CN113542634A

CN113542634A - High dynamic range image sensor pixel circuit and working method thereof

Info

Publication number: CN113542634A
Application number: CN202110949016.7A
Authority: CN
Inventors: 郭同辉
Original assignee: SmartSens Technology Shanghai Co Ltd
Current assignee: SmartSens Technology Shanghai Co Ltd
Priority date: 2021-08-18
Filing date: 2021-08-18
Publication date: 2021-10-22

Abstract

A high dynamic range image sensor pixel circuit and its working method, in the field of sensor, receive the optical signal in order to produce the electric charge through the photoelectric conversion component; the charge storage and transfer circuit stores the charges according to the charge output signal to generate an integral charge signal and outputs the integral charge signal according to the control selection signal; the charge output signal is a modulation signal comprising n periods, and the corresponding time lengths of the periods are different from the 1 st period to the nth period; the releasing circuit releases part of charges generated by the photoelectric conversion element according to the weak closing signal at least in the process that the charge storage and transfer circuit stores the charges to generate the integrated charge signal; the charge readout circuit outputs an electric signal according to the integrated charge signal to obtain an image signal; the photosensitive dynamic range of the pixel circuit of the high dynamic range image sensor is increased, and the flexibility of photosensitive dynamic range adjustment is increased.

Description

High dynamic range image sensor pixel circuit and working method thereof

Technical Field

The application belongs to the field of sensors, and particularly relates to a high dynamic range image sensor pixel circuit and a working method thereof.

Background

Image sensors have been widely used in digital cameras, mobile phones, medical devices, automobiles, and other applications. In particular, the rapid development of the technology for manufacturing a Complementary Metal Oxide Semiconductor (CMOS) image sensor has made people have higher requirements for the quality of an output image of the image sensor.

The pixel circuit of the image sensor generally adopts the working principle that a photodiode is adopted to convert a received optical signal into a photoelectric charge signal, the maximum amount of photoelectric charge which can be collected by the photodiode in the exposure process is called charge saturation capacity, and the higher the charge saturation capacity is, the higher the dynamic range of object information collected by the pixel is. In the pixel circuit of the image sensor in the related art, the photo response of the photodiode is generally linear, and is called a linear image sensor; the photodiode reaches a saturation state quickly along with the increase of the exposure, image information of a strong light environment is difficult to collect, and the dynamic range of the collected image information is generally lower than 80 dB. In nature, human eyes are sensitive to weak light, namely the sensitivity is high when the weak light is perceived; but is not sensitive to strong light, namely the sensitivity is low when strong light is sensed. The human eyes can perceive the light rays in a logarithmic curve relationship, the relationship effectively improves the capability of the eyes to perceive the light rays, and the dynamic range of the relationship can reach more than 100 dB. It follows that the above-described linear image sensor is clearly less capable of capturing images than the human eye.

Disclosure of Invention

The present application aims to provide a high dynamic range image sensor pixel circuit and a working method thereof, and aims to solve the defect of a conventional high dynamic range image sensor pixel circuit that the photosensitive dynamic range is low.

The embodiment of the application provides a high dynamic range image sensor pixel circuit, including:

a photoelectric conversion element configured to receive a light signal to generate electric charges;

a charge storage transfer circuit connected to the photoelectric conversion element, configured to store the charge to generate an integrated charge signal, and output the integrated charge signal according to a control selection signal;

the charge storage and transfer circuit stores the charges according to a charge output signal to generate an integrated charge signal, wherein the charge output signal is a modulation signal comprising n periods, and the time lengths corresponding to the periods are different from the 1 st period to the nth period;

a release circuit connected to the photoelectric conversion element and the input terminal of the charge storage transfer circuit, configured to release a part of the charge generated by the photoelectric conversion element according to a weak off signal at least during the storage of the charge by the charge storage transfer circuit to generate the integrated charge signal;

and the charge readout circuit is connected with the output end of the charge storage transfer circuit and is configured to output an electric signal according to the integrated charge signal so as to obtain an image signal.

The embodiment of the present application further provides a working method of the pixel circuit of the high dynamic range image sensor, including:

inputting a charge output signal, the charge storage conversion circuit storing charge according to the charge output signal to generate an integrated charge signal;

inputting a weak off signal during inputting the charge output signal to cause a release circuit to release a part of the charge generated by the photoelectric conversion element in accordance with the weak off signal at least during storing of the charge by the charge storage transfer circuit to generate the integrated charge signal; the charge output signal is a modulation signal comprising n periods, and the corresponding time lengths of the periods are different from the 1 st period to the nth period;

and inputting a control selection signal to enable the charge storage transfer circuit to output the integrated charge signal according to the control selection signal so as to obtain an image signal.

