Time-of-flight image sensor pixel circuit and readout method thereof, and image sensor

Abstract

The present application relates to a time-of-flight image sensor pixel circuit, a readout method thereof, and an image sensor. In the time-of-flight image sensor pixel circuit, a plurality of transfer transistors are arranged in pairs with a photosensitive element as the center. The photocharge signals transmitted by the paired transfer transistors are charge signals integrated in the same period. The photocharge signals output by the paired transfer transistors are output by corresponding output control units and then merged into a total photocharge signal for image processing. Assisted by the readout method of the time-of-flight image sensor pixel circuit, by dividing the phase signal into multiple depth subframes for readout, the transmission distance of the photocharge signal can be shortened to the greatest extent while maintaining the original pixel size, and the depth measurement accuracy can be increased without sacrificing the operating frequency, while improving the signal-to-noise ratio of the image sensor.

Description

Technical Field

[0001] The present application relates to the technical field of image sensors, and in particular to a time-of-flight image sensor pixel circuit and a readout method thereof, and an image sensor. Background Art

[0002] Image sensors are a crucial component of digital cameras. Depending on the component, they can be categorized into two main types: CCD (Charge Coupled Device) image sensors and CMOS (Complementary Metal Oxide Semiconductor) image sensors. CMOS image sensors are widely used in various fields due to their advantages, such as low power consumption, low cost, and ease of standardized production.

[0003] Time-of-flight (TOF) image sensor devices are mainly used in systems that acquire 3D images. They use the time it takes for light to travel from a light source to an object and then reflect back to the image sensor based on optical time-of-flight to measure the distance between the imaging target and the image sensor device. By using each pixel of the time-of-flight image sensor for distance measurement, high-precision depth images can be obtained.

[0004] As the functions of image sensors increase day by day, the requirements for the hardware properties of image sensors are gradually increasing, and the size of the image sensor pixel circuit is also increasing in line with the requirements of the hardware properties. Due to the increase in the size of the image sensor pixel circuit, in the pixel structure, the transmission path of the photocharge signal output by the photosensitive element to the floating diffusion point is longer than that of traditional image sensors. Accordingly, the transmission time of the photocharge signal output by the photosensitive element to the floating diffusion point is longer than that of traditional image sensors. Summary of the Invention

[0005] In order to at least overcome the above-mentioned deficiencies in the prior art, the present application aims to provide a time-of-flight image sensor pixel circuit and a readout method thereof, and an image sensor.

[0006] In a first aspect, an embodiment of the present application provides a time-of-flight image sensor pixel circuit, which includes a pixel unit and several output control units. Specifically, the pixel unit includes a photosensitive element and several transfer transistors, wherein control ends of the several transfer transistors respectively receive transmission control signals, input ends of the several transfer transistors are respectively connected to the photosensitive element, and output ends of the several transfer transistors are respectively connected to the several output control units; the several transfer transistors are arranged in pairs with the photosensitive element as the center, and the photocharge signals transmitted by the paired transfer transistors are photocharge signals integrated in the same period; several floating diffusion points are further included between the several output control units and the several transfer transistors, the several output control units are respectively connected to the several floating diffusion points, the several transfer transistors are used to control the photocharge signals output by the photosensitive element to be transmitted to the several floating diffusion points, and the several output control units are used to respectively receive the photocharge signals output by the several floating diffusion points for image processing; wherein the photocharge signals output by the paired transfer transistors are output by the corresponding output control units and then merged into a total photocharge signal for image processing.

[0007] In a possible implementation, adjacent pixel units share one of the plurality of output control units.

[0008] In a possible implementation, the plurality of transfer transistors and the photosensitive element are arranged in the same plane; the plurality of transfer transistors are spaced apart from each other and symmetrically distributed around the photosensitive element, and a center line between the transfer transistors arranged in pairs passes through the photosensitive element.

[0009] In one possible implementation, the pixel unit includes four transfer transistors, which are centered on the photosensitive element and include: a first transfer transistor located at a 315° angle in the clockwise direction of the photosensitive element, a second transfer transistor located at a 45° angle in the clockwise direction of the photosensitive element, a third transfer transistor located at a 135° angle in the clockwise direction of the photosensitive element, and a fourth transfer transistor located at a 225° angle in the clockwise direction of the photosensitive element; the center connection of the second transfer transistor and the fourth transfer transistor passes through the photosensitive element, the center connection of the third transfer transistor and the first transfer transistor passes through the photosensitive element, and the center connection of the second transfer transistor and the fourth transfer transistor intersects with the center connection of the third transfer transistor and the first transfer transistor at the photosensitive element.

[0010] In a possible implementation, the photosensitive element includes a first photosensitive area and a second photosensitive area, and an electron concentration on an N-type semiconductor side in the second photosensitive area is higher than an electron concentration on an N-type semiconductor side in the first photosensitive area.

[0011] In a possible implementation, a ratio of an electron concentration on the N-type semiconductor side in the second light sensing region to an electron concentration on the N-type semiconductor side in the first light sensing region is 2:1.

[0012] In one possible implementation, the pixel unit also includes a switching transistor arranged between the photosensitive element and the several transfer transistors, the switching transistor includes a first switching transistor and a second switching transistor, the first switching transistor and the second switching transistor are spaced apart from each other and symmetrically arranged, and the switching transistors between the paired transfer transistors are the same.

[0013] In a possible implementation, the pixel unit further includes a plurality of storage modules, wherein the storage modules are connected to the photosensitive element and the transfer transistor, and the storage modules are arranged between the photosensitive element and the transfer transistor.

[0014] In one possible implementation, the storage module includes a storage transistor and a storage capacitor, the transfer transistor is connected in series to the switching transistor through the storage transistor, and the storage capacitor is connected to the output end of the storage transistor; the storage transistor is used to transfer the photocharge signal output by the photosensitive element to the storage capacitor after the switching transistor is turned on.

[0015] In one possible implementation, a central modulation pump gate is provided on one side of the photosensitive element, and the central modulation pump gate is used to generate an electric field in the direction from the far end of the photosensitive element to the center of the photosensitive element, and accelerate the transfer of electrons from the center of the photosensitive element to the far end of the photosensitive element.

