WO2022007449A1 - Image sensor pixel circuit, image sensor, and depth camera - Google Patents

Abstract

Disclosed in the present application is an image sensor pixel circuit, comprising: a charge generation unit used for converting an incident optical signal into an electrical signal and comprising a photodiode and a plurality of exposure control transistors; a charge storage unit connected to the charge generation unit and configured to store the electrical signal; a charge transfer unit connected to the charge storage unit and configured to transfer the electrical signal to a readout unit; and the readout unit configured to transfer the electrical signal of the charge storage unit as a pixel and read a signal of the pixel, wherein a plurality of electrical signals are respectively obtained by means of the plurality of exposure control transistors from a signal generated by the photodiode, and charges accumulated by the photodiode are alternately stored in corresponding charge storage units by means of the plurality of exposure control transistors. The pixel structure of the present application can support a global exposure mode, and noise can be reduced, so as to satisfy high-precision and long-distance measurement.

Description

An image sensor pixel circuit, an image sensor and a depth camera

This application claims the priority of the Chinese patent application filed on July 9, 2020 with the application number of 202010659009.9 and the invention titled "An Image Sensor Pixel Circuit, Image Sensor and Depth Camera", the entire contents of which are by reference Incorporated in this application.

technical field

The present application relates to the technical field of image sensors, and in particular, to an image sensor pixel circuit, an image sensor and a depth camera.

Background technique

The full name of TOF is Time-of-Flight, that is, time of flight. TOF ranging technology is a technology that achieves precise ranging by measuring the round-trip flight time of light pulses between the transmitting/receiving device and the target object. In TOF technology, the technology of directly measuring the time of flight of light is called d-TOF (direct-TOF); the emitted light signal is periodically modulated, and the phase delay of the reflected light signal relative to the emitted light signal is measured. The measurement technology that calculates the time of flight from the phase delay is called i-TOF (Indirect-TOF) technology. In i-TOF (Indirect-TOF) technology, it can be divided into continuous wave (Continuous Wave, CW) modulation and demodulation method and pulse modulation (Pulse Modulated, PM) modulation and demodulation method according to different modulation and demodulation types.

CW modulation usually uses sine wave modulation, and the demodulation end detects the waveform phase change after being reflected by the target object. This measurement method first binds the light flight distance information and the phase information of the light intensity change, and then converts the phase information. For the light intensity information detectable by the photodetector, the measurement of the time of flight of light is realized indirectly.

PW modulation measures distance directly by calculating the ratio of the number of electrons collected by different taps based on the time difference between transmitting and receiving the pulsed beam. The transmitter emits short pulse beams. On the one hand, due to the high energy of the transmitter, the interference of the background light is reduced to a certain extent, which can improve the measurement accuracy; on the other hand, due to the lower duty cycle, the laser power consumption can be reduced. However, the transmitter needs to generate high-frequency and high-intensity pulses, which requires high laser drive performance, and cannot use multi-frequency modulation like the CW modulation method. When performing long-distance ranging, a laser pulse with a wider pulse width is required, and the accuracy will also be reduced. as the pulse width decreases.

Chinese Patent Application Publication No. 201910385779.6 provides a time-of-flight depth camera and a distance measurement method for single-frequency modulation and demodulation, which extends the measurement distance with the same pulse width compared to the existing PM-iTOF measurement scheme; The CW-iTOF measurement scheme only needs one exposure to output the signal volume of three taps to obtain one frame of depth information, thus significantly reducing the overall measurement power consumption and increasing the measurement frame rate.

However, Chinese Patent Application Publication No. 201910386369.3 provides a time-of-flight depth camera and a distance measurement method for multi-frequency modulation and demodulation, which gets rid of the existing PM-iTOF measurement scheme in which the pulse width is proportional to the measurement distance and power consumption. The contradiction is negatively correlated with the measurement accuracy; the expansion of the measurement distance is no longer limited by the pulse width, so that the lower measurement power consumption and higher measurement accuracy can still be maintained under the condition of a longer measurement distance.

In the above modulation type TOF image sensor, light is irradiated to the target object within the measurement range, and then the time required for the reflected pulse of light to reach the receiver from the target object is calculated to obtain distance information. When the TOF image sensor adopts a multi-tap demodulation pixel structure, the taps receiving equal signals may have different sensitivities from each other in each unit pixel, which is prone to errors in distance information. Therefore, it is necessary to propose a technical solution to solve the above problems.

SUMMARY OF THE INVENTION

The purpose of the present application is to provide an image sensor pixel circuit, an image sensor and a depth camera to solve at least one of the above background technical problems.

An embodiment of the present application provides an image sensor pixel circuit, including: a charge generation unit for converting an incident optical signal into an electrical signal, which includes a photodiode and a plurality of exposure control transistors; a charge storage unit, connected to the charge a generating unit configured to store the electrical signal; a charge transfer unit, connected to the charge storage unit, configured to transfer the electrical signal to a readout unit; a readout unit configured to transfer the charge storage unit The electrical signal of the photodiode is transmitted as a pixel signal and the signal of the pixel is read; wherein, the signal generated by the photodiode obtains a plurality of electrical signals through the plurality of exposure control transistors, and the plurality of exposure control transistors alternately The charges accumulated by the photodiodes are stored in the corresponding charge storage cells.