Compared with the prior art, the embodiment of the invention has the following beneficial effects: a discharging circuit discharges a part of charges generated by the photoelectric conversion element according to a weak off signal at least in a process that the charge storage and transfer circuit stores the charges to generate an integrated charge signal; the charge in the photoelectric conversion element irradiated by strong light is released partially through the release circuit, the charge output signal is a modulation signal comprising n periods, the corresponding time duration of each period is different from the 1 st period to the nth period, the photosensitive capacity of the pixel irradiated by the strong light is suppressed, the photosensitive sensitivity is reduced, the photosensitive capacity and the sensitivity of the pixel irradiated by weak light are kept unchanged, the photosensitive dynamic range of the pixel circuit of the high dynamic range image sensor is increased, and the flexibility of adjusting the photosensitive dynamic range of the pixel circuit of the high dynamic range image sensor is increased.

Drawings

In order to more clearly illustrate the technical invention in the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and other drawings can be obtained by those skilled in the art without inventive efforts.

Fig. 1 is a schematic structural diagram of a pixel circuit of a high dynamic range image sensor according to an embodiment of the present disclosure;

FIG. 2 is a schematic diagram of an exemplary circuit for a pixel circuit of a high dynamic range image sensor according to an embodiment of the present disclosure;

FIG. 3 is a timing diagram of key signals corresponding to the pixel circuit of the high dynamic range image sensor of FIG. 2;

FIG. 4 is a schematic diagram of a potential well of a pixel circuit of a high dynamic range image sensor operating under strong light according to an embodiment of the present application

FIG. 5 is a schematic diagram of a potential well of a pixel circuit of a high dynamic range image sensor operating under low light illumination according to an embodiment of the present application

Fig. 6 is a diagram illustrating a photo-response curve of a pixel circuit of a high dynamic range image sensor according to the related art and an embodiment of the present application.

Detailed Description

In order to make the technical problems, technical solutions and advantageous effects to be solved by the present application clearer, the present application is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present application and are not intended to limit the present application.

It will be understood that when an element is referred to as being "secured to" or "disposed on" another element, it can be directly on the other element or be indirectly on the other element. When an element is referred to as being "connected to" another element, it can be directly connected to the other element or be indirectly connected to the other element.

It will be understood that the terms "length," "width," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," and the like, refer to an orientation or positional relationship illustrated in the drawings for convenience in describing the present application and to simplify description, and do not indicate or imply that the referenced device or element must have a particular orientation, be constructed and operated in a particular orientation, and thus should not be construed as limiting the present application.

Furthermore, the terms "first", "second" and "first" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of the present application, "a plurality" means two or more unless specifically limited otherwise.

In a related art image sensor pixel circuit, in order to improve the dynamic range of the collected image information, a method of high-dose implantation of impurity ions is generally adopted in a photodiode, so as to increase the charge saturation capacity of the photodiode, but the defects are that the full depletion potential of the photodiode is increased, image smear is easily generated, and dark current is also increased. Another related technique for improving the dynamic range of the collected image information is to set two modes of high conversion gain and low conversion gain in the pixel to respectively deal with the image collection in the low-light environment and the high-light environment, although the two modes can be switched back and forth according to the illumination environment, only one of the two modes can be used at the same time in the working process of the pixel, so that the detail information of the low-light and high-light illumination images cannot be simultaneously obtained.

Fig. 1 shows a schematic structural diagram of a pixel circuit of a high dynamic range image sensor according to a preferred embodiment of the present application, and for convenience of description, only the parts related to the embodiment are shown, and detailed descriptions are as follows:

the high dynamic range image sensor pixel circuit described above includes a photoelectric conversion element 11, a charge storage transfer circuit 12, a discharge circuit 13, and a charge readout circuit 14.

The photoelectric conversion element 11 is configured to receive a light signal to generate electric charges.

A charge storage transfer circuit 12 connected to the photoelectric conversion element 11, configured to store the charge to generate an integrated charge signal, and output the integrated charge signal according to a control selection signal; the charge storage and transfer circuit 12 stores the charge according to a charge output signal to generate an integrated charge signal, where the charge output signal is a modulation signal including n cycles, and the time duration corresponding to each cycle is different from the 1 st cycle to the nth cycle.

And the release circuit 13 is connected with the photoelectric conversion element 11 and the input end of the charge storage and transfer circuit 12 and is configured to release part of charges generated by the photoelectric conversion element according to a weak closing signal at least in the process that the charge storage and transfer circuit 12 stores the charges to generate an integrated charge signal.

And the charge readout circuit 14 is connected with the output end of the charge storage transfer circuit 12 and is configured to output an electric signal according to the integrated charge signal so as to obtain an image signal.

In an example, from the 1 st cycle to the nth cycle, the integration duration corresponding to each cycle gradually decreases or the integration duration corresponding to each cycle gradually increases; in this embodiment, the integration duration corresponding to each period is selected to be gradually reduced. Each period comprises a charge output signal turn-off stage and a charge output signal turn-on stage; the integration duration is the duration of the charge output signal turn-off phase of the cycle, and in this embodiment, each cycle includes a charge output signal turn-off phase and a charge output signal turn-on phase in sequence.

The integral duration corresponding to each period is gradually increased, so that the pixel light sensitivity of strong light irradiation is further suppressed, the light sensitivity is further reduced, the pixel light sensitivity and the sensitivity of weak light irradiation are enhanced, the high dynamic range image sensor pixel circuit is more in line with the logarithmic curve relationship of the perception characteristics of human eyes to light, the light perception capability is effectively improved, and the light perception dynamic range of the high dynamic range image sensor pixel circuit is further increased.