[0016] In a possible implementation, the central modulation pump gate is coaxially arranged with the photosensitive element.

[0017] In a possible implementation, the ratio of the projected area of ​​the central modulation pump gate on the photosensitive element to the surface area of ​​the photosensitive element on a side facing the central modulation pump gate is 1:3.

[0018] In a possible implementation, the shape of the central modulation pump gate is the same as the shape of the photosensitive element.

[0019] In a possible implementation, the pixel unit further includes a pixel reset transistor, one end of which is connected to the photosensitive element, and is configured to clear the charge of the photosensitive element before the photosensitive element transmits a charge signal.

[0020] In one possible implementation, the several output control units include a reset module and an output module, and the reset module and the output module are respectively connected to the several floating diffusion points, the reset module is used to reset the voltage of the several floating diffusion points, and the output module is used to output the photocharge signals transmitted by the several floating diffusion points.

[0021] In one possible implementation, the reset module includes the reset transistor, one end of the reset transistor is connected to the high-level voltage signal, the other end of the reset transistor is connected to the floating diffusion point, and is connected to the output module through the floating diffusion point, the control end of the reset transistor is connected to the reset control signal, and the reset transistor is used to reset the voltages of the several floating diffusion points according to the reset control signal.

[0022] In one possible implementation, the reset module includes the dual conversion gain control module, which is connected between the reset transistor and the plurality of floating diffusion points. The dual conversion gain control module includes a dual conversion gain control transistor and a capacitor. The dual conversion gain control module is used to implement switching between a low conversion gain mode and a high conversion gain mode.

[0023] In one possible implementation, the output module includes a source follower transistor, the first end of the source follower transistor is connected to a high-level voltage signal, the control end of the source follower transistor is connected to a floating diffusion point, the second end of the source follower transistor is connected to an output line, and the source follower transistor is used to amplify and output the photocharge signals transmitted by the several floating diffusion points.

[0024] In a possible implementation, the output module further includes a row selection transistor, the second end of the source follower transistor is connected to the first end of the row selection transistor, and the second end of the row selection transistor is connected to the output line.

[0025] In a second aspect, an embodiment of the present application also provides a readout method for a time-of-flight image sensor pixel circuit, which is used for reading out the photocharge signal of the above-mentioned time-of-flight image sensor pixel circuit. Specifically: the process of the pixel unit collecting the phase signal is divided into multiple depth subframes. Within a depth subframe, the phase signals output by the paired transmission transistors are the same. The phase signals output by the paired transmission transistors are merged as a phase signal information. The same phase signals of multiple depth subframes are synthesized to obtain all the phase information. The phase difference is calculated based on the phase of the light wave at the time of emission and the phase of the light wave when it is received, thereby calculating the propagation time of the light wave to capture the distance information between itself and the object and obtain a three-dimensional image.

[0026] In a possible implementation, the process of the pixel unit collecting the phase signal is divided into four depth subframes, and the pixel unit includes the first transfer transistor, the second transfer transistor, the third transfer transistor and the fourth transfer transistor, wherein: in the first depth subframe, the first transfer transistor transmits the phase signal value of the first phase, the second transfer transistor transmits the phase signal value of the second phase, the third transfer transistor transmits the phase signal value of the first phase, and the fourth transfer transistor transmits the phase signal value of the second phase; in the second depth subframe, the first transfer transistor transmits the phase signal value of the fourth phase, the second transfer transistor transmits the phase signal value of the third phase, and the third transfer transistor transmits the phase signal value of the fourth phase. The first transmission transistor transmits the phase signal value of the fourth phase, and the fourth transmission transistor transmits the phase signal value of the third phase; in a third depth subframe, the first transmission transistor transmits the phase signal value of the second phase, the second transmission transistor transmits the phase signal value of the first phase, the third transmission transistor transmits the phase signal value of the second phase, and the fourth transmission transistor transmits the phase signal value of the first phase; in a fourth depth subframe, the first transmission transistor transmits the phase signal value of the third phase, the second transmission transistor transmits the phase signal value of the fourth phase, the third transmission transistor transmits the phase signal value of the third phase, and the fourth transmission transistor transmits the phase signal value of the fourth phase.

[0027] In a third aspect, an embodiment of the present application further provides an image sensor, comprising the above-mentioned time-of-flight image sensor pixel circuit.

[0028] Based on the above content, the time-of-flight image sensor pixel circuit and its readout method, and the image sensor provided in the embodiments of the present application arrange a number of transfer transistors in pairs with a photosensitive element as the center. The photocharge signals transmitted by the paired transfer transistors are charge signals integrated in the same period. The photocharge signals output by the paired transfer transistors are output by the corresponding output control units and then merged into a total photocharge signal for image processing. At the same time, the readout method of the time-of-flight image sensor pixel circuit is supplemented by dividing the phase signal into multiple depth subframes to read out the phase signal. This can shorten the transmission distance of the photocharge signal to the greatest extent while maintaining the original pixel size, increase the depth measurement accuracy without sacrificing the operating frequency, and improve the signal-to-noise ratio of the image sensor. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] 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 in the embodiments. 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 paying any creative work.

[0030] Figure 1 A circuit diagram of a time-of-flight image sensor pixel circuit provided in this application;

[0031] Figure 2 This is a schematic diagram of the structure of the time-of-flight image sensor pixel circuit provided by this application;

[0032] Figure 3 This is another structural schematic diagram of the time-of-flight image sensor pixel circuit provided by this application;

[0033] Figure 4 for Figure 3 A schematic cross-sectional view of a time-of-flight image sensor pixel circuit along line PP';

[0034] Figure 5 A schematic diagram of the phase signal distribution of the pixel circuit of the time-of-flight image sensor provided by this application in the first depth subframe;

[0035] Figure 6 A schematic diagram of the phase signal distribution of the pixel circuit of the time-of-flight image sensor provided by this application in the first depth subframe;

[0036] Figure 7 A schematic diagram of the phase signal distribution of the pixel circuit of the time-of-flight image sensor provided by this application in the first depth subframe;

[0037] Figure 8This is a schematic diagram of the phase signal distribution of the time-of-flight image sensor pixel circuit provided by this application in the first depth subframe. DETAILED DESCRIPTION

[0038] In order to make the purpose, technical solutions and advantages of the embodiments of the present application clearer, 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. It should be understood that the drawings in the present application only serve the purpose of illustration and description and are not used to limit the scope of protection of the present application. In addition, it should be understood that the schematic drawings are not drawn to scale. The flowcharts used in this application illustrate the operations implemented according to some embodiments of the embodiments of the present application. It should be understood that the operations of the flowcharts can be implemented out of sequence, and steps without logical context can be reversed or implemented simultaneously. In addition, those skilled in the art, under the guidance of the contents of this application, can add one or more other operations to the flowchart, or remove one or more operations from the flowchart.

