signals for output.

[0014] Furthermore, the main body of the first charge transfer transistor 102 and the second charge transfer transistor 104 is a slanted rectangular structure that matches the photodiode 101;

[0015] One side of the gate of the first charge transfer transistor 102 and one side of the gate of the second charge transfer transistor 104 are respectively in contact with the two waistlines of the cone-shaped structure 107.

[0016] The other side of the gate of the first charge transfer transistor 102 is in contact with the first storage node 103, and the other side of the gate of the second charge transfer transistor 104 is in contact with the second storage node 105.

[0017] Furthermore, the first storage node 103 is a right-angled triangle structure matched with the first charge transfer transistor 102 and the photodiode 101, and the second storage node 105 is a right-angled triangle structure matched with the second charge transfer transistor 104 and the photodiode 101.

[0018] The inclined side of the first storage node 103 is in contact with the other side of the gate of the first charge transfer transistor 102, and the inclined side of the second storage node 105 is in contact with the other side of the gate of the second charge transfer transistor 104.

[0019] Furthermore, the m trapezoidal structures 106 are arranged at equal intervals.

[0020] Furthermore, the trapezoidal structure 106 is a trapezoidal structure or a bell-shaped structure.

[0021] Furthermore, the trapezoidal structure 106 is a trapezoidal structure with a long base length of 1.5μm to 3μm.

[0022] Furthermore, the angle between the waistline of the cone-shaped structure 107 and the charge transfer direction is 15° to 60°.

[0023] Furthermore, the photodiode 101 is a clamping photodiode.

[0024] A 5T pixel structure, based on the aforementioned dual-port CMOS image sensor pixel structure, is characterized by:

[0025] It also includes a floating diffuse node reset transistor 108, a source follower transistor 109, and a row select transistor 110;

[0026] The external power supply voltage of the first storage node 103;

[0027] The floating diffusion node reset transistor 108 is used to reset the floating diffusion node to the power supply voltage. Its source is connected to the gate of the second storage node 105, the source follower transistor 109, and the drain of the second charge transfer transistor 104. The gate of the floating diffusion node reset transistor 108 is used to connect to an external input signal, and its drain is connected to the power supply.

[0028] The source follower transistor 109 is used to buffer and amplify the signal of the second storage node 105. Its source is connected to the drain of the row select transistor 110, and its drain is connected to the power supply.

[0029] The row selection transistor 110 is used to select whether the signal of the second storage node 105 is read by the external readout circuit of the pixel. Its gate is connected to the external input signal and its source is used for pixel output.

[0030] The first charge transfer transistor 102 is a photodiode reset transistor, the second charge transfer transistor 104 is a transfer transistor, and the drain of the first charge transfer transistor 102 is the first storage node 103.

[0031] The advantages of this invention over the prior art are:

[0032] Compared with the prior art, the present invention has the following beneficial technical effects:

[0033] 1. The photodiode in the dual-port CMOS image sensor pixel structure provided by this invention is a special polygonal structure with axisymmetric design. Both the trapezoidal and conical structures can generate charge transfer electric fields, so that when any transfer transistor is turned on, the charge transfer electric field is modulated in the direction of the turned-on transfer transistor, thereby achieving a high charge transfer speed in any direction, enhancing the time resolution capability of the CMOS image sensor, and can be applied to 5T pixels and other pixel structures with dual charge transfer ports.

[0034] 2. In the pixel structure of the dual-port CMOS image sensor provided by the present invention, the charge transfer transistor and the storage node are designed according to the shape of the photodiode, which can effectively reduce the waste of pixel area;

[0035] 3. In the pixel structure of the dual-port CMOS image sensor provided by the present invention, the trapezoidal structure of the photodiodes is arranged at equal intervals, so that the charge distribution is uniform and the charge transfer speed is further improved. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of a traditional 4T pixel structure circuit.

[0037] Figure 2 This is a schematic diagram of a traditional 5T pixel structure circuit.