The integral duration corresponding to each period is gradually reduced, so that the light sensitivity of the pixel irradiated by weak light is further suppressed, the light sensitivity is further reduced, the light sensitivity and the sensitivity of the pixel irradiated by strong light are enhanced, and the pixel circuit of the high dynamic range image sensor is suitable for scenes sensed by strong light.

In a specific implementation, the proportion of the integration time duration of adjacent cycles is between 1/2 and 9/10, for example, the integration time duration of the next cycle is set to 4/5 of the previous integration time duration. In another example, the integration time lengths of the periods may be distributed in an arithmetic progression.

In one example, the charge output signal is a pulse modulated signal comprising n pulse periods, the charge output signal having a total duration T, which is a positive number.

By way of example and not limitation, the pulse duration of the pulse modulation signal is a, a is a positive number and is constant.

By setting the charge output signal to a pulse modulation signal comprising n pulse periods, the pixel photosensibility can be adjusted, further increasing the photosensing dynamic range of the high dynamic range image sensor pixel circuit.

Here, the output end of the photoelectric conversion element 11 may be a cathode of the photoelectric conversion element 11, for example, the photoelectric conversion element 11 is selected to be a photodiode. Accordingly, the photoelectric conversion element 11 receives the optical signal and generates a negative charge, thereby improving the mobility of the charge.

In a specific implementation, the charge readout circuit 14 is further configured to clear the charge in the photoelectric conversion element 11 according to the clear signal.

The charges in the photoelectric conversion elements 11 are cleared by the charge readout circuit 14 in accordance with the clear signal, so that the charges in the photoelectric conversion elements 11 are cleared each time an image signal is acquired, improving the acquisition accuracy of the image signal.

The discharge circuit 13 is also configured to clear the electric charges in the photoelectric conversion element 11 in accordance with the anti-electric-charge-crosstalk control signal when the electric-charge readout circuit 14 clears the electric charges in the photoelectric conversion element 11; and the control voltage corresponding to the anti-charge crosstalk control signal is equal to the control voltage corresponding to the weak turn-off signal when part of charges generated by the photoelectric conversion element are released.

The electric charges in the photoelectric conversion elements 11 are cleared by the discharge circuits 13, and the electric charges in the photoelectric conversion elements 11 are further cleared, improving the acquisition accuracy of the image signals. The control voltage corresponding to the anti-electric-charge crosstalk control signal is equal to the control voltage corresponding to when a part of the electric charges generated by the photoelectric conversion element is released according to the weak off signal, so that ground interference caused by potential variation of the release circuit 13 is prevented.

The embodiment of the invention also provides a working method of the pixel circuit of the high dynamic range image sensor, which comprises the steps 101a and 101b and the step 102.

Step 101 a: in the exposure process, inputting a charge output signal, and storing the charge by a charge storage conversion circuit according to the charge output signal to generate an integral charge signal;

step 101 b: inputting a weak off signal in inputting a charge output signal to cause the discharging circuit 13 to discharge a part of the charge generated by the photoelectric conversion element in accordance with the weak off signal at least in the process of storing the charge by the charge storage transfer circuit to generate an integrated charge signal;

the charge output signal is a modulation signal comprising n periods, and the corresponding time duration of each period is different from the 1 st period to the nth period.

By way of example and not limitation, step 101a and step 101b may be preceded by step 100 a.

Step 100 a: inputting a clear signal before starting exposure, the charge readout circuit 14 clearing the charges in the photoelectric conversion element 11 in accordance with the clear signal; the exposure is started.

In performing step 100a, the method of operating a high dynamic range image sensor pixel circuit further includes step 100 b.

Step 100 b: inputting an anti-charge crosstalk control signal, and clearing the charges in the photoelectric conversion element 11 by the release circuit 13 according to the anti-charge crosstalk control signal;

and the control voltage corresponding to the anti-charge crosstalk control signal is equal to the control voltage corresponding to the weak turn-off signal when part of charges generated by the photoelectric conversion element are released.

Step 102: the control selection signal is input to cause the charge storage transfer circuit 12 to output the integrated charge signal in accordance with the control selection signal to obtain an image signal.

In an example, from the 1 st cycle to the nth cycle, the integration duration corresponding to each cycle gradually decreases or the integration duration corresponding to each cycle gradually increases;

each period sequentially comprises a charge output signal turn-off stage and a charge output signal turn-on stage; the integration duration is the duration of the charge output signal off phase of the cycle.

In a specific implementation, the proportion of the integration time duration of adjacent periods is between 1/2 and 9/10; alternatively, the integration time lengths of the periods are distributed in an arithmetic progression.

In one example, the charge output signal is a pulse modulated signal comprising n pulse periods, the total duration of the charge output signal being T, T being a positive number.

In a further example, the pulse duration of the pulse modulated signal is a; a is a positive number and is a constant.

In particular implementations, step 102 may include steps a1 through a 2.