[0039] In addition, the described embodiments are only a part of the embodiments of the present application, rather than all of the embodiments. The components of the embodiments of the present application generally described and shown in the drawings here can be arranged and designed in various configurations. Therefore, the following detailed description of the embodiments of the present application provided in the drawings is not intended to limit the scope of the claimed application, but merely represents the selected embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of the present application.

[0040] As described in the background technology section, in order to improve the hardware properties of an image sensor, it is usually necessary to increase the size of the image sensor pixel circuit. The purpose of doing so is to improve the sensitivity of the image sensor. However, due to the increase in the size of the image sensor pixel circuit, it is necessary to increase the size of the photosensitive element. Typically, the size of the photosensitive element occupies 50%-60% of the total size of the pixel unit, resulting in a longer transmission path for the photocharge signal output by the photosensitive element to be transmitted to the floating diffusion point than in a traditional image sensor. Accordingly, the transmission time for the photocharge signal output by the photosensitive element to be transmitted to the floating diffusion point is longer than in a traditional image sensor. Therefore, the embodiments of the present application provide a time-of-flight image sensor pixel circuit and an image sensor.

[0041] Please refer to Figure 1 , Figure 1 FIG. 1 shows a circuit diagram of a time-of-flight image sensor pixel circuit provided by the present application. Figure 1 As shown, the time-of-flight image sensor pixel circuit includes a pixel unit and several output control units, and the several pixel units share the same output control unit.

[0042] Please refer to Figure 2 , Figure 2 FIG1 is a schematic diagram showing the structure of the time-of-flight image sensor pixel circuit provided by the present application. Figure 2 As shown, the pixel unit includes a photosensitive element and a plurality of transfer transistors TX, the control ends of the plurality of transfer transistors TX respectively receive transmission control signals, the input ends of the plurality of transfer transistors TX are respectively connected to the photosensitive elements, and the output ends of the plurality of transfer transistors TX are respectively connected to a plurality of output control units. It can be understood that the output end of any transfer transistor TX can correspond to an output control unit adjacent thereto; thus, according to the transmission control signal and the timing control of turning on and off the switching transfer transistor TX, the photocharge signal read into the photosensitive element after exposure can be transmitted in batches; wherein, the plurality of transfer transistors TX are arranged in pairs with the photosensitive element as the center, and the photocharge signal transmitted by the paired transfer transistors TX is a charge signal integrated in the same period, with the transmission control signal as the center. Figure 2 For example, the middle connection line of the paired transfer transistors TX passes through the transfer transistors TX, and the signals actually transmitted by the two are the same; it can be understood that the turn-on and turn-off times of the paired transfer transistors TX are synchronized. It is worth noting that because the photocharge signals transmitted by the paired transfer transistors TX are charge signals integrated over the same period, during image processing, the photocharge signals transmitted by the multiple paired transfer transistors TX can be combined into a total photocharge signal after being output by the corresponding output control units.

[0043] Several floating diffusion points FD are also included between the several output control units and the several transfer transistors TX. The several output control units and the several transfer transistors TX are respectively connected to the several floating diffusion points FD. It can be understood that any transfer transistor TX can be correspondingly connected to a floating diffusion point FD adjacent to it; the several transfer transistors TX are used to control the transmission of the photocharge signals output by the photosensitive elements to the several floating diffusion points FD, and the several output control units are used to respectively receive the photocharge signals output by the several floating diffusion points FD for image processing.

[0044] It is understood that the photosensitive element referred to in the embodiments of the present application can be any photosensitive structure for converting visible light into a photocharge signal, such as any one of a photodiode, a grating, or a photoconductor. As a more preferred embodiment, the photosensitive element in the embodiments of the present application is a photodiode. A plurality of transfer transistors TX are arranged in pairs with the photosensitive element as the center, and the photocharge signals transmitted by the paired transfer transistors TX are charge signals integrated in the same period. The purpose of such an arrangement is that after the photosensitive element receives the light signal and converts the light signal into a photocharge signal, since the plurality of transfer transistors TX are arranged in pairs with the photosensitive element as the center, the photocharge signals can be transmitted to the signal output module in batches by timing control of the opening and closing of different transfer transistors TX.

[0045] Furthermore, the adjacent pixel units share one of the output control units. In other words, the adjacent pixel units are controlled by the signals of the output control units and transmit the photocharge signals through the output control units. It can be understood that the adjacent pixel units are not limited to a specific number and adjacent direction. Figure 1 In the embodiment, four adjacent pixel units share one of the output control units.

[0046] like Figure 2 In the illustrated embodiment, within the pixel unit of the time-of-flight image sensor pixel circuit, several transfer transistors TX are coplanarly arranged with a photosensitive element. These transfer transistors TX are spaced and symmetrically distributed around the photosensitive element, with the center line connecting the paired transfer transistors TX passing through the photosensitive element. In other words, the distances between the transfer transistors TX and the photosensitive element are equidistant, resulting in a substantially consistent timing for the photocharge signal to travel from the photosensitive element to the paired transfer transistors TX, further improving the transmission efficiency of the photocharge signal.