In some embodiments, the plurality of exposure control transistors are first, second, and third exposure control transistors, and the first, second, and third exposure control transistors are obtained through the first, second, and third exposure control transistors, respectively. an electrical signal; the charge storage unit includes first, second, and third charge storage units, which are respectively connected with the first, second, and third exposure control transistors to store the first, second, and third electric signal.

In some embodiments, the charge transfer unit includes first, second, and third charge transfer units, which are respectively connected to the first, second, and third charge storage units to transfer the charges stored in each charge storage unit. to the readout unit.

In some embodiments, the readout unit includes first, second, and third readout units, which are respectively connected to the first, second, and third transmission units to read the first, second, and third transmission units. The charge stored by the third charge storage unit.

In some embodiments, the first, second and third charge transfer units are connected to the same readout unit; the readout unit includes a reset transistor, a source follower transistor, a selection transistor and a floating diffusion node; the The stored charges of the first, second, and third charge storage units are sequentially transferred to the same floating diffusion node through the first, second, and third charge transfer units in a time-sharing manner.

In some embodiments, an anti-overflow transistor is further included, and the source of the anti-overflow transistor is connected to the charge generation unit, so as to prevent electrons of the charge generation unit from overflowing to the charge storage unit after exposure is completed.

In some embodiments, the readout unit includes a reset transistor, a source follower transistor, a select transistor and a floating diffusion node; wherein the floating diffusion node is connected to the source of the charge transfer unit and the reset transistor, respectively pole, the reset transistor is configured to reset the voltage of the floating diffusion node according to a reset control signal.

In some embodiments, the readout unit further includes a conversion gain control transistor and a double conversion gain capacitor; wherein the conversion gain control transistor is connected between the reset transistor and the floating diffusion node, and the The double conversion gain capacitor is connected to the drain of the conversion gain control transistor, and the conversion gain control is realized by controlling the gate voltage of the conversion gain control transistor.

An embodiment of the present application further provides an image sensor, including: a row decoder/driver, a column decoder, a pixel column unit, and a pixel array; wherein the pixel array includes a plurality of pixels, and the pixels include an image sensor a pixel circuit; the image sensor pixel circuit includes: a charge generation unit for converting an incident optical signal into an electrical signal, which includes a photodiode and a plurality of exposure control transistors; a charge storage unit, connected to the charge generation unit, is configured to store the electrical signal; a charge transfer unit, connected to the charge storage unit, is configured to transfer the electrical signal to a readout unit; a readout unit is configured to transfer the electrical signal of the charge storage unit The signal is transmitted and read as the pixel; wherein, the signal generated by the photodiode obtains a plurality of electrical signals through the plurality of exposure control transistors, and the plurality of exposure control transistors alternately connect the photoelectric The charges accumulated by the diodes are stored in the corresponding charge storage cells.

The embodiment of the present application also provides a depth camera, including an emission module, a collection module, and a control and processor; wherein the emission module includes a light source and a light source driver; the collection module includes an image sensor; the The control and processor are respectively connected with the launch module and the acquisition module, and synchronize the trigger signals of the launch module and the acquisition module to calculate that the light beam is emitted by the launch module and sent by the acquisition module. The time required for group reception; the image sensor includes: a row decoder/driver, a column decoder, a pixel column unit, and a pixel array; wherein the pixel array includes a plurality of pixels, and the pixels include an image sensor a pixel circuit, the image sensor pixel circuit includes: a charge generation unit for converting an incident optical signal into an electrical signal, which includes a photodiode and a plurality of exposure control transistors; a charge storage unit, connected to the charge generation unit, is configured to store the electrical signal; a charge transfer unit, connected to the charge storage unit, is configured to transfer the electrical signal to a readout unit; a readout unit is configured to transfer the electrical signal of the charge storage unit The signal is transmitted and read as the pixel; wherein, the signal generated by the photodiode obtains a plurality of electrical signals through the plurality of exposure control transistors, and the plurality of exposure control transistors alternately connect the photoelectric The charges accumulated by the diodes are stored in the corresponding charge storage cells.

An embodiment of the present application provides an image sensor pixel circuit, including: a charge generation unit for converting an incident optical signal into an electrical signal, which includes a photodiode and a plurality of exposure control transistors; a charge storage unit, connected to the charge a generating unit configured to store the electrical signal; a charge transfer unit, connected to the charge storage unit, configured to transfer the electrical signal to a readout unit; a readout unit configured to transfer the charge storage unit The electrical signal of the photodiode is transmitted as a pixel signal and the signal of the pixel is read; wherein, the signal generated by the photodiode obtains a plurality of electrical signals through the plurality of exposure control transistors, and the plurality of exposure control transistors alternately The charges accumulated by the photodiodes are stored in the corresponding charge storage units, and the charges accumulated by the photodiodes during the exposure process are transferred to different charge storage units for storage through exposure control transistors with different taps, and after the exposure ends The stored charge is transferred to the corresponding floating diffusion node/the same floating diffusion node through the multiplexing transistor, so that the pixel structure can support the global exposure mode, and the noise can be reduced through the subsequent correlated double sampling circuit, so as to achieve high precision , distance measurement.