Step A1: inputting a control selection signal, and outputting an integrated charge signal by the charge storage transfer circuit 12 according to the control selection signal;

step A2: the charge readout circuit 14 outputs an electrical signal from the integrated charge signal.

In particular implementations, step a1 may be preceded by step a 0.

Step A0: a reset control signal is input, and the charge readout circuit 14 outputs a reset signal in accordance with the reset control signal.

In a specific implementation, in the step 102, the method for operating the pixel circuit of the high dynamic range image sensor further includes the step 102 b:

step 102 b: stopping exposure, inputting a charge release signal, and outputting and clearing the charges in the photoelectric conversion element 11 by the release circuit 13 according to the charge release signal; wherein the input charge-release signal is maintained during the outputting of the integrated charge signal.

By removing the electric charges in the photoelectric conversion element 11 before exposure and after stopping exposure, the acquisition accuracy of the image signal is improved.

In a specific implementation, the reset control signal, the anti-charge crosstalk control signal, the charge output signal, the control selection signal, and the charge release signal may be output by the control logic.

Fig. 2 shows an exemplary circuit structure of a pixel circuit of a high dynamic range image sensor provided by an embodiment of the present invention, and for convenience of description, only the parts related to the embodiment of the present invention are shown, and detailed as follows:

the charge readout circuit 14 includes a reset transistor 104, a source follower transistor 105, and a pixel selection transistor 106.

The source of the reset transistor 104 and the gate of the source follower transistor 105 are commonly connected to the input terminal of the charge readout circuit 14, and the input terminal of the charge readout circuit 14 serves as an integrated charge signal input terminal;

the gate of the reset transistor 104 is connected to a reset signal control line to serve as a reset control signal input terminal of the charge readout circuit 14; the drain of the reset transistor 104 is connected to a first power supply VDD, and the drain of the source follower transistor 105 is connected to a second power supply, where the first power supply VDD and the second power supply are the same or different power supplies; the source of the source follower transistor 105 is connected to the drain of the pixel selection transistor 106, the source of the pixel selection transistor 106 is connected to the output terminal of the charge readout circuit 14, and the output terminal of the charge readout circuit 14 serves as an output terminal of an electric signal.

The charge storage transfer circuit 12 includes a charge transfer transistor 102, a charge storage device 108, and a charge selection transistor 103.

The source of the charge transfer transistor 102 is connected to the charge input terminal of the charge storage transfer circuit 12, the charge input terminal is connected to the output terminal of the photoelectric conversion element, the drain of the charge transfer transistor 102 is connected to the first terminal of the charge storage device 108 and the source of the charge selection transistor 103, the drain of the charge selection transistor 103 is connected to the integrated charge signal output terminal of the charge storage transfer circuit 12; the gate of the charge transfer transistor 102 is connected to a charge input signal control line as a charge output signal input terminal, and the gate of the charge selection transistor 103 is connected to a control selection signal control line as a control selection signal input terminal. The second terminal of the charge storage device 108 is coupled to a first voltage, where the first voltage may be represented as the second terminal of the charge storage device 108 being coupled to ground or a specified voltage.

The discharging circuit 13 includes a charge discharging transistor 107. In one example, the charge discharging transistor 107 is input with a weak off signal through a control line during an exposure phase, input with an anti-crosstalk control signal before exposure, and output with a charge discharging signal during a readout phase. The control voltage corresponding to the anti-crosstalk control signal is equal to the control voltage corresponding to the weak turn-off signal and is smaller than the control voltage corresponding to the charge release signal.

The photoelectric conversion element 11 includes, but is not limited to, a buried Photodiode (Pinned Photodiode), a polysilicon gate Photodiode (photo diode), a Current assisted Photodiode (Current Assistance Photodiode); charge storage devices include, but are not limited to, MOS transistor type capacitors, polysilicon gate-insulator-polysilicon gate type capacitors, metal-insulator-metal type capacitors.

The description of fig. 2 is further described below in conjunction with the working principle:

the one-frame operation process for collecting photoelectric signals by the pixel circuit of the high dynamic range image sensor comprises three stages: the pixel reset phase, the photoelectric charge signal collection phase and the photoelectric signal output phase.

The pixel reset phase is the first phase in the operation of collecting photoelectric signals of a high dynamic range image sensor pixel circuit for one frame, specifically, the reset transistor 104, the charge transfer transistor 102 and the charge selection transistor 103 are turned on, and the charge release transistor 107 is set to be in a weak off state (inputting an anti-crosstalk control signal), so as to clear the charges in the photodiode 101 and the charge storage device 108; after the charges in the photodiode 101 and the charge storage device 108 are cleared, the reset transistor 104, the charge transfer transistor 102, and the charge selection transistor 103 are turned off, and the second stage photo charge signal collection stage is performed.