[0038] Figure 3 A top view of a simplified structure for a traditional dual-port pixel;

[0039] Figure 4 A circuit schematic diagram of an embodiment of the pixel structure of a dual-port CMOS image sensor provided by the present invention;

[0040] Figure 5 A top view of an embodiment of the pixel structure of a dual-port CMOS image sensor provided by the present invention (the source and drain of the first charge transfer transistor and the second charge transfer transistor are not shown);

[0041] Figure 6 The circuit schematic diagram is for an embodiment of the 5T pixel structure provided by the present invention;

[0042] Figure 7 A simplified top view of an embodiment of the 5T pixel structure provided by the present invention;

[0043] The annotations in the attached figures are explained as follows:

[0044] 101-Photodiode, 102-First charge transfer transistor, 103-First storage node, 104-Second charge transfer transistor, 105-Second storage node, 106-Trapezoidal structure, 107-Tapered structure, 108-Floating diffused node reset transistor, 109-Source follower transistor, 110-Row select transistor. Detailed Implementation

[0045] To make the objectives, advantages, and features of the present invention clearer, the following detailed description of a dual-port CMOS image sensor pixel structure proposed by the present invention, in conjunction with the accompanying drawings and specific embodiments, is provided. In the description of the present invention, it should be noted that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0046] like Figure 4 As shown, a pixel structure of a dual-port CMOS image sensor includes a photodiode 101, a first charge transfer transistor 102, a first storage node 103, a second charge transfer transistor 104, and a second storage node 105.

[0047] The anode of the photodiode 101 is grounded, and the cathode is coupled to the source of the first charge transfer transistor 102 and the source of the second charge transfer transistor 104. This is used to convert the acquired light pixel signal into a photogenerated charge signal, and under the action of the charge transfer electric field, to rapidly transfer the photogenerated charge signal to the direction of the first charge transfer transistor 102 or the second charge transfer transistor 104 when the first charge transfer transistor 102 or the second charge transfer transistor 104 is turned on.

[0048] The drain of the first charge transfer transistor 102 is coupled to the first storage node 103, and its gate is connected to an external input signal. It is used to transfer the photogenerated charge signal from the photodiode 101 to the first storage node 103 under the influence of the external input signal. The drain of the second charge transfer transistor 104 is coupled to the second storage node 105, and its gate is connected to an external input signal. It is used to transfer the charge signal from the photodiode 101 to the second storage node 105 under the influence of the external input signal. The first storage node 103 and the second storage node 105 are used to collect the transferred photogenerated charge signal and convert it into a voltage signal for output.

[0049] The dual-port CMOS image sensor pixel structure provided by this invention is an axisymmetric structure. For example... Figure 5As shown, the photodiode 101 has an axisymmetric polygonal structure, which consists of m trapezoidal structures 106 and one cone-shaped structure 107, where m ≥ 1 and m is an integer. The m trapezoidal structures 106 are arranged at equal intervals, generating a charge transfer electric field from their short bases to their long bases. The long bases of the trapezoidal structures 106 and the cone-shaped structure 107 are in contact, transferring the photogenerated charge signal to the cone-shaped structure 107. The cone-shaped structure 107 is coupled to the source of the first charge transfer transistor 102 and the source of the second charge transfer transistor 104, respectively. Its two waistlines are in contact with the gates of the first charge transfer transistor 102 and the second charge transfer transistor 104, respectively. It generates a charge transfer electric field inside, transferring the photogenerated charge signal towards its central region. When an external high-voltage signal is input to the gate of the first charge transfer transistor 102 or the second charge transfer transistor 104, the photogenerated charge signal is rapidly transferred towards the first charge transfer transistor 102 or the second charge transfer transistor 104. The photodiode 101 is a clamping photodiode, which is the most widely used in image sensors.

[0050] Because, within a certain scale range, the clamping potential generated by the photodiode 101 of a CMOS image sensor pixel in a narrower region is smaller than that in a wider region, the trapezoidal structure 106 can generate a charge transfer electric field from the short base to the long base. In this embodiment, the trapezoidal structure 106 is a trapezoidal structure; in other embodiments, it can also be a bell-shaped structure, etc. To achieve a higher charge transfer speed, the length of the short base of the trapezoidal structure 106 is determined by the minimum width of the photodiode 101 required by the process, and the length of the long base is 1.5 μm to 3 μm, which can be optimized according to different process conditions.

[0051] In this embodiment, the cone-shaped structure 107 also utilizes the principle of generating a charge transfer electric field based on the shape design of the photodiode 101 to achieve... Figure 5 For a vertical charge transfer electric field to achieve a better vertical charge transfer effect, the angle between its waistline and the charge transfer direction should be greater than 15° and less than 60°.