[0047] As a preferred embodiment, Figure 2As shown, the pixel unit further includes four transfer transistors TX. Within the same plane, the four transfer transistors TX are centered around the photosensitive element, including a second transfer transistor TX2 located at a 45° clockwise angle from the photosensitive element, a third transfer transistor TX3 located at a 135° clockwise angle from the photosensitive element, a fourth transfer transistor TX4 located at a 225° clockwise angle from the photosensitive element, and a first transfer transistor TX1 located at a 315° clockwise angle from the photosensitive element. The center connection between the second transfer transistor TX2 and the fourth transfer transistor TX4 passes through the photosensitive element, while the center connection between the third transfer transistor TX3 and the first transfer transistor TX1 passes through the photosensitive element. Furthermore, the center connection between the second transfer transistor TX2 and the fourth transfer transistor TX4 intersects with the center connection between the third transfer transistor TX3 and the first transfer transistor TX1 at the photosensitive element. This arrangement minimizes the electron transfer distance while maintaining the original pixel size, effectively increasing the full well capacity of the image sensor and thereby improving the image sensor's ability to suppress background light under strong background light.

[0048] Please continue to refer to Figure 2 ,like Figure 2 As shown, the photosensitive element includes a first photosensitive area PDN and a second photosensitive area APDN. The electron concentration on the N-type semiconductor side of the second photosensitive area APDN is higher than that on the N-type semiconductor side of the first photosensitive area PDN. The closer to the floating diffusion point FD, the greater the electron doping concentration of the photosensitive area, while the farther away from the floating diffusion point FD, the lower the doping concentration of the photosensitive area. Specifically, during operation, the closer to the floating diffusion point FD, the greater the potential of the photosensitive area, while the farther away from the floating diffusion point FD, the lower the potential of the photosensitive area. As a result, there is a potential difference between the first light sensing area PDN and the second light sensing area APDN, that is, the potential of the second light sensing area APDN is actually higher than the potential of the first light sensing area PDN. By forming a structure in which the concentration of doped electrons decreases as the distance from the floating diffusion point FD increases, the movement speed of the photoelectrons is increased, and the number of photoelectrons retained in the photodiode is reduced, thereby improving the photoelectron collection efficiency of the photodiode, reducing image delay or information loss, and accelerating the transmission speed of the photoelectrons, thereby accelerating the transmission of the photocharge signal from the first light sensing area PDN to the second light sensing area APDN.

[0049] In a preferred embodiment, the ratio of the electron concentration on the N-type semiconductor side of the second photosensitive region APDN to the electron concentration on the N-type semiconductor side of the first photosensitive region PDN is 2:1. As the distance from the floating diffusion point FD decreases, the electron concentration increases linearly from the second photosensitive region APDN to the first photosensitive region PDN, and the corresponding potential also increases accordingly. This forms an internal electric field in the photosensitive region facing away from the floating diffusion point FD. This internal electric field promotes the directional movement of photoelectrons in the photosensitive region, reduces photoelectron retention, and thus achieves a relatively ideal photoelectron transmission speed.

[0050] Please continue to refer to Figure 2 The pixel unit also includes a switching transistor arranged between the photosensitive element and the transmission transistor TX, further including a first switching transistor PGA and a second switching transistor PGB, the first switching transistor PGA and the second switching transistor PGB are spaced apart and symmetrically arranged, and the switching transistors between the two transmission transistors TX farthest from each other are the same: Figure 2 For example, the switching transistors between the second transfer transistor TX2 and the fourth transfer transistor TX4 are both second switching transistors PGB, and the switching transistors between the third transfer transistor TX3 and the first transfer transistor TX1 are both first switching transistors PGA. The purpose of this setting is that the two transfer transistors TX that are farthest apart can receive the same photocharge signal transmitted by the photosensitive element at the same time (that is, when the first switching transistor PGA is turned on), so that the image sensor can transmit the same photocharge signal through the two transfer transistors TX at the same phase at the same time. After all signals are transmitted, the photocharge signals output by the paired transfer transistors TX are merged into a total photocharge signal for image processing.

[0051] Please refer to the Figure 1 and Figure 2The pixel unit also includes a plurality of storage modules SN, which are connected to the photosensitive element and the transfer transistor TX. The storage modules SN are arranged in pairs between the photosensitive element and the transfer transistor TX, so that the storage modules SN and the transfer transistor TX are arranged around the photosensitive element together. In some embodiments, the distances between the storage modules SN and the photosensitive element are fixed, that is, the distances between different storage modules SN and the photosensitive element are equal. The storage module SN is arranged on the side of the transfer transistor TX close to the photosensitive element and is electrically connected to the transfer transistor TX and the floating diffusion point FD. The photosensitive element acquires the light signal and converts it into a photocharge signal. The storage module SN is used to receive the photocharge signal transmitted by the photosensitive element. Different transfer transistors TX are turned on at different timings, and the storage module SN is used to store the photocharge signal. After the transfer transistor TX receives the transmission control signal, it is turned on, and the photocharge signal stored in the storage module SN is transmitted to the floating diffusion point FD.

[0052] The storage module SN includes a storage transistor SG and a storage capacitor CN. The transmission transistor TX is connected in series with the switching transistor through the storage transistor SG; the storage capacitor CN is connected to the output end of the storage transistor SG. The storage transistor SG is used to transmit the photocharge signal output from the photosensitive element to the storage capacitor CN after the switching transistor is turned on, and output the photocharge signal stored in the storage capacitor CN to the floating diffusion point FD through the transmission transistor TX at different timing nodes.

[0053] In some embodiments, please refer to Figure 3 and Figure 4 , Figure 3 is another structural schematic diagram of a time-of-flight image sensor pixel circuit provided by the present application, wherein: Figure 4 yes Figure 3 The cross-sectional diagram along the PP' line in FIG. 1 further shows: the photosensitive element is Figure 3 and Figure 4Specifically, the photosensitive element PD can be any of a photodiode, a grating, or a photoconductor. Specifically, a central modulation pump gate (CMPG) is provided on one side (the light-receiving side) of the photosensitive element PD. The central modulation pump gate (CMPG) not only generates an electric field from the far end to the center of the photosensitive element PD when the switching transistor is off, but also lowers the electric field when the switching transistor is on, thereby accelerating the transfer of electrons from the center to the far end of the photosensitive element PD. Since the switching transistor is connected in series with the transfer transistor TX, when both are on, electrons from the photosensitive element PD are transferred through the transfer transistor TX to the floating diffusion FD and then output through several output control units. The central modulation pump gate (CMPG) can generate a higher pulse voltage output with a lower input voltage. Because the electric field modulation capability of the central modulation pump gate (CMPG) is higher than that of the switching transistor gate, when the central modulation pump gate (CMPG) is at the on-voltage, the pump gate generates a higher electric field strength on the photodiode, thereby increasing the charge transfer speed on the photosensitive element PD.

