A pixel circuit, image sensor and detection device

Abstract

The present invention discloses a pixel circuit, characterized in that it includes: a photoelectric conversion unit, comprising at least two taps, for receiving a light signal and converting the light signal into an electrical signal; at least two transmission gates, respectively connected to the taps, for controlling the transmission of the electrical signal; at least two floating diffusion capacitors, respectively connected to the transmission gates, for storing the electrical signal; at least two reset transistors, respectively connected to the floating diffusion capacitors, for resetting the floating diffusion capacitors; wherein each floating diffusion capacitor is equal to the coupling capacitance of the transmission gate and the reset transistor.

Description

Technical Field

[0001] The present application relates to the technical field of image sensors, and in particular to a three-dimensional image sensor. Background Art

[0002] With the development of LiDAR technology, Time of Flight (TOF) has received more and more attention. The TOF principle is to continuously send light pulses to the target, then use a sensor to receive the light returned from the object, and obtain the distance of the target by detecting the flight (round-trip) time of the light pulse.

[0003] Direct Time of Flight (DTOF) and Indirect Time of Flight (ITOF) are detection methods developed based on TOF. Both detection methods have their own advantages during use and have received more and more widespread attention.

[0004] Among them, indirect time-of-flight detection mainly obtains the phase difference relationship between the transmitted wave and the reflected echo of the detected object, and uses the phase difference relationship to obtain the distance information of the detected object. Generally, due to the limitation of the pixel structure, this sensor has measurement errors, which limits the ranging accuracy.

[0005] Therefore, developing a three-dimensional image sensor that reduces measurement errors is an urgent problem to be solved. Summary of the invention

[0006] The purpose of the present application is to provide a pixel circuit, an image sensor and a detection device to reduce measurement errors and improve ranging accuracy in view of the deficiencies in the above-mentioned prior art.

[0007] To achieve the above purpose, the technical solution adopted in the embodiment of the present application is as follows:

[0008] In a first aspect, the present application provides a pixel circuit, characterized in that it includes: a photoelectric conversion unit, comprising at least two taps, for receiving a light signal and converting the light signal into an electrical signal; at least two transmission gates, respectively connected to the taps, for controlling the transmission of the electrical signal; at least two floating diffusion capacitors, respectively connected to the transmission gates, for storing the electrical signal; at least two reset transistors, respectively connected to the floating diffusion capacitors, for resetting the floating diffusion capacitors; wherein each floating diffusion capacitor is equal to the coupling capacitance of the transmission gate and the reset transistor.

[0009] Optionally, the pixel circuit further includes: at least two readout transistors, respectively connected to the floating diffusion capacitors, for reading electrical signals of the floating diffusion capacitors; wherein each floating diffusion capacitor is equal to a coupling capacitance of the readout transistor.

[0010] Optionally, the pixel circuit is characterized in that the circuit also includes: at least two source followers, respectively connected to the floating diffusion capacitors, for reading out the electrical signals of the floating diffusion capacitors; wherein each floating diffusion capacitor is equal to the coupling capacitance of the readout transistor.

[0011] In a second aspect, the present application provides an image sensor, characterized in that it includes: a pixel array and a processing circuit; the pixel array is composed of a plurality of pixel units, and the pixel unit includes: a photoelectric conversion unit, comprising at least two taps, for receiving a light signal and converting the light signal into an electrical signal; at least two transmission gates, respectively connected to the taps, for controlling the transmission of the electrical signal; at least two floating diffusion capacitors, respectively connected to the transmission gates, for storing the electrical signal; at least two reset transistors, respectively connected to the floating diffusion capacitors, for resetting the floating diffusion capacitors; wherein each floating diffusion capacitor is equal to the coupling capacitance of the transmission gate and the reset transistor.

[0012] Optionally, the pixel circuit further includes: at least two readout transistors, respectively connected to the floating diffusion capacitors, for reading electrical signals of the floating diffusion capacitors; wherein each floating diffusion capacitor is equal to a coupling capacitance of the readout transistor.

[0013] Optionally, the pixel circuit is characterized in that the circuit also includes: at least two source followers, respectively connected to the floating diffusion capacitors, for reading out the electrical signals of the floating diffusion capacitors; wherein each floating diffusion capacitor is equal to the coupling capacitance of the readout transistor.