Description of drawings

In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the accompanying drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only These are some embodiments of the present application, and for those of ordinary skill in the art, other drawings can also be obtained from these drawings without any creative effort.

FIG. 1 is a schematic diagram of a TOF depth camera according to an embodiment of the present application.

Figure 2 is Figure 2 in the accompanying drawings of Chinese Patent Application Publication No. 201910385779.6;

Figure 3 is Figure 3 in the accompanying drawings of Chinese Patent Application Publication No. 201910386369.3;

FIG. 4 is a partial diagram of an image sensor according to an embodiment of the present application;

5 is a block diagram of a pixel circuit of an image sensor according to an embodiment of the present application;

FIG. 6 is a circuit diagram of a pixel circuit of an image sensor according to an embodiment of the present application;

FIG. 7 is a circuit diagram of a pixel circuit of an image sensor according to another embodiment of the present application;

FIG. 8 is a circuit diagram of a pixel circuit of an image sensor according to still another embodiment of the present application.

detailed description

In order to make the technical problems, technical solutions and beneficial effects to be solved by the embodiments of the present application more clearly understood, the present application will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application, but not to limit the present application.

It should be noted that when an element is referred to as being "fixed to" or "disposed on" another element, it can be directly on the other element or 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 indirectly connected to the other element. In addition, the connection can be used for both the fixing function and the circuit connection function.

It is to be understood that the terms "length", "width", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top" , "bottom", "inside", "outside", etc. indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, which are only for the convenience of describing the embodiments of the present application and simplifying the description, rather than indicating or implying that The device or element must have a specific orientation, be constructed and operate in a specific orientation, and therefore should not be construed as a limitation of the present application.

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

As shown in FIG. 1 , FIG. 1 is a schematic structural diagram of a TOF depth camera 10 . The TOF depth camera 10 includes an emission module 11 , an acquisition module 12 and a control and processor 13 . The emission module 11 provides the emission beam 30 to the target space to illuminate the object 20 in the space, at least part of the emission beam 30 is reflected by the object 20 to form a reflected beam 40, and at least part of the reflected beam 40 is collected by the acquisition module 12; The processor 13 is respectively connected with the emission module 11 and the collection module 12, and synchronizes the trigger signals of the emission module 11 and the collection module 12 to calculate the time required for the light beam to be emitted by the emission module 11 and received by the collection module 12, That is, the flight time t between the emitted light beam 30 and the reflected light beam 40, and further, the distance D of the corresponding point on the object can be calculated by the following formula:

where c is the speed of light; t is the flight time between the emitted beam and the reflected beam.

The emission module 11 includes a light source, a light source driver (not shown in the figure), and the like. Among them, the light source can be a light source such as a light emitting diode (LED), an edge emitting laser (EEL), a vertical cavity surface emitting laser (VCSEL), etc., or a light source array composed of multiple light sources, and the light beam emitted by the light source can be visible light, infrared light, ultraviolet light, etc.

The acquisition module 12 includes an image sensor 121, a lens unit, a filter (not shown), and the like. The lens unit receives at least part of the light beams reflected back by the object and guides the at least part of the light beams to the image sensor 121, and the filter is a narrow-band filter matching the wavelength of the light source, used to suppress background light noise in the remaining wavelength bands or Stray light. The image sensor 121 may be an image sensor array composed of a charge coupled element (CCD), a complementary metal oxide semiconductor (CMOS), an avalanche diode (AD), a single photon avalanche diode (SPAD), etc. The size of the image sensor array represents the depth The resolution of the camera, such as 320×240, etc. Generally, connected to the image sensor 121 also includes a readout circuit (not shown in the figure) composed of one or more of a signal amplifier, a time-to-digital converter (TDC), an analog-to-digital converter (ADC) and other devices. ).

The image sensor 121 includes at least one pixel, and each pixel includes a plurality of taps (for storing and reading or discharging charge signals generated by incident photons under the control of the corresponding electrodes), such as including 2 taps, to use for reading charge signal data. The specific description of the image sensor 121 will be described in detail later with reference to FIG. 4 .

The control and processor 13 can be an independent dedicated circuit, such as a dedicated SOC chip, FPGA chip, ASIC chip, etc. composed of CPU, memory, bus, etc., or a general-purpose processing circuit, such as when the TOF depth camera is integrated into a In smart terminals such as mobile phones, televisions, and computers, the processing circuit of the terminal can be used as at least a part of the control and processor 13 .

In some embodiments, the control and processor 13 is used to provide a modulation signal (emission signal) required when the light source emits laser light, and the light source emits a pulsed beam to the object to be measured under the control of the modulation signal; in addition, the control and processor 13 The demodulation signal (collection signal) of the taps in each pixel of the image sensor 121 is also provided. Under the control of the demodulation signal, the tap collects the charge signal generated by the pulsed beam including the reflected back of the object to be tested, and calculates the phase based on the charge signal. difference to obtain the distance of object 20. Generally, in addition to the reflected pulse beam reflected back by the object to be tested, there will also be some background light, interference light and other light beams. Specifically, the modulation and demodulation method, control, processing and other functions performed by the control and processor 13 may adopt the solutions described in Chinese Patent Application Publication Nos. 201910385779.6 and 201910386369.3. It can be understood that, for the convenience of description, the PM-iTOF modulation and demodulation method is used as an example for description in the embodiments of the present application, but is not limited to PM-iTOF modulation and demodulation.