[0054] In some embodiments, the time-of-flight image sensor pixel circuit also includes a storage module SN connected in series between the switching transistor and the transfer transistor TX. Since the storage module SN includes a storage transistor SG and a storage capacitor CN, the transfer transistor TX is connected in series with the switching transistor through the storage transistor SG. Therefore, when the switching transistor and the transfer transistor TX are both turned on, the electrons of the photosensitive element PD can be transferred to the storage capacitor CN through the storage transistor SG for storage first, and then transferred to the floating diffusion point FD through the transfer transistor TX and then output through several output control units.

[0055] In some embodiments, the central modulated pump gate CMPG is coaxially arranged with the photosensitive element PD. In other words, the central modulated pump gate CMPG is located in the center of one side of the photosensitive element PD. Under this condition, when the central modulated pump gate CMPG is at the turn-on voltage, the electric field intensity generated by the pump gate on the photodiode is greater than when the central modulated pump gate CMPG is not located in the center of one side of the photosensitive element PD, thereby further improving the charge transfer speed on the photosensitive element PD.

[0056] Furthermore, as a preferred solution, the ratio of the projected area of ​​the central modulated pump gate CMPG on the photosensitive element PD to the surface area of ​​the photosensitive element PD on the side relative to the central modulated pump gate CMPG is 1:3. At this time, the electric field intensity generated by the pump gate on the photodiode is the largest, which can maximize the charge transfer speed on the photosensitive element PD.

[0057] Optionally, the shape of the central modulation pump gate CMPG is the same as the shape of the photosensitive element PD. When the photosensitive element PD is circular, the central modulation pump gate CMPG is also circular, and so on. It can be understood that the shape of the photosensitive element PD can be any shape, and the embodiments of the present application are not limited to this.

[0058] In the time-of-flight image sensor pixel circuit provided by this application, Figure 2 As shown, the pixel unit further includes a pixel reset transistor AB, one end of which is connected to the photosensitive element, and the other end of which can be connected to either a negative voltage or a ground signal, for clearing the charge of the photosensitive element before the photosensitive element transmits a charge signal. Specifically, a first end of the pixel reset transistor AB is connected to the ground signal, a second end of the pixel reset transistor AB is connected to the photosensitive element, and a control end of the pixel reset transistor AB is controlled by a pixel reset signal. Before the photosensitive element transmits a charge signal, the pixel reset signal controls the pixel reset transistor AB to turn on. At this time, the photosensitive element is connected to the ground signal through the pixel reset transistor AB, so that the photosensitive element clears excess charge by grounding the ground signal before reading the charge signal, thereby completing the charge clearing operation on the photosensitive element.

[0059] like Figure 1 As shown, in the pixel circuit of the time-of-flight image sensor, the output control units further include a reset module and an output module. The reset module and the output module are respectively connected to the floating diffusion points FD. The reset module is used to reset the voltage of the floating diffusion points FD, and the output module is used to output the photocharge signals transmitted by the floating diffusion points FD. Specifically:

[0060] The reset module at least includes a reset transistor RST, one end of which is connected to a high-level voltage signal VDD, the other end of which is connected to a floating diffusion point FD, and is connected to a signal output module through the floating diffusion point FD, and a control end of the reset transistor RST is connected to a reset control signal, for resetting the voltage of the floating diffusion point FD according to the reset control signal: when the reset control signal outputs a high level to the reset transistor RST, the reset transistor is controlled to be turned on, the reset transistor RST imports the high-level voltage signal VDD, and resets the potential of the floating diffusion point FD and the potential of the signal output module; when the reset control signal outputs a low level to the reset transistor RST, the reset transistor is controlled to be turned off, and the photosensitive element begins to receive and output the photocharge signal converted from the light signal after this step.

[0061] Optionally, in the time-of-flight image sensor pixel circuit provided by the present application, the reset module also includes a dual conversion gain control module (not shown), which is connected between the reset transistor RST and the floating diffusion point FD. The dual conversion gain control module includes at least one dual conversion gain control transistor and a capacitor. The dual conversion gain control module is used to switch between a low conversion gain mode and a high conversion gain mode, thereby controlling the exposure dynamic range of the time-of-flight image sensor. Through the above setting, the conversion gain can be increased with a smaller integral capacitor under low illumination conditions to improve sensitivity; under high illumination conditions, a larger integral capacitor can be used to increase the stored charge, reduce the conversion gain to improve the dynamic range, and control the size of the dynamic range to further reduce the interference of excessive background light on the pixel circuit of the time-of-flight image sensor, thereby ensuring that the signal output module can be normally turned on and the charge signal transmitted by the photosensitive element can be smoothly read.

[0062] The output module includes at least a source follower transistor SF. In some embodiments, the output module also includes a source follower transistor SF and a row selection transistor RS. The first end of the source follower transistor SF is connected to the high-level voltage signal VDD, the control end is connected to the floating diffusion point FD, and the second end can be connected to the output line bitline or connected in series with the row selection transistor RS. Specifically, the second end of the source follower transistor SF is connected to the first end of the row selection transistor RS, and the second end of the row selection transistor RS is connected to the output line bitline. The output line bitline is connected through the row selection transistor SF. The source follower transistor SF is used to amplify the photocharge signal transmitted by the output floating diffusion point FD; at the same time, the control end of the source follower transistor SF is also connected to the output end of the reset transistor RST; in some embodiments, the source follower transistor SF is connected to the row selection transistor RS, so that the photocharge signal output by the floating diffusion point FD can be introduced with the help of the source follower transistor SF and amplified and then exported to the corresponding output end through the row selection transistor SF. It is worth noting that Figure 1 This is only a typical example of the time-of-flight image sensor pixel circuit provided in this application. In fact, the signal output module may also include only a source follower transistor SF, which can amplify and directly derive the charge signal transmitted by any floating diffusion point FD through the source follower transistor SF. Similarly, other amplifier devices with different gains can be used instead. Figure 2 The row selection transistor SF shown in the figure can be, for example, a junction field effect transistor or a differential amplifier, as long as it can amplify and output the voltage signal input from the floating diffusion point FD. The embodiment of the present application does not impose any specific limitation on this.