In the second stage photoelectric charge signal collection stage, the photodiode 101 receives and converts the optical signal into a photoelectric charge signal, and collects it in the photodiode 101. A process of collecting charges by the photodiode 101, which is called an exposure period, in which the charge transfer transistor 102 has a plurality of on and off actions, a period from when the charge transfer transistor 102 is turned off to when the charge transfer transistor 102 is turned on and turned off next time is called a micro-exposure period (period T), the micro-exposure period in the exposure period is gradually shortened, N micro-exposure periods are counted, N is a natural number greater than 2, and the sum of the N micro-exposure periods is an exposure period (total period T); each turn-on and turn-off of the charge transfer transistor 102 transfers the photo-charge in the photodiode 101 to the charge storage device 108 in what is referred to as a modulation of the charge by the charge transfer transistor 102. After the photodiode 101 is exposed, the third stage of outputting the photoelectric signal is performed.

A third phase of photoelectric signal output, specifically, turning on the pixel selection transistor 106, and the reset transistor 104 gives an on operation to clear the charges in the parasitic capacitance of the floating diffusion active region FD, and then the high dynamic range image sensor pixel circuit outputs a reset signal; then, the charge selection transistor 103 gives an on operation, transfers the charges in the charge storage device 108 into the floating diffusion active region FD, and then the high dynamic range image sensor pixel circuit outputs an optoelectronic signal. And after the photoelectric signal is output, finishing one frame operation of collecting the photoelectric signal. The collected image signal is equal to the difference between the reset signal and the photoelectric signal.

In the second stage of the photoelectric charge signal collection stage, the exposure period is divided into a plurality of micro exposure steps, and the charge saturation capacity of the photodiode 101 is lower than that of the photodiode in the related art, so that the fully depleted potential of the photodiode 101 is lower, the image smear problem is not easy to generate, and the dark current is also reduced. The charge transfer transistor 102 has a function of modulating charges, in which charges in the photodiode 101 irradiated with strong light are partially discharged to the positive electrode of the power supply through the charge discharging transistor 107, and charges in the photodiode 101 irradiated with weak light are collected as a whole. It can be seen that the photosensitivity of the pixel irradiated by strong light is suppressed, the photosensitivity is reduced, while the photosensitivity and sensitivity of the pixel irradiated by weak light remain unchanged, so that the photoelectric response of the photodiode 101 is in a nonlinear mode, and such a sensor is called a nonlinear image sensor. Therefore, the signal acquisition of the strong light illumination intensity range can be effectively expanded, the pixel photosensitive dynamic range is improved compared with the related technology, and the risks of image smear and dark current are reduced.

Fig. 3 shows a schematic diagram of a specific timing operation method for acquiring a photoelectric signal by a pixel circuit of a high dynamic range image sensor, where fig. 3 is a timing diagram corresponding to the pixel circuit of the high dynamic range image sensor shown in fig. 2. It is understood that the timing diagram is an example, and those skilled in the art can make corresponding adjustments according to common knowledge in the art to obtain other example timing diagrams.

In fig. 3, a frame of operation flow for acquiring the photo-electric signal by the pixel circuit of the high dynamic range image sensor includes three stages: a pixel reset phase 201, a photo charge signal collection phase 202 and a photo signal output phase 203. The transistor has a high potential at its gate terminal indicating that the transistor is in an on state and a low potential at its gate terminal indicating that the transistor is in an off state; the read timing is the operation timing of reading signals in the pixel circuit in the photoelectric signal processing system module, and the high level represents the signals output by the pixel circuit.

The pixel reset phase 201 is described as follows:

the pixel reset stage 201 is the first stage in the operation of collecting a photoelectric signal by a pixel circuit of the high dynamic range image sensor in one frame, and specifically, the operation is to set the gate Anti of the charge release transistor 107 to be a secondary low potential so that the charge release transistor 107 is in a weak off state; the gate SG of the charge storage device 108 and the gate RS of the pixel selection transistor 106 are held at low potentials, respectively; setting the gate RST of the reset transistor 104, the gate terminal TX of the charge transfer transistor 102, and the gate TXb of the charge select transistor 103 high, turning on the reset transistor 104, the charge transfer transistor 102, and the charge select transistor 103, clearing the charge in the photodiode 101 and the charge storage device 108; after the clearing is completed, the gate RST of the reset transistor 104, the gate TX of the charge transfer transistor 102, and the gate TXb of the charge select transistor 103 are set to low potential, and the charge transfer transistor 102, the charge select transistor 103, and the reset transistor 104 are turned off. Subsequently, the gate terminal SG of the charge storage device 108 is set to the high potential, the pixel reset phase 201 ends, and the next phase photoelectric charge signal collection phase is entered.

The photoelectric charge signal collection phase 202 is described as follows:

the second phase is a photo charge signal collection phase 202, in which the photodiode 101 receives a light signal, converts the light signal into electric charge, and collects the electric charge in the photodiode 101; the gate Anti of the charge discharging transistor 107 is kept in the next low potential state (a weak off signal is input) in order to keep the charge discharging transistor 107 in the weak off state; the gate RST of the reset transistor 104, the gate TXb of the charge select transistor 103, and the gate RS of the pixel select transistor 106 are all held in a low state.