Specifically, referring to Fig. 2 in Chinese Patent Application Publication No. 201910385779.6, the embodiment of Fig. 2 exemplarily shows the laser emission signal (modulated signal), received signal and collected signal (decoded signal) within two frame periods T The timing diagram of the modulation signal). The meaning of each signal is: Sp represents the pulse emission signal of the light source, each pulse emission signal represents a pulse beam; Sr represents the reflected light signal of the pulsed light reflected back by the object, and each reflected light signal represents the reflected light from the object to be measured. There is a certain delay relative to the pulse emission signal on the time line (horizontal axis in the figure), and the delayed time t is the flight time of the pulse beam to be calculated; S1 represents the pulse of the first tap of the pixel Acquisition signal, S2 represents the pulse acquisition signal of the second tap of the pixel, S3 represents the pulse acquisition signal of the third tap, and each pulse acquisition signal represents that the tap collects the charge signal (electron) generated by the pixel in the time period corresponding to the signal.

The entire frame period T is divided into two time periods Ta and Tb, where Ta represents the time period during which each tap of the pixel performs charge collection and storage, and Tb represents the time period during which the charge signal is read out. In Ta, each tap collects electrons generated on the pixel during its pulse period as the reflected light signal is reflected back to the pixel by the object. The first tap, the second tap, and the third tap perform charge collection and storage in sequence, respectively, to obtain the charge amounts q1, q2, and q3, so as to complete one pulse period Tp. In Figure 2, two pulse periods Tp are included in a single frame period, and a total of two laser pulse signals are emitted. Therefore, in the Tb period, each tap collects and reads out the total charge amount corresponding to the charge amount of the optical signal collected twice. It can be understood that in a single frame period, the pulse period Tp or the number of times the laser pulse signal is emitted can be K times, and K is not less than 1, and can also be as high as tens of thousands of times, or even higher. The specific number is based on actual In addition, the number of pulses in different frame periods can also be different.

Therefore, the total charge collected and read out by each tap in the Tb time period is the sum of the charges corresponding to the optical signals collected multiple times by each tap in the entire frame period T, and the total charge of each tap in a single frame period can be It is expressed as follows:

Qi=∑qi,i=1,2,3 (2)

According to formula (2), the total charge amount in a single frame period of the first tap, the second tap, and the third tap can be obtained as Q1, Q2 and Q3.

In the traditional modulation and demodulation method, the measurement range is limited to a single pulse width time, that is, it is assumed that the reflected light signal is collected by the first tap and the second tap, and the third tap is used to collect the ambient light signal. For the collected total charge, the control and processor can calculate the total light flight distance of the pulsed light signal from emission to reflection to the pixel according to the following formula:

Among them, c is the speed of light; _Th is the pulse width of the single exposure laser; Q1, Q2, Q3 are the total charge of the three taps respectively.

如果返回的激光落在第二抽头和第三抽头曝光使能信号之间，或者落在第三抽头和下一个周期的第一抽头 之间，那么超出了测量范围，以至于得到了错误的结果，如果需要测量更远的距离，就必须增加激光的脉宽，但这样会导致测量精度降低。 It can be understood that in the actual situation, the first tap and the second tap will collect ambient light signals in addition to the reflected light signals, and according to formula (2), it can be seen that when all the returned laser light falls on Second tap, so the maximum distance that can be detected

If the returned laser falls between the second tap and the third tap exposure enable signal, or between the third tap and the first tap of the next cycle, the measurement range is exceeded and an erroneous result is obtained , if you need to measure longer distances, you must increase the pulse width of the laser, but this will reduce the measurement accuracy.

In order to improve the measurement distance, according to the schematic diagram of optical signal emission and collection shown in Figure 2, the tap for collecting background light can not be fixed. At this time, the reflected light signal can not only fall into the first tap and the second tap. It can also be allowed to fall between the second tap and the third tap enable, or even allow it to fall between the third tap and the first tap in the next pulse period Tp (for at least two pulse periods) above Tp). The term "falls on the tap" as used herein means that it can be captured by the tap.

Considering that the amount of charge collected by the tap of the received reflected light signal is greater than that of the tap containing only the background light signal, the control and processor 13 judges the three total charge amounts Q1, Q2 and Q3 obtained to determine the obtained amount of charge. For a three-tap image sensor, there are three possibilities for taps containing the reflected light signal laser electronics and/or acquiring taps containing only the background signal:

(1) If Q1 and Q2 collect the reflected light signal, and Q3 collects the background light signal, the expression is as follows:

(2) If Q2 and Q3 collect the reflected light signal, and Q1 collects the background light signal, the expression is as follows:

(3) If Q1 and Q3 collect the reflected light signal, and Q2 collects the background light signal, the expression is as follows:

扩大到

因而相对于传统调制解调方法其测量距离扩大了3倍。 Compared with formula (3), it can be clearly seen that the measurement distance has been extended, and the maximum measurement flight distance is determined by the traditional modulation and demodulation method.