[0014] In a third aspect, the present application provides a detection device, characterized in that it includes: a transmitting module for transmitting a detection signal; a receiving module for receiving the detection signal; the receiving module includes an image sensor, the image sensor includes: a pixel array composed of a plurality of pixel units, the pixel unit includes: a photoelectric conversion unit, comprising at least two taps, for receiving an optical signal and converting the optical signal into an electrical signal; at least two transmission gates, respectively connected to the taps, for controlling the transmission of the electrical signal; at least two floating diffusion capacitors, respectively connected to the transmission gates, for storing the electrical signal; at least two reset transistors, respectively connected to the floating diffusion capacitors, for resetting the floating diffusion capacitors; wherein each floating diffusion capacitor is equal to the coupling capacitance of the transmission gate and the reset transistor.

[0015] Optionally, the pixel circuit further includes: at least two readout transistors, respectively connected to the floating diffusion capacitors, for reading electrical signals of the floating diffusion capacitors; wherein each floating diffusion capacitor is equal to a coupling capacitance of the readout transistor.

[0016] Optionally, the pixel circuit is characterized in that the circuit also includes: at least two source followers, respectively connected to the floating diffusion capacitors, for reading out the electrical signals of the floating diffusion capacitors; wherein each floating diffusion capacitor is equal to the coupling capacitance of the readout transistor.

[0017] The beneficial effects of this application are:

[0018] The present application provides a pixel circuit, characterized in that it includes: a photoelectric conversion unit, including at least two taps, for receiving a light signal and converting the light signal into an electrical signal; at least two transmission gates, respectively connected to the taps, for controlling the transmission of the electrical signal; at least two floating diffusion capacitors, respectively connected to the transmission gates, for storing the electrical signal; at least two reset transistors, respectively connected to the floating diffusion capacitors, for resetting the floating diffusion capacitors; wherein each floating diffusion capacitor is equal to the coupling capacitance of the transmission gate and the reset transistor.

[0019] By designing each floating diffusion capacitor to be equal to the coupling capacitor of the transmission gate and the reset transistor, the distance measurement error caused by the two floating diffusion capacitors is reduced, thereby improving the distance measurement accuracy of the pixel unit, the image sensor and the detection device. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other related drawings can be obtained based on these drawings without paying creative work.

[0021] Figure 1 A TOF ranging principle diagram of the prior art provided in the embodiment of the present application;

[0022] Figure 2 A block diagram of an image sensor provided in an embodiment of the present application;

[0023] Figure 3 A structural block diagram of a pixel unit provided in an embodiment of the present application;

[0024] Figure 4A circuit diagram of a pixel unit provided in an embodiment of the present application;

[0025] Figure 5 A timing diagram of a pixel unit provided in an embodiment of the present application;

[0026] Figure 6 A circuit diagram of a pixel unit provided in an embodiment of the present application. DETAILED DESCRIPTION

[0027] In order to make the purpose, technical solution and advantages of the embodiments of the present application clearer, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. The components of the embodiments of the present application described and shown in the drawings here can be arranged and designed in various different configurations.

[0028] Therefore, the following detailed description of the embodiments of the present application provided in the accompanying drawings is not intended to limit the scope of the present application for which protection is sought, but merely represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without creative work are within the scope of protection of the present application.

[0029] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0030] Figure 1 It is a general TOF distance measurement principle diagram. The flight time between the sensor and the object to be measured is obtained by the phase difference between the emitted light signal and the echo signal, and then the distance information is calculated. As shown in the figure, a beam of emitted light signal is emitted from the light source 101. The emitted light signal can be a laser pulse signal modulated by a pseudo-random sequence or an ordinary laser pulse signal. After the emitted light signal is reflected by the object 102, it is focused on the pixel of the image sensor 104 by the lens 103. The signal received by the image sensor is the echo signal and the background light signal or only the background light signal, and it is assumed that the background light signal is uniform within a certain time. The pixel circuit in the image sensor 104 performs signal elimination or signal extraction on the received signal, removes the background light part, and obtains a pure echo signal. Finally, the phase shift of the echo signal relative to the emitted light signal in the returned light by each pixel can detect the distance of the surrounding objects.