The process by which the photodiode 101 collects charge is referred to as an exposure period T. A grid TX of the charge transmission transistor 102 inputs a charge output signal, the charge output signal is a pulse modulation signal with pulse duration a and period T, and the total duration of the charge output signal is T; a. t and T are both positive numbers; namely: in this exposure period T, the charge transfer transistor 102 has a plurality of on and off actions, and a period from when the charge transfer transistor 102 is turned off to when the charge transfer transistor 102 is turned on and off next time is referred to as a micro-exposure period T (period T of the pulse modulation signal), which is N micro-exposure periods (e.g., T1, T2, … …, tN shown in fig. 3), where N is a natural number greater than 2, and the sum of the N micro-exposure periods is one exposure period T (total period T of the charge output signal), that is: t1+ T2+ … … + tN ═ T. From the 1 st period to the nth period, the integral duration corresponding to each period is gradually reduced; each period sequentially comprises a charge output signal turn-off stage and a charge output signal turn-on stage; the integration duration is the duration of the charge output signal off phase of the cycle.

The duration of each pulse of the high potential of the charge transfer transistor 102 during the exposure period T may range from 0.05us to 0.5us, and each pulse of the high potential of the charge transfer transistor 102 transfers the charge in the photodiode 101 to the charge storage device 108, which is called the modulation of the charge by the charge transfer transistor 102. After the photodiode 101 is exposed, the third stage of outputting the photoelectric signal is performed.

The photo signal output stage 203 is described as follows:

a third stage 203 for outputting the photoelectric signal, in which the gate Anti of the charge discharging transistor 107 is set to a high potential, and the charge discharging transistor 107 is turned on; setting the gate RS of the pixel selection transistor 106 to a high potential, and turning on the pixel selection transistor 106; setting the gate RST of the reset transistor 104 to a high potential clears the charges in the parasitic capacitance of the floating diffusion active region FD connected to the drain of the charge selection transistor 103, the source of the reset transistor 104, and the gate of the source follower transistor 105.

After the clearing is finished, setting the grid RST of the reset transistor 104 to be low potential, and then, indicating that the pixel outputs a reset signal R by a pulse SHR in a read time sequence; next, the gate terminal TXb of the charge selection transistor 103 is set to a high potential, followed by setting the gate SG of the charge storage device 108 to a high potential, the electric charge in the charge storage device 108 is transferred to the floating diffusion active region FD, the gate TXb of the charge selection transistor 103 is set to a low potential after the photoelectric charge transfer is completed, and then the pulse SHS in the read timing indicates that the high dynamic range image sensor pixel circuit outputs the photoelectric signal S. After the photoelectric signal S is output, the gate Anti of the charge releasing transistor 107 and the gate RS of the pixel selecting transistor 106 are set to low potentials, respectively, and one frame of operation for the pixel to acquire the photoelectric signal is completed.

The image signal Sig collected by the high dynamic range image sensor pixel circuit is equal to the reset signal R minus the photo signal S.

In fig. 3, a potential well diagram of a pixel circuit of a high dynamic range image sensor illuminated by strong light in a charge modulation mode of the charge transmission transistor 102 is shown in fig. 4, which is a potential well diagram of the N micro-exposure periods at the end of one of the micro-exposure periods, and a device portion in a dashed box 109 marked in fig. 1 is drawn in the potential well diagram.

In fig. 4, the potential states of the power supply Vdd, the potential of the photodiode 101, and the potential of the SD region, and when the charge releasing transistor 107 and the charge transmitting transistor 102 are off are indicated, respectively.

Where Vtx _ off is a channel potential at the time when the charge transfer transistor 102 is off, Vanti _ off is a channel potential at the time of a weak off state of the charge discharging transistor 107, Vsd is a reset potential of the SD region (charge storage device), and Vdd is a potential of the positive electrode of the first power supply; vanti _ off is higher than Vtx _ off by more than 0.1V, which may be 0.2V, 0.5V, for example. Excessive charges are collected in the photodiode 101 irradiated with strong light, charges exceeding the Vanti _ off preset potential are discharged to the positive electrode Vdd of the first power source, and the more the intensity of the light is stronger, the more charges the photodiode 101 is discharged.

A potential well diagram of a photodiode 101 of a low light illuminated high dynamic range image sensor pixel circuit is shown in fig. 5, which is a potential well diagram of a time when one of N micro-exposure time periods is about to end, during an exposure period T, the photodiode 101 collects a small amount of charges, the small amount of photoelectric charges are not enough to make the potential well potential of the photodiode 101 lower than a preset potential Vanti _ off, so that no charges are discharged to a positive electrode Vdd of a first power supply, and the photoelectric charges are all collected.

Therefore, the high dynamic range image sensor pixel circuit is of a nonlinear photoelectric response type, different micro-exposure time periods and different exposure times in the high dynamic range image sensor pixel circuit are different, collected photoelectric charges are different, and charges released to the positive electrode Vdd of the first power supply are different, so that a nonlinear response curve is formed.