[0052] The gates of the first charge transfer transistor 102 and the second charge transfer transistor 104 are primarily oblique rectangles that match the photodiode 101. One side of the gate of the first charge transfer transistor 102 and one side of the gate of the second charge transfer transistor 104 are respectively in contact with the two waistlines of the cone-shaped structure 107. The other side of the gate of the first charge transfer transistor 102 is in contact with the first storage node 103, transferring the charge signal to the first storage node 103 when a high voltage is applied to the gate; the other side of the gate of the second charge transfer transistor 104 is in contact with the second storage node 105, transferring the charge signal to the second storage node 105 when a high voltage is applied to the gate.

[0053] To ensure an appropriate overall shape for the pixel structure, the first storage node 103 is a right-angled triangle structure matching the first charge transfer transistor 102 and the photodiode 101, and the second storage node 105 is a right-angled triangle structure matching the second charge transfer transistor 104 and the photodiode 101. The hypotenuse of the first storage node 103 contacts the other side of the gate of the first charge transfer transistor 102, and the hypotenuse of the second storage node 105 contacts the other side of the gate of the second charge transfer transistor 104.

[0054] The dual-port CMOS image sensor pixel structure provided in this embodiment can change the charge transfer electric field in the direction of the open port when any port of the pixel structure is open. The working principle is as follows:

[0055] The photogenerated charge signal generated in the trapezoidal structure 106 region is first transferred to the cone-shaped structure 107 region under the drive of the charge transfer electric field. If the first charge transfer transistor 102 is turned on and the second charge transfer transistor 104 is turned off, under the action of the charge transfer electric field generated by the shape change, the region of the cone-shaped structure 106 that is far from the first charge transfer transistor 102 is transferred vertically to the central region of the cone-shaped structure 107, and then transferred to the first storage node 103 under the action of the electric field generated by the voltage applied to the gate of the transfer transistor 102. Similarly, if the first charge transfer transistor 102 is turned off and the second charge transfer transistor 104 is turned on, the photogenerated charge in the cone-shaped structure 107 region can be transferred to the second storage node 105.

[0056] This embodiment also provides a 5T pixel structure, such as Figure 6 As shown, based on the original dual-port CMOS image sensor pixel structure, a floating diffusion node reset transistor 108, a source follower transistor 109, and a row selection transistor 110 are added.

[0057] The first storage node 103 is connected to an external power supply voltage. A floating diffusion node reset transistor 108 is used to reset the floating diffusion node to the power supply voltage. Its source is connected to the second storage node 105, the gate of the source follower transistor 109, and the drain of the second charge transfer transistor 104. The gate of the floating diffusion node reset transistor 108 is used to connect to an external input signal, and its drain is connected to the power supply. The source follower transistor 109 is used to buffer and amplify the signal from the second storage node 105. Its source is connected to the drain of the row select transistor 110, and its drain is connected to the power supply. The row select transistor 110 is used to select whether the signal from the second storage node 105 is read by the pixel external readout circuit. Its gate is connected to an external input signal, and its source is used for pixel output.

[0058] The first charge transfer transistor 102 serves as the photodiode reset transistor for the 5T pixel, the second charge transfer transistor 104 serves as the transfer transistor for the 5T pixel, the drain of the first charge transfer transistor 102 serves as the first storage node 103 and is connected to an external power supply, and the second storage node 105 serves as the floating diffusion node for the 5T pixel. For example... Figure 7 As shown, the floating diffusion node reset transistor 108, the source follower transistor 109, and the row select transistor 110 are concentrated in one side of the pixel structure.

[0059] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention 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 or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the present invention.