Expanded to

Therefore, compared with the traditional modulation and demodulation method, the measurement distance is expanded by 3 times.

Although the above-mentioned modulation and demodulation method achieves three times the measurement distance, it still cannot satisfy the measurement of longer distances without increasing the pulse width.

Referring to the embodiment shown in FIG. 3 of Chinese Patent Application Publication No. 201910386369.3, in the embodiment of FIG. 3, the multi-frequency spreading method in the CW modulation method is borrowed, which can meet the measurement of longer distances.

The embodiment of FIG. 3 describes a method for multi-frequency modulation and demodulation, that is, different modulation and demodulation frequencies are used in adjacent frames. For ease of explanation, two adjacent frame periods and two modulation frequencies are used as examples for description. In the adjacent frame period, the number of pulse transmissions K=2 (it can also be multiple times, and the number of frames can also be different), the number of taps of the pixel N=3, the pulse periods are Tp1, Tp2, and the pulse widths are Th1, respectively. Th2, the accumulated charges of the three taps per pulse are q11, q12, q21, q22, q31, and q32 respectively. Assuming that the distance of the object remains unchanged during the exposure time of adjacent frames, after receiving the total charge of each tap, the control and processor 13 uses the modulation and demodulation method shown in FIG. 2 to measure the distance in each frame period respectively, The measurement distance in each frame period is calculated by the above judgment method, and then the final distance can be obtained by using the least common multiple method.

Assuming that in the embodiment shown in FIG. 3, Tp=9ns, the maximum measured flight distance using the modulation and demodulation method in FIG. 2 is 1.35m; if Tp=15ns, the maximum measured flight distance is 2.25m. If a multi-frequency modulation and demodulation method is used based on the modulation and demodulation method shown in FIG. 2, for example, in some embodiments, Tp1=9ns, Tp2=15ns, the least common multiple of 9ns and 15ns is 45ns, and the maximum value corresponding to 45ns is 45ns. The long-distance measurement target distance can reach 6.75m.

Based on the aforementioned modulation and demodulation method, the embodiment of the present application proposes an image sensor including multi-tap pixels, which can implement the above-mentioned modulation and demodulation method, thereby realizing long-distance and high-precision measurement of the TOF depth camera.

Referring to FIG. 4 , FIG. 4 is a partial schematic layout diagram of an image sensor according to an embodiment of the present application. The image sensor 121 shown in FIG. 4 receives a part of the reflected light 40 reflected by the object 20, and calculates the charge signals Q1, Q2 and Q3 accumulated by the three taps according to the reflected light. According to the charge amount, the three possibilities described above can be used. The distance of the object is calculated by a linear expression; it can be understood that the embodiment of the present application uses three taps as an example for description, but is not limited to three taps.

For ease of illustration, pixel array 42 in image sensor 121 in FIG. 4 is shown as 9 pixels arranged in a 3×3 array; in practice, pixel arrays may contain thousands or numbers in multiple rows and columns megapixel. In particular embodiments, each pixel in pixel array 42 may have the same configuration, so each pixel is represented by the same reference numeral "41" as shown in FIG. 4 .

In addition to the pixel array 42 , the image sensor 121 in the embodiment of FIG. 4 may also include a row decoder/driver 47 , a column decoder 53 and a pixel column unit 54 . The pixel column unit 54 includes circuitry for correlated double sampling (CDS) and column-specific analog-to-digital converters (ADCs) used in 3D imaging devices. In some embodiments, each column in a pixel may have one ADC. In particular embodiments, row decoder/driver 47, column decoder 53 and pixel column unit 54 may be part of control and processor 13 shown in FIG. In the embodiment of FIG. 4, row decoder/driver 47 is shown as providing 8 different signals as input to each pixel 41 in a row of pixels to control the pixels in pixel array 42 and thereby enable the generation of column specific PIXOUT ( pixel output signal) 50, 51, 52. The arrows numbered 44, 45, 46 in FIG. 4 show that a specific signal is input to each pixel 41 in the corresponding row. These signals include: overflow prevention signal (DRN), reset signal (RST), charge storage signal (SG), three-tap exposure control signals (MG1, MG2, MG3) and row select signal (SEL).

In some embodiments, a row select (SEL) signal is used to select the appropriate row of pixels. Row decoder/driver 47 may decode via row address/control input 47 to enable it to select the appropriate row using the SEL signal and provide corresponding RST, TG and other signals to the row selected for decoding. The RST signal may be applied to pixels in selected rows to reset the pixels to a predetermined high voltage level. The DRN signal releases the electrons collected by the photodiode (PD) to the power supply after exposure, preventing the collected electrons from overflowing into the three-tap charge storage section.