[0063] Because a time-of-flight image sensor is an important component of a distance measurement device, it can capture the distance information between itself and an object and obtain a three-dimensional image. A time-of-flight image sensor typically uses the time-of-flight method to calculate the distance information to the object. Specifically, after the time-of-flight image sensor transmits a light wave to the object, it measures the propagation time for the light wave to be transmitted to the object and for the reflected light wave to be received by the time-of-flight image sensor, thereby calculating the distance information to the target object. Specifically, the propagation time of the light wave from the time-of-flight image sensor to the time-of-flight image sensor can be: the time-of-flight image sensor first calculates the phase difference between the phase of the light wave at the time of transmission and the phase of the light wave at the time of reception, and then calculates the propagation time of the light wave based on the phase difference.

[0064] In the time-of-flight image sensor pixel circuit provided in the embodiment of the present application, each pixel unit needs to collect four phase signals of the light wave before the propagation time of the light wave can be calculated. However, usually the photosensitive element can only collect one or two phase signals at a time and transmit them to the corresponding storage module SN. If the four phase signals of the light wave are to be collected, the photosensitive element needs to collect the phase signals at least twice and transmit them to the corresponding storage module SN. Therefore, the present application also provides a readout method for the time-of-flight image sensor pixel circuit:

[0065] Specifically, the embodiment of the present application divides the process of collecting phase signals by the pixel unit into multiple depth subframes. Within a depth subframe, the phase signals output by the paired transmission transistors TX are the same. The phase signals output by the paired transmission transistors TX are merged as a phase signal information, and the same phase signals of multiple depth subframes are synthesized to obtain all the phase signals, so that the phase difference can be calculated based on the phase of the light wave at the time of emission and the phase of the light wave when it is received, and the propagation time of the light wave is calculated to capture the distance information between itself and the object, thereby obtaining a three-dimensional image.

[0066] The purpose of synthesizing the same phase signals of multiple depth subframes to obtain all phase signals is to reduce or eliminate noise and mismatch signals (Mismatch) when the phase signals are transmitted to the floating diffusion point FD through synthesis when the paired transmission transistors TX transmit the same phase signals in multiple depth subframes.

[0067] by Figure 2Taking the time-of-flight image sensor pixel circuit shown as an example, the pixel unit includes four storage modules: a second transfer transistor TX2, a third transfer transistor TX3, a fourth transfer transistor TX4, and a first transfer transistor TX1. The two transfer transistors TX farthest from each other transmit the same phase signal. In other words, the first transfer transistor TX1 and the third transfer transistor TX3 transmit the same phase signal, and the second transfer transistor TX2 and the fourth transfer transistor TX4 transmit the same phase signal. Since the photosensitive element can only transmit one or two phase signals at a time and transmit them to the corresponding transfer transistor TX, the embodiment of the present application decomposes the process of the pixel unit transmitting the phase signal into four depth sub-frames. Figure 5-8 Schematic diagram of the phase signal distribution in four different depth subframes, where any black dotted box is used to represent a single pixel unit, such as Figure 5-8 As shown, the embodiment of the present application takes the upper left pixel unit as an example, and the specific steps of the readout method are as follows:

[0068] In the first depth subframe, as Figure 5 As shown, the first transfer transistor TX1 transmits the phase signal value of the first phase A, and the second transfer transistor TX2 transmits the phase signal value of the second phase B. As mentioned above, the switching transistor between the first transfer transistor TX1 and the third transfer transistor TX3 is the first switching transistor PGA. Therefore, within the same depth subframe, the phase signal values ​​transmitted by the first transfer transistor TX1 and the third transfer transistor TX3 should be the same. The switching transistor between the second transfer transistor TX2 and the fourth transfer transistor TX4 is the second switching transistor PGB. Therefore, within the same depth subframe, the phase signal values ​​transmitted by the second transfer transistor TX2 and the fourth transfer transistor TX4 are also the same. Therefore, the third transfer transistor TX3 transmits the phase signal value of the first phase A, and the fourth transfer transistor TX4 transmits the phase signal value of the second phase B.

[0069] In the second depth subframe, as Figure 6As shown, the first transfer transistor TX1 transmits the phase signal value of the fourth phase D, and the second transfer transistor TX2 transmits the phase signal value of the third phase C. As previously mentioned, the switching transistor between the first transfer transistor TX1 and the third transfer transistor TX3 is the first switching transistor PGA. Therefore, within the same depth subframe, the phase signal values ​​transmitted by the first transfer transistor TX1 and the third transfer transistor TX3 should be the same. The switching transistor between the second transfer transistor TX2 and the fourth transfer transistor TX4 is the second switching transistor PGB. Therefore, within the same depth subframe, the phase signal values ​​transmitted by the second transfer transistor TX2 and the fourth transfer transistor TX4 are also the same. Therefore, the third transfer transistor TX3 transmits the phase signal value of the fourth phase D, and the fourth transfer transistor TX4 transmits the phase signal value of the third phase C.

[0070] In the third depth subframe, as Figure 7 As shown, the first transfer transistor TX1 transmits the phase signal value of the second phase B, and the second transfer transistor TX2 transmits the phase signal value of the first phase A. As mentioned above, the switching transistor between the first transfer transistor TX1 and the third transfer transistor TX3 is the first switching transistor PGA. Therefore, within the same depth subframe, the phase signal values ​​transmitted by the first transfer transistor TX1 and the third transfer transistor TX3 should be the same. The switching transistor between the second transfer transistor TX2 and the fourth transfer transistor TX4 is the second switching transistor PGB. Therefore, within the same depth subframe, the phase signal values ​​transmitted by the second transfer transistor TX2 and the fourth transfer transistor TX4 are also the same. Therefore, the third transfer transistor TX3 transmits the phase signal value of the second phase B, and the fourth transfer transistor TX4 transmits the phase signal value of the first phase A.