[0031] Generally, in practical applications, the light source 101 usually emits light using a sine wave, and the phase shift between the sine wave received by the image sensor 104 and the sine wave emitted by the light source 101 is proportional to the distance of the object 102. Assume that the emitted sine wave signal is

[0032]

[0033] After a delay of t1, the signal received by the image sensor 104 is

[0034]

[0035] When the image sensor 104 receives signals at four times with equal delay intervals, that is, at 0°, 90°, 180°, and 270° phase delays, the expressions of the four different signals are shown in formula (1-3);

[0036]

[0037] When the light signal emitted by the light source 101 is received and integrated by the detector, an electrical signal is obtained.

[0038] Equations 1-4, 1-5, 1-6, and 1-7, where B is the background light noise.

[0039]

[0040]

[0041] Subtracting Q0 from Q180 in the above four formulas can eliminate the background light noise. Similarly, subtracting Q90 from Q270 can also eliminate the background light noise. The signals after subtraction are shown in formulas 1-8 and 1-9;

[0042]

[0043] In order to obtain the phase wΔt, it is also necessary to eliminate A R The influence of amplitude on phase. So divide formula 1-9 by formula 1-10 to get formula 1-10;

[0044]

[0045] From this, the phase can be calculated and the distance R can be obtained, see formula 1-11;

[0046]

[0047] From the above theoretical formula, it can be known that the image sensor needs to obtain the number of charges received at the phases of 0°, 90°, 180°, and 270°. Generally, the image sensor needs to obtain the number of charges at a specific phase by sampling the voltage in the pixel unit.

[0048] Combine the following Figure 2 The structure of the TOF image sensor is described below. Figure 2 As shown, the TOF image sensor includes a pixel unit 201, a pixel array 202, a row selection circuit 203, a readout circuit 204, and a signal processing module 205. The pixel array 202 is a two-dimensional array of multiple pixel units 201. The row selection circuit 203 is used to select the row pixels in the pixel array 202. For example, the row selection circuit 203 selects the first row of pixel units, and then the signal of the row of pixel units is read out by the readout circuit 204, and the readout circuit 204 can be a sample and hold circuit. The readout circuit can be connected to the signal processing module 205 to process the readout row signal to obtain the TOF ranging result.

[0049] Figure 3 The structure block diagram of a general TOF pixel circuit includes a photoelectric conversion unit 310, a modulation gate 320, and a charge storage device 330. The photoelectric conversion unit 310 is used to receive a light signal and generate photogenerated electrons; the modulation gate 320 is used to control the transmission of the photogenerated electrons; and the charge storage device 330 is used to store the photogenerated electrons.

[0050] For example, Figure 4 As shown, a TOF pixel circuit diagram includes a photodiode 401, a reset transistor 402, a first transfer transistor 403, a second transfer transistor 404, a first floating diffusion capacitor 405 (Floating Diffusion, FD1), a second floating diffusion capacitor 406 (Floating Diffusion, FD2), a first source follower 407, a second source follower 408, a first selection transistor 409 (Select Tianssion, SEL1), and a second selection transistor 410 (Select Tianssion, SEL2).

[0051] Combine the following Figure 5 , which explains in detail the working principle of the TOF pixel circuit.

[0052] When the gate TX1 of the first transmission transistor 403 is at a high level, the gate TX2 of the second transmission transistor 404 is at a low level, at which time the first transmission transistor 403 is turned on and the second transmission transistor 404 is turned off, and the photogenerated electrons generated in the photodiode 401 are stored on the capacitor 405 through the first modulation transistor 403;

[0053] When the gate TX1 of the first transmission transistor 403 is at a low level, the gate TX2 of the second transmission transistor 404 is at a high level, at which time the first transmission transistor 403 is turned off and the second transmission transistor 404 is turned on, and the photogenerated electrons generated in the photodiode 401 are stored on the capacitor 406 through the second modulation transistor 404;

[0054] Therefore, capacitor 405 and capacitor 409 store information of the reflected signal at different phases, respectively. For example, capacitor 405 and capacitor 409 store charge information of the reflected signal at phases of 0° and 180°.

[0055] It should be noted that the pixel circuit of the image sensor provided in the embodiment of the present application may be a two-tap structure or a four-tap structure, and the present application does not make any specific limitation on this.