The pixel column unit 54 may receive the PIXOUT signals 50, 51, 52 from the pixels in the row and process these signals to calculate the amount of charge Q1, Q2 and Q3 from which the distance of the object is calculated. Column selection allows sequential reception of pixel output from each pixel in the row selected by the corresponding SEL signal. Control and processor 13 may provide appropriate row address inputs to select rows of pixels and may also provide appropriate column address inputs to column decoder 53 to enable pixel column unit 54 to receive outputs from individual pixels in the selected row (PIXOUT).

FIG. 5 is a schematic block diagram of a pixel circuit of an image sensor according to an embodiment of the present application. For the convenience of description, a three-tap pixel is used as an example for description in the embodiment of the present application. In the embodiment of FIG. 4 , each pixel 41 in pixel array 42 may have the pixel configuration in FIG. 5 .

Referring to FIG. 5 , the image sensor pixel circuit 100 includes a charge generation unit 60 , a charge storage unit 70 , a charge transfer unit 80 and a readout unit 90 .

The charge generation unit 60 is used for converting the incident optical signal into an electrical signal, and includes a photodiode (PD) and a plurality of exposure control transistors, and the plurality of exposure control transistors alternately store the charges accumulated by the photodiodes to corresponding charges storage unit. Specifically, in the embodiment of the present application, the electrical signal generates the first electrical signal according to the first exposure control transistor (MG1), generates the second electrical signal according to the second exposure control transistor (MG2), and generates the second electrical signal according to the third exposure control transistor (MG3) A third electrical signal is generated. During global exposure, MG1, MG2 and MG3 alternately store the charges accumulated by the photodiodes to the corresponding charge storage units.

The charge storage unit 70 is connected to the charge generation unit and is configured to store the electrical signal. In this embodiment of the present application, the charge storage unit includes a first charge storage unit SG1, a second charge storage unit SG2, and a third charge storage unit SG3, which are connected with the first exposure transistor MG1, the second exposure transistor MG2, and the third exposure transistor respectively. MG3 is connected and configured to store the first electrical signal, the second electrical signal and the third electrical signal, respectively. In some embodiments, the charge storage unit may be a capacitor, a PN junction or others, which are not limited herein.

The charge transfer unit 80 is connected to the charge storage unit and is configured to transfer the electrical signal to the readout unit. In the embodiment of the present application, the charge transfer unit 80 includes a first charge transfer unit TG1, a second charge transfer unit TG2, and a third charge transfer unit TG3, so as to connect the first charge storage unit SG1 and the second charge storage unit SG2 respectively and a third charge storage unit SG3 for transferring the charges stored in the charge storage unit to the readout unit. In some embodiments, the charge transfer unit may be an electron transfer transistor.

The readout unit 90 is configured to transmit the electrical signal of the charge storage unit as a pixel and to read the signal of the pixel. Specifically, in the embodiment of the present application, the readout unit 90 is configured to transmit the charges stored in the first charge storage unit SG1 , the second charge storage unit SG2 and the third charge storage unit SG3 as pixels and read the pixels signal of.

In some embodiments, the image sensor pixel circuit 100 further includes an overflow prevention transistor (DRN), and the source of the overflow prevention transistor is connected to the charge generation unit, and is configured to prevent electrons from the charge generation unit from overflowing to the charge storage unit. Specifically, in the embodiment of the present application, the source of the overflow prevention transistor is connected to the photodiode, so as to prevent the photodiode from collecting electrons and overflowing to the charge storage unit after exposure.

In some embodiments, the readout cell 90 includes a reset transistor (RST), a source follower transistor (SF), a select transistor (SEL), and a floating diffusion node (FD). The floating diffusion node is respectively connected to the charge transfer unit and the source of the reset transistor, and the reset transistor is configured to reset the voltage of the floating diffusion node according to the reset control signal. Once the pixel is selected by the selection transistor SEL and reset by the reset transistor RST, the charge storage unit is turned off to transfer electrons to the floating diffusion node FD. At this time, the voltage at the floating diffusion node FD is output as PIXOUT and transferred to the ADC unit to be converted into a digital signal.

In some embodiments, the readout unit 90 includes a first readout unit 901 , a second readout unit 902 and a third readout unit 903 . As shown in FIG. 6 , the first readout unit 901 is connected to the first charge transfer unit, the second readout unit 902 is connected to the second charge transfer unit, and the third readout unit 903 is connected to the third charge transfer unit, so as to read the first charge transfer unit respectively. Charges of a charge storage unit, a second charge storage unit, and a third charge storage unit.

The first readout unit 901 is taken as an example for description below. The drain of the reset transistor (RST) of the first readout unit 901 is connected to a voltage source, and the voltage of the floating diffusion node (FD) is reset according to the reset control signal. The charge transfer unit transfers the electrons stored in the first charge storage unit to the floating diffusion node FD, and the gate of the source follower transistor (SF) of the first readout unit is connected to the floating diffusion node (FD), and the drain thereof is connected The voltage source, the source follower transistor amplifies the voltage signal of the floating diffusion node as the output of PIXOUT1, and transmits it to the ADC unit to be converted into an appropriate digital signal. It can be understood that the structures and transmission modes of the second readout unit, the third readout unit and the first readout unit are the same, and details are not described herein again.