[0071] In the fourth depth subframe, as Figure 8 As shown, the first transfer transistor TX1 transmits the phase signal value of the third phase C, and the second transfer transistor TX2 transmits the phase signal value of the fourth phase D. As previously mentioned, the switching transistor between the first transfer transistor TX1 and the third transfer transistor TX3 is the first switching transistor PGA. Therefore, within the same depth subframe, the phase signal values ​​transmitted by the first transfer transistor TX1 and the third transfer transistor TX3 should be the same. The switching transistor between the second transfer transistor TX2 and the fourth transfer transistor TX4 is the second switching transistor PGB. Therefore, within the same depth subframe, the phase signal values ​​transmitted by the second transfer transistor TX2 and the fourth transfer transistor TX4 are also the same. Therefore, the third transfer transistor TX3 transmits the phase signal value of the third phase C, and the fourth transfer transistor TX4 transmits the phase signal value of the fourth phase D.

[0072] according to Figure 5-8As shown, as a preferred embodiment, the phase angle of the first phase A is 0 degrees, the phase angle of the second phase B is 180 degrees, the phase angle of the third phase C is 90 degrees, and the phase angle of the fourth phase D is 180 degrees.

[0073] The phase signals of the four depth subframes collected by the pixel unit are synthesized to obtain the same phase information. The phase difference is calculated based on the phase of the light wave when it is emitted and the phase when the light wave is received. The propagation time of the light wave is calculated to capture the distance information between itself and the object, so that a three-dimensional image can be obtained.

[0074] The present application also provides an image sensor, comprising the time-of-flight image sensor pixel circuit described in the above embodiment.

[0075] To summarize, the time-of-flight image sensor pixel circuit provided by the present application arranges a number of transfer transistors in pairs with a photosensitive element as the center. The photocharge signals transmitted by the paired transfer transistors are charge signals integrated in the same period. The photocharge signals output by the paired transfer transistors are output by the corresponding output control units and then merged into a total photocharge signal for image processing. At the same time, it is supplemented by the readout method of the time-of-flight image sensor pixel circuit. By dividing the phase signal into multiple depth subframes, the transmission distance of the photocharge signal can be shortened to the greatest extent while maintaining the original pixel size. The depth measurement accuracy can be increased without sacrificing the operating frequency, and the signal-to-noise ratio of the image sensor can be improved.

[0076] The embodiments described above are only some of the embodiments of the present application, rather than all of the embodiments. The components of the embodiments of the present application generally described and shown in the accompanying drawings can be arranged and designed in various different configurations. Therefore, the detailed description of the embodiments of the present application provided in the accompanying drawings is not intended to limit the scope of protection of the present application, but merely represents selected embodiments of the present application. Based on this, the scope of protection of the present application shall be based on the scope of protection of the claims. In addition, based on the embodiments of the present application, all other embodiments that can be obtained by those skilled in the art without making creative work should fall within the scope of protection of the present application.

Claims

1. A time-of-flight image sensor pixel circuit, comprising a pixel unit and a plurality of output control units, characterized in that: The pixel unit includes a photosensitive element and a plurality of transmission transistors, wherein control ends of the plurality of transmission transistors respectively receive transmission control signals, input ends of the plurality of transmission transistors are respectively connected to the photosensitive element, and output ends of the plurality of transmission transistors are respectively connected to the plurality of output control units; The plurality of transmission transistors are arranged in pairs with the photosensitive element as the center, and the photocharge signals transmitted by the transmission transistors arranged in pairs are photocharge signals integrated in the same period; A plurality of floating diffusion points are further included between the plurality of output control units and the plurality of transfer transistors. The plurality of output control units and the plurality of transfer transistors are respectively connected to the plurality of floating diffusion points. The plurality of transfer transistors are used to control the transmission of the photocharge signals output by the photosensitive elements to the plurality of floating diffusion points. The plurality of output control units are used to respectively receive the photocharge signals output by the plurality of floating diffusion points for image processing. The photocharge signals output by the transfer transistors arranged in pairs are output by the corresponding output control units and then merged into a total photocharge signal for image processing.

2. The time-of-flight image sensor pixel circuit according to claim 1, wherein: The adjacent pixel units share one of the output control units.

3. The time-of-flight image sensor pixel circuit according to claim 1 , wherein: The plurality of transmission transistors and the photosensitive element are arranged on the same plane; The plurality of transmission transistors are spaced apart from each other and symmetrically arranged with the photosensitive element as the center, and a center line between the transmission transistors arranged in pairs passes through the photosensitive element.

4. The time-of-flight image sensor pixel circuit according to claim 3, wherein: The pixel unit includes four transfer transistors, which are centered on the photosensitive element and include: a first transfer transistor located at an angle of 315° clockwise to the photosensitive element, a second transfer transistor located at an angle of 45° clockwise to the photosensitive element, a third transfer transistor located at an angle of 135° clockwise to the photosensitive element, and a fourth transfer transistor located at an angle of 225° clockwise to the photosensitive element; The center connection of the second transfer transistor and the fourth transfer transistor passes through the photosensitive element, the center connection of the third transfer transistor and the first transfer transistor passes through the photosensitive element, and the center connection of the second transfer transistor and the fourth transfer transistor intersects the center connection of the third transfer transistor and the first transfer transistor at the photosensitive element.

5. The time-of-flight image sensor pixel circuit according to claim 1, wherein: The photosensitive element includes a first photosensitive area and a second photosensitive area, and the electron concentration on the N-type semiconductor side in the second photosensitive area is higher than the electron concentration on the N-type semiconductor side in the first photosensitive area.