Claims

1. A pixel structure for a dual-port CMOS image sensor, characterized in that: It includes a photodiode (101), a first charge transfer transistor (102), a first storage node (103), a second charge transfer transistor (104), and a second storage node (105); The photodiode (101) is used to convert the collected light pixel signal into a photogenerated charge signal. Its anode is grounded, and its cathode is coupled to the source of the first charge transfer transistor (102) and the source of the second charge transfer transistor (104). The photodiode (101) is an axisymmetric polygonal structure composed of m trapezoidal structures (106) and a conical structure (107), where m ≥ 1 and m is an integer. The interior of each of the m trapezoidal structures (106) generates a charge transfer electric field from its short base to its long base, and its long base contacts the long base of the conical structure (107), transferring the photogenerated charge signal to the conical structure (107). The cone-shaped structure (107) is coupled to the source of the first charge transfer transistor (102) and the source of the second charge transfer transistor (104), respectively. Its two waistlines are in contact with the gates of the first charge transfer transistor (102) and the second charge transfer transistor (104), respectively. A charge transfer electric field is generated inside it, which transfers the photogenerated charge signal towards its central region. When the first charge transfer transistor (102) or the second charge transfer transistor (104) is turned on, the photogenerated charge signal is rapidly transferred towards the first charge transfer transistor (102) or the second charge transfer transistor (104). The drain of the first charge transfer transistor (102) is coupled to the first storage node (103), and the gate is connected to the external input signal. It is used to transmit the photogenerated charge signal from the photodiode (101) to the first storage node (103) when a high voltage signal is input from the outside. The drain of the second charge transfer transistor (104) is coupled to the second storage node (105), and the gate is connected to an external input signal. It is used to transmit the photogenerated charge signal from the photodiode (101) to the second storage node (105) when a high voltage signal is input from the outside. The first storage node (103) and the second storage node (105) are used to collect the transferred photogenerated charge signals and convert them into voltage signals for output.

2. The pixel structure of the dual-port CMOS image sensor according to claim 1, characterized in that: The gate bodies of the first charge transfer transistor (102) and the second charge transfer transistor (104) are rectangular structures that match the photodiode (101); One side of the gate of the first charge transfer transistor (102) and one side of the gate of the second charge transfer transistor (104) are respectively in contact with the two waistlines of the cone-shaped structure (107); The other side of the gate of the first charge transfer transistor (102) is in contact with the first storage node (103), and the other side of the gate of the second charge transfer transistor (104) is in contact with the second storage node (105).

3. The pixel structure of the dual-port CMOS image sensor according to claim 2, characterized in that: The first storage node (103) is a right-angled triangle structure matched with the first charge transfer transistor (102) and photodiode (101), and the second storage node (105) is a right-angled triangle structure matched with the second charge transfer transistor (104) and photodiode (101); The slant of the first storage node (103) is in contact with the other side of the gate of the first charge transfer transistor (102), and the slant of the second storage node (105) is in contact with the other side of the gate of the second charge transfer transistor (104).

4. The pixel structure of the dual-port CMOS image sensor according to claim 3, characterized in that: m of the aforementioned trapezoidal structures (106) are arranged at equal intervals.

5. The dual-port CMOS image sensor pixel structure according to claim 4, characterized in that: The trapezoidal structure (106) is either a trapezoidal structure or a bell-shaped structure.

6. The pixel structure of the dual-port CMOS image sensor according to claim 5, characterized in that: The trapezoidal structure (106) is a trapezoidal structure with a long base length of 1.5μm to 3μm.

7. The pixel structure of the dual-port CMOS image sensor according to claim 6, characterized in that: The waistline of the cone-shaped structure (107) has an angle of 15° to 60° with the charge transfer direction.

8. The pixel structure of the dual-port CMOS image sensor according to claim 7, characterized in that: The photodiode (101) is a clamping photodiode.

9. A 5T pixel structure, based on the dual-port CMOS image sensor pixel structure according to any one of claims 1-8, characterized in that: It also includes a floating diffuse node reset transistor (108), a source follower transistor (109), and a row select transistor (110); The first storage node (103) is connected to an external power supply voltage; The floating diffusion node reset transistor (108) is used to reset the second storage node (105) to the power supply voltage. Its source is connected to the second storage node (105), the gate of the source follower transistor (109), and the drain of the second charge transfer transistor (104). The gate of the floating diffusion node reset transistor (108) is used to connect to the external input signal, and its drain is connected to the power supply. The source follower transistor (109) is used to buffer and amplify the signal of the second storage node (105), and its source is connected to the drain of the row select transistor (110), and its drain is connected to the power supply. The row selection transistor (110) is used to select whether the signal of the second storage node (105) is read out by the external readout circuit of the pixel. Its gate is connected to the external input signal and its source is used for pixel output.

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