By separating the three readout units, and each readout unit has a separate floating diffusion node FD, the parasitic capacitance value on the floating diffusion node FD is small, and a large conversion gain can be achieved. However, this will increase the number of transistors in a single pixel, thereby reducing the fill factor. In addition, by fixing the background light in time, the charge stored in the memory cell is transferred to the corresponding floating diffusion node FD. The amount of charge is theoretically the same, but due to process production deviations, such as the gain deviation of the source follower (SF), the gain error between multiple time windows will increase, making subsequent calibration more difficult.

In some embodiments, reference is made to FIG. 7 . The first charge transfer unit, the second charge transfer unit and the third charge transfer unit are connected to the same readout unit. A reset transistor (RST), a source follower transistor (SF), a select transistor (SEL) and a floating diffusion node (FD) are shared as part of the readout cell. The stored charges of the first charge storage unit, the second charge storage unit and the third charge storage unit in the circuit shown in FIG. 7 are sequentially transferred to the first charge transfer unit, the second charge transfer unit and the third charge transfer unit in a time-sharing manner The same floating diffusion node, and the voltage at the floating diffusion node FD is output through the PIXOUT of the same readout circuit, and then transmitted to the ADC unit in turn. It can be understood that the voltage acquisition method at the floating diffusion node FD is the same as the voltage acquisition method of the floating diffusion node FD in the first readout unit in the embodiment shown in FIG. 6 , and details are not described herein again.

By connecting the first charge transfer unit, the second charge transfer unit and the third charge transfer unit to the same readout unit, the number of transistors in the pixel is greatly reduced, the fill factor of the pixel is improved, and the Process deviation brings gain error in depth, reducing subsequent correction work.

Referring to FIG. 8 , in some embodiments, in order to address the complexity of iTOF application scenarios, such as outdoor strong ambient light. The readout unit further includes a conversion gain control transistor (LG) and a double conversion gain capacitor (CLG), the conversion gain control transistor is connected between the reset transistor and the floating diffusion node, and the double conversion gain capacitor is connected between the fixed level and the conversion gain control transistor. drain, so that the conversion gain control is achieved by controlling the gate voltage of the conversion gain control transistor. It can be understood that the capacitance of the double conversion gain can be realized by MIM, MOM, MOS capacitance, parasitic capacitance, and the like.

In the first frame, the conversion gain control transistor is turned off, and the integral capacitance of the floating diffusion node is the parasitic capacitance brought by the reset transistor (RST), the source follower transistor (SF), the select transistor (SEL), and the floating diffusion node to the liner. It is composed of the bottom junction capacitance, and the capacitance value is relatively small, so that a high conversion gain can be achieved; after obtaining the image frame with high conversion gain of each of the three taps, the gain control transistor is enabled, and the integral capacitance of the floating diffusion node is now floating. On the basis of the original, a double conversion gain capacitor CLG is added, and the value of the integral capacitor becomes larger, which reduces the conversion gain of the pixel. The three taps obtain their respective image frames with low conversion gain in turn. For fusion, 3D depth information with high dynamic range can be achieved.

In some embodiments, readout unit 90 also includes a correlated double sampling (CDS) circuit (not shown) in which the output of a pixel can be measured twice: once under known conditions, and the other Once under unknown conditions, the value measured under known conditions can be subtracted from the value measured under unknown conditions to generate a value with a known relationship to the measured physical quantity, representing the photoelectrons of a particular portion of the pixel receiving light charge. By using correlated double sampling, noise can be reduced by removing the pixel's reference voltage, such as the reset pixel voltage, from the pixel's signal voltage at the end of each integration period.

It should be noted that, in the image sensor of the embodiment of FIG. 4 , the pixels in the pixel array 42 are the pixels described in any one of the embodiments of FIG. 5 to FIG. 8 . For details, please refer to the description of FIGS. It is not repeated in the embodiment of the image sensor shown. Likewise, in the depth camera of the embodiment of FIG. 1 , the image sensor 121 included in the acquisition module is the image sensor described in the solution of the embodiment of FIG. 4 , and details are not repeated here.

It can be understood that the above content is a further detailed description of the present application in conjunction with specific/preferred embodiments, and it cannot be considered that the specific implementation of the present application is limited to these descriptions. For those of ordinary skill in the technical field of the present application, without departing from the concept of the present application, they can also make several substitutions or modifications to the described embodiments, and these substitutions or modifications should be regarded as It belongs to the protection scope of this application. In the description of this specification, reference to the terms "one embodiment," "some embodiments," "preferred embodiment," "example," "specific example," or "some examples" or the like is meant to be used in conjunction with the description. A particular feature, structure, material, or characteristic described by an example or example is included in at least one embodiment or example of the present application.

In this specification, schematic representations of the above terms are not necessarily directed to the same embodiment or example. Furthermore, the particular features, structures, materials or characteristics described may be combined in any suitable manner in any one or more embodiments or examples. Furthermore, those skilled in the art may combine and combine the different embodiments or examples described in this specification, as well as the features of the different embodiments or examples, without conflicting each other. Although the embodiments of the present application and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the scope as defined by the appended claims.