6. The time-of-flight image sensor pixel circuit according to claim 5, wherein: The ratio of the electron concentration on the N-type semiconductor side in the second light sensing region to the electron concentration on the N-type semiconductor side in the first light sensing region is 2:

1.

7. The time-of-flight image sensor pixel circuit according to claim 1, wherein: The pixel unit also includes a switching transistor arranged between the photosensitive element and the several transfer transistors, the switching transistor including a first switching transistor and a second switching transistor, the first switching transistor and the second switching transistor are spaced apart from each other and symmetrically arranged, and the switching transistors between the paired transfer transistors are the same.

8. The time-of-flight image sensor pixel circuit according to claim 1, wherein: The pixel unit further includes a plurality of storage modules, which are connected to the photosensitive element and the transfer transistor respectively, and the storage modules are arranged between the photosensitive element and the transfer transistor.

9. The time-of-flight image sensor pixel circuit according to claim 8, wherein: The storage module includes a storage transistor and a storage capacitor. The transfer transistor is connected in series with a switching transistor through the storage transistor, and the storage capacitor is connected to the output end of the storage transistor. The storage transistor is used to transfer the photocharge signal output by the photosensitive element to the storage capacitor after the switching transistor is turned on.

10. The time-of-flight image sensor pixel circuit according to claim 1, wherein: A central modulation pump gate is provided on one side of the photosensitive element, and the central modulation pump gate is used to generate an electric field from the far end of the photosensitive element to the center of the photosensitive element, and accelerate the transfer of electrons from the center of the photosensitive element to the far end of the photosensitive element. 11 . The time-of-flight image sensor pixel circuit according to claim 10 , wherein the central modulation pump gate is coaxially arranged with the photosensitive element.

12. The time-of-flight image sensor pixel circuit according to claim 11, wherein the ratio of the projected area of ​​the central modulation pump gate on the photosensitive element to the surface area of ​​the photosensitive element on a side facing the central modulation pump gate is 1:

3. 13 . The time-of-flight image sensor pixel circuit according to claim 12 , wherein a shape of the central modulation pump gate is the same as a shape of the photosensitive element.

14. The time-of-flight image sensor pixel circuit according to claim 1, wherein: The pixel unit further includes a pixel reset transistor, one end of which is connected to the photosensitive element and is used to clear the charge of the photosensitive element before the photosensitive element transmits a charge signal.

15. The time-of-flight image sensor pixel circuit according to claim 1, wherein: The several output control units include a reset module and an output module, which are respectively connected to the several floating diffusion points. The reset module is used to reset the voltage of the several floating diffusion points, and the output module is used to output the photocharge signals transmitted by the several floating diffusion points.

16. The time-of-flight image sensor pixel circuit according to claim 15, wherein: The reset module includes a reset transistor, one end of which is connected to a high-level voltage signal, the other end of which is connected to the floating diffusion point and connected to the output module through the floating diffusion point, the control end of the reset transistor is connected to a reset control signal, and the reset transistor is used to reset the voltages of the plurality of floating diffusion points according to the reset control signal.

17. The time-of-flight image sensor pixel circuit according to claim 15, wherein: The reset module includes a dual conversion gain control module connected between the reset transistor and the plurality of floating diffusion points. The dual conversion gain control module includes a dual conversion gain control transistor and a capacitor. The dual conversion gain control module is used to achieve switching between a low conversion gain mode and a high conversion gain mode.

18. The time-of-flight image sensor pixel circuit according to claim 15, wherein: The output module includes a source follower transistor, the first end of the source follower transistor is connected to a high-level voltage signal, the control end of the source follower transistor is connected to a floating diffusion point, the second end of the source follower transistor is connected to an output line, and the source follower transistor is used to amplify and output the photocharge signals transmitted by the several floating diffusion points.

19. The time-of-flight image sensor pixel circuit according to claim 15, wherein: The output module further includes a row selection transistor. The second end of the source follower transistor is connected to the first end of the row selection transistor, and the second end of the row selection transistor is connected to the output line.

20. A readout method for a time-of-flight image sensor pixel circuit, used for reading out a photocharge signal from the time-of-flight image sensor pixel circuit according to any one of claims 1 to 19, characterized in that: The process of collecting phase signals by the pixel unit is divided into multiple depth subframes. Within a depth subframe, the phase signals output by the paired transmission transistors are the same. The phase signals output by the paired transmission transistors are combined as a phase signal information. The same phase signals of multiple depth subframes are synthesized to obtain the entire phase information. The phase difference is calculated based on the phase of the light wave at the time of emission and the phase of the light wave at the time of reception, thereby calculating the propagation time of the light wave, capturing the distance information between itself and the object, and obtaining a three-dimensional image.

21. The method for reading out a pixel circuit of a time-of-flight image sensor according to claim 20, wherein: The process of collecting the phase signal by the pixel unit is divided into four depth subframes, and the pixel unit includes a first transmission transistor, a second transmission transistor, a third transmission transistor and a fourth transmission transistor, wherein: In a first depth subframe, the first transmission transistor transmits a phase signal value of a first phase, the second transmission transistor transmits a phase signal value of a second phase, the third transmission transistor transmits the phase signal value of the first phase, and the fourth transmission transistor transmits the phase signal value of the second phase; In a second depth subframe, the first transmission transistor transmits a phase signal value of a fourth phase, the second transmission transistor transmits a phase signal value of a third phase, the third transmission transistor transmits the phase signal value of the fourth phase, and the fourth transmission transistor transmits the phase signal value of the third phase; In a third depth subframe, the first transmission transistor transmits the phase signal value of the second phase, the second transmission transistor transmits the phase signal value of the first phase, the third transmission transistor transmits the phase signal value of the second phase, and the fourth transmission transistor transmits the phase signal value of the first phase; In a fourth depth subframe, the first transmission transistor transmits the phase signal value of the third phase, the second transmission transistor transmits the phase signal value of the fourth phase, the third transmission transistor transmits the phase signal value of the third phase, and the fourth transmission transistor transmits the phase signal value of the fourth phase.

22. An image sensor, characterized in that: The method comprises a plurality of time-of-flight image sensor pixel circuits according to any one of claims 1 to 19.

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