The photoelectric response curve of the present invention and the related art is schematically shown in fig. 6. In fig. 6, Qmax is the highest charge amount that can be detected by the pixel floating diffusion active region FD, Pmax1 is the highest exposure amount that can be detected by the related art linear photo-responsive image sensor, and Pmax2 is the highest exposure amount that can be detected by the non-linear photo-responsive image sensor of the present invention.

In the region 501, the pixel circuit of the high dynamic range image sensor works in a low-light environment, the exposure of the pixel is low, and all photoelectric charges are collected;

in the 502 region, a pixel circuit of the high dynamic range image sensor works in an environment with common light intensity, and a small amount of charges are released in the pixel exposure process;

in the region 503, the high dynamic range image sensor pixel circuit operates in a high intensity lighting environment, the pixel exposure is high, and a large portion of photoelectric charges are released.

As shown in fig. 6, the photoelectric response curve of the high dynamic range image sensor pixel circuit is in a nonlinear mode, so that the photoelectric response sensitivity of the high-intensity light illuminating pixel is reduced, and the capability of the pixel for sensing high-intensity light information is improved, thereby effectively improving the dynamic range of the image acquired by the high dynamic range image sensor pixel circuit.

In the working method of the pixel circuit of the high dynamic range image sensor, because the exposure period is divided into a plurality of micro-exposure time length steps, the charge quantity generated in each micro-exposure time length stage is small, and the charge saturation capacity of the photodiode 101 is lower than that of the photodiode 101 in the related technology, the completely depleted potential of the photodiode 101 is lower, and the image smear problem is not easy to generate, namely, the charges in the photodiode are easy to derive, and the dark current is also reduced.

The charge transfer transistor 102 has a function of modulating charges, in which a part of charges in the photodiode 101 irradiated with strong light is discharged to the first power supply positive electrode Vdd through the charge discharging transistor 107, and charges in the photodiode 101 irradiated with weak light are all collected. Therefore, the light sensitivity of the pixel irradiated by strong light is suppressed, the light sensitivity is reduced, and the light sensitivity and the sensitivity of the pixel irradiated by weak light are kept unchanged.

In the embodiment of the invention, the photoelectric conversion element receives the optical signal to generate the electric charge; the charge storage and transfer circuit stores the charges according to the charge output signal to generate an integral charge signal and outputs the integral charge signal according to the control selection signal; the charge output signal is a modulation signal comprising n periods, and the corresponding time lengths of the periods are different from the 1 st period to the nth period; the releasing circuit releases part of charges generated by the photoelectric conversion element according to the weak closing signal at least in the process that the charge storage and transfer circuit stores the charges to generate the integrated charge signal; the charge readout circuit outputs an electric signal according to the integrated charge signal to obtain an image signal; because the charge in the photoelectric conversion element irradiated by strong light is released by the release circuit, and the charge in the photoelectric conversion element irradiated by weak light is completely collected, the light sensitivity of the pixel irradiated by strong light is suppressed, the light sensitivity is reduced, and the light sensitivity and the sensitivity of the pixel irradiated by weak light are kept unchanged, so that the light sensitivity dynamic range of the pixel circuit of the high dynamic range image sensor is increased. And from the 1 st period to the nth period, the corresponding time lengths of the periods are different, so that the flexibility of adjusting the photosensitive dynamic range is improved.

It should be understood that, the sequence numbers of the steps in the foregoing embodiments do not imply an execution sequence, and the execution sequence of each process should be determined by its function and inherent logic, and should not constitute any limitation to the implementation process of the embodiments of the present application.

The above-mentioned embodiments are only used for illustrating the technical solutions of the present application, and not for limiting the same; although the present application has been described in detail with reference to the foregoing embodiments, it should be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; such modifications and substitutions do not substantially depart from the spirit and scope of the embodiments of the present application and are intended to be included within the scope of the present application.

Claims

1. A high dynamic range image sensor pixel circuit, comprising:

the charge storage and transfer circuit stores the charges according to a charge output signal to generate the integrated charge signal, wherein the charge output signal is a modulation signal comprising n periods, and the time lengths corresponding to the periods are different from the 1 st period to the nth period;

a release circuit connected to the photoelectric conversion element and the input terminal of the charge storage transfer circuit, and configured to release a part of the charge generated by the photoelectric conversion element according to a weak off signal at least during the process of storing the charge by the charge storage transfer circuit to generate the integrated charge signal;

2. The high dynamic range image sensor pixel circuit of claim 1, wherein from cycle 1 to cycle n, the integration duration for each of said cycles gradually decreases or the integration duration for each of said cycles gradually increases;

each period sequentially comprises a charge output signal turn-off stage and a charge output signal turn-on stage; the integration duration is the duration of the charge output signal turn-off phase of the cycle.

3. The high dynamic range image sensor pixel circuit of claim 2, wherein a proportion of integration durations adjacent said periods is between 1/2-9/10; or the integral time length of each period is distributed in an arithmetic progression.

4. The high dynamic range image sensor pixel circuit of claim 1, wherein the charge output signal is a pulse modulated signal comprising n pulse periods, the charge output signal total duration is T, T being a positive number, the pulse duration of the pulse modulated signal is a; a is a positive number and is a constant.