Furthermore, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. Those of ordinary skill in the art will readily appreciate that the above disclosures, processes, machines, now existing or later developed, that perform substantially the same functions or achieve substantially the same results as the corresponding embodiments described herein can be utilized. Manufacture, composition of matter, means, method or step. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

Claims (10)

An image sensor pixel circuit, comprising: a charge generating unit for converting an incident optical signal into an electrical signal, which includes a photodiode and a plurality of exposure control transistors; a charge storage unit, connected to the charge generation unit, configured to store the electrical signal; a charge transfer unit, connected to the charge storage unit, configured to transfer the electrical signal to the readout unit; a readout unit configured to transmit an electrical signal of the charge storage unit as a pixel and to read a signal of the pixel; Wherein, the signal generated by the photodiode obtains a plurality of electrical signals through the plurality of exposure control transistors, respectively, and the plurality of exposure control transistors alternately store the charges accumulated by the photodiode into the corresponding charge storages unit. 1. The image sensor pixel circuit of claim 1, wherein the plurality of exposure control transistors are first, second, and third exposure control transistors, and the first, second, and third exposure control transistors pass through the first, second, and third exposure control transistors. obtaining first, second, and third electrical signals respectively; the charge storage unit includes first, second, and third charge storage units, which are respectively connected with the first, second, and third exposure control transistors to store the first, second and third electrical signals. The image sensor pixel circuit according to claim 2, wherein the charge transfer unit comprises first, second and third charge transfer units, which are respectively connected to the first, second and third charge storage units, The charges stored in each charge storage unit are transferred to the readout unit. The pixel circuit of the image sensor according to claim 3, wherein the readout unit comprises first, second and third readout units, which are respectively connected to the first, second and third transmission units to The charges stored in the first, second and third charge storage units are read. The image sensor pixel circuit according to claim 3, wherein: the first, second and third charge transfer units are connected to the same readout unit; the readout unit comprises a reset transistor, a source follower transistor, A transistor and a floating diffusion node are selected; the stored charges of the first, second, and third charge storage units are sequentially transferred to the same floating diffusion node through the first, second, and third charge transfer units in a time-sharing manner. 1. The image sensor pixel circuit according to claim 1, further comprising an anti-overflow transistor, the source of the anti-overflow transistor is connected to the charge generation unit, so as to avoid the leakage of the charge generation unit after exposure is completed. Electrons overflow into the charge storage cells. The image sensor pixel circuit of claim 1, wherein the readout unit comprises a reset transistor, a source follower transistor, a selection transistor and a floating diffusion node; wherein the floating diffusion node is respectively connected to the A charge transfer unit and a source of the reset transistor configured to reset the voltage of the floating diffusion node according to a reset control signal. The image sensor pixel circuit of claim 7, wherein the readout unit further comprises a conversion gain control transistor and a double conversion gain capacitor; wherein the conversion gain control transistor is connected to the reset transistor and the Between the floating diffusion nodes, the double conversion gain capacitor is connected to the drain of the conversion gain control transistor, and the conversion gain control is realized by controlling the gate voltage of the conversion gain control transistor. An image sensor, comprising: a row decoder/driver, a column decoder, a pixel column unit, and a pixel array; wherein the pixel array includes a plurality of pixels, and the pixels include an image sensor pixel circuit , the image sensor pixel circuit includes: a charge generation unit for converting an incident optical signal into an electrical signal, which includes a photodiode and a plurality of exposure control transistors; a charge storage unit, connected to the charge generation unit, configured In order to store the electrical signal; a charge transfer unit, connected to the charge storage unit, is configured to transfer the electrical signal to a readout unit; the readout unit is configured to use the electrical signal of the charge storage unit as a pixel transmitting and reading the signals of the pixels; wherein, the signals generated by the photodiodes respectively obtain a plurality of electrical signals through the plurality of exposure control transistors, and the photodiodes are alternately accumulated by the plurality of exposure control transistors The charge is stored in the corresponding charge storage unit. A depth camera, comprising an emission module, a collection module, and a control and processor, characterized in that: the emission module includes a light source and a light source driver; the collection module includes an image sensor; the control and processor Connect to the launch module and the acquisition module respectively, and synchronize the trigger signals of the launch module and the acquisition module to calculate the light beams sent by the launch module and received by the acquisition module. time; wherein the image sensor includes: a row decoder/driver, a column decoder, a pixel column unit, and a pixel array; wherein the pixel array includes a plurality of pixels, and the pixels include an image sensor pixel circuit , the image sensor pixel circuit includes: a charge generation unit for converting an incident optical signal into an electrical signal, which includes a photodiode and a plurality of exposure control transistors; a charge storage unit, connected to the charge generation unit, configured In order to store the electrical signal; a charge transfer unit, connected to the charge storage unit, is configured to transfer the electrical signal to a readout unit; the readout unit is configured to use the electrical signal of the charge storage unit as a pixel transmitting and reading the signals of the pixels; wherein, the signals generated by the photodiodes respectively obtain a plurality of electrical signals through the plurality of exposure control transistors, and the photodiodes are alternately accumulated by the plurality of exposure control transistors The charge is stored in the corresponding charge storage unit.

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