5. The high dynamic range image sensor pixel circuit of claim 1, wherein the number n of cycles of the charge output signal is between 4-10.

6. The high dynamic range image sensor pixel circuit of claim 1, wherein the charge readout circuit is further configured to clear at least the charge in the photoelectric conversion element in accordance with a clear signal.

7. The high dynamic range image sensor pixel circuit of claim 6, wherein when the charge readout circuit clears at least the charge in the photoelectric conversion element, the release circuit is further configured to transfer the charge in the photoelectric conversion element in accordance with an anti-charge crosstalk control signal;

8. The high dynamic range image sensor pixel circuit of claim 1, wherein the charge readout circuit comprises a reset transistor, a source follower transistor, and a pixel select transistor;

the source electrode of the reset transistor and the grid electrode of the source following transistor are connected to the input end of the charge readout circuit in a common mode, and the input end of the charge readout circuit serves as the integrated charge signal input end;

the grid electrode of the reset transistor is connected to a reset signal control line to serve as a reset control signal input end of the charge readout circuit;

the drain electrode of the reset transistor is connected to a first power supply, the drain electrode of the source follower transistor is connected to a second power supply, and the first power supply and the second power supply are the same or different power supplies;

the source electrode of the source following transistor is connected with the drain electrode of the pixel selection transistor, the source electrode of the pixel selection transistor is connected with the output end of the charge readout circuit, and the output end of the charge readout circuit is used as the output end of the electric signal.

9. The high dynamic range image sensor pixel circuit of any of claims 1-8, wherein the charge storage transfer circuit comprises a charge transfer transistor, a charge storage device, and a charge selection transistor;

a source of the charge transfer transistor is connected to a charge input terminal of the charge storage transfer circuit, the charge input terminal is connected to an output terminal of the photoelectric conversion element, a drain of the charge transfer transistor is connected to a first terminal of the charge storage device and a source of the charge selection transistor, and a drain of the charge selection transistor is connected to an integrated charge signal output terminal of the charge storage transfer circuit;

the grid electrode of the charge transmission transistor is connected to the charge input signal control line to serve as a charge output signal input end, and the grid electrode of the charge selection transistor is connected to the control selection signal control line to serve as a control selection signal input end;

the second terminal of the charge storage device is coupled to a first voltage.

10. The high dynamic range image sensor pixel circuit of claim 9, wherein the release circuit comprises a charge crosstalk resistant transistor, a source of the charge crosstalk resistant transistor is connected to the output of the photoelectric conversion element, a drain of the charge crosstalk resistant transistor is connected to a control power supply, and a gate of the charge crosstalk resistant transistor is connected to a charge crosstalk resistant transistor control line;

wherein the channel potential of the anti-charge crosstalk transistor is controlled based on the anti-charge crosstalk transistor control line to be greater than the channel potential when the charge transfer transistor is turned off to discharge the part of the charge.

11. A method of operating a high dynamic range image sensor pixel circuit as claimed in any one of claims 1 to 10, comprising:

inputting the charge output signal in the exposure process, and storing the charge by a charge storage conversion circuit according to the charge output signal to generate an integral charge signal;

inputting the weak off signal during inputting the charge output signal to cause a release circuit to release a part of the charge generated by the photoelectric conversion element in accordance with the weak off signal at least during storing of the charge by the charge storage transfer circuit to generate the integrated charge signal;

the charge output signal is a modulation signal comprising n periods, and the corresponding time duration of each period is different from the 1 st period to the nth period;

12. The method of operating a high dynamic range image sensor pixel circuit as recited in claim 11, wherein the charge storage conversion circuit further comprises, prior to storing charge to generate an integrated charge signal:

inputting a clear signal before starting exposure, and clearing the charges in the photoelectric conversion element by the charge readout circuit according to the clear signal; the input of the clear signal is stopped, and the exposure is started.

13. The operating method of a high dynamic range image sensor pixel circuit according to claim 12, wherein an anti-charge crosstalk control signal is inputted during the period in which the clear signal is inputted, and the discharging circuit clears the charge in the photoelectric conversion element in accordance with the anti-charge crosstalk control signal;

14. The method of operating a high dynamic range image sensor pixel circuit as claimed in claim 11, wherein said inputting a control select signal to cause a charge storage transfer circuit to output said integrated charge signal further comprises, before said integrated charge signal is output and after the end of the exposure:

stopping exposure, inputting a charge release signal, and clearing the charges in the photoelectric conversion element by the release circuit according to the charge release signal; wherein the charge release signal remains input during the outputting of the integrated charge signal.

15. The method of claim 11, wherein inputting the control selection signal to obtain the image signal comprises:

inputting a control selection signal, wherein the charge storage transfer circuit outputs the integrated charge signal according to the control selection signal;

the charge readout circuit outputs an electrical signal according to the integrated charge signal.

16. The method of operating a high dynamic range image sensor pixel circuit of any of claims 11-15, further comprising, before the charge storage transfer circuit outputs the integrated charge signal and after generating the integrated charge signal:

a reset control signal is input, and the charge readout circuit outputs a reset signal according to the reset control signal.