[0056] Taking the phase of the modulation signal as 0° as an example, generally, the work of the image processor can be divided into: reset, integration and readout. During reset, the gate of the reset transistor 402 and the gate of the second transfer transistor 404 are opened, and the charge on the FD is reset; during integration, the gate of the reset transistor 402 is closed; during readout, the gate of the gate transistor 410 is opened, and finally the voltage of FD1 is output through the readout signal line. In this process, each signal jump will cause a change in FD1, that is, it will cause the coupling capacitance on FD, which will further affect the readout FD1 voltage. Since the voltage on FD1 is a representation of the number of electrons in the integration time, the jump of these signals will also cause errors in the readout voltage. In addition, when the reset transistor 402, the gate transistor 410, and the second transfer transistor 404 are turned on, there will be channel charges in the channel region. At the moment of turning on or off, this charge flows into or out of the MOS switch, thereby changing the voltage of the corresponding node. These errors will cause measurement errors of the image sensor.

[0057] In addition, since the TOF image sensor includes at least two floating diffusion capacitors, such as two-tap or four-tap pixels, in the two-tap pixel structure, the difference in coupling capacitance between the two taps will cause inconsistent integrated charges, thereby affecting the ranging error.

[0058] The following takes two tap pixels as an example, combined with Figure 6 The specific implementation of the pixel circuit provided in this application is described.

[0059] like Figure 6 The pixel circuit shown comprises:

[0060] A two-tap pixel includes a photoelectric conversion part 601, which is used to receive an optical signal and convert the optical signal into an electrical signal. Pixel taps, namely, an electron well PGA and an electron well PGB, are arranged on both sides of the photoelectric conversion part. Different taps collect electrons at different phases. For example, PGA collects electrons at a phase of 0°, and PGB collects electrons at a phase of 180°.

[0061] Two transmission gates, such as Figure 6 As shown, exemplarily, the left side is the first transmission gate TX1, and the right side is the second transmission gate TX2, and accordingly, it also includes a first floating diffusion capacitor FD1 and a second floating diffusion capacitor FD2. When the gate of the transmission gate is turned on, electrons can be transferred from the electron well PGA through the transmission gate TX1 to FD1 for storage, and accordingly, electrons can be transferred from the electron well PGB through the transmission gate TX2 to FD2 for storage. Exemplarily, at a 0° phase, electrons can be transferred from the electron well PGA through the transmission gate TX1 to FD1 for storage, and at a 180° phase, electrons can be transferred from the electron well PGB through the transmission gate TX2 to FD2 for storage.

[0062] In addition, the pixel circuit further includes a first reset transistor 602, a second reset transistor 603, a first source follower 604, and a second source follower 605. When the electrons are stored in FD1 or FD2, the current will pass through the first source follower 604 and the first selection transistor 606 to output Vout_A, or through the second source follower 605 and the second selection transistor 607 to output Vout_B.

[0063] When the pixel circuit starts to work, the gate signal of the first reset transistor 602 or the second reset transistor 603 jumps, that is, when FD1 or FD2 is reset, the jump on the signal line will generate a coupling capacitance for FD1 or FD2. It should be noted that the source of the first reset transistor 602 or the second reset transistor 603 is connected to the power supply, and the disturbance of the signal on the power supply line will also cause the coupling capacitance of FD1 or FD2. Similarly, the gate signal of the first selection transistor 606 or the second selection transistor 607 jumps, that is, when the electrical signal on FD1 or FD2 is read out, the jump on the signal line will also generate a coupling capacitance for FD1 or FD2.

[0064] Therefore, it can be seen that the coupling capacitance of FD1 and the coupling capacitance of FD2 are disturbed by the signals on different transistors, which will cause the coupling capacitance of FD1 and FD2 to be asymmetric. Therefore, the pixel circuit proposed in this application ensures that the coupling capacitance of FD1 and FD2 is equal through circuit and layout design. In this way, the ranging errors of the readout pixels of the two TAPs are equal on the left and right, thereby improving the accuracy of the pixels.

[0065] It should be noted that the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, an element defined by the sentence "comprises a ..." does not exclude the existence of other identical elements in the process, method, article or device including the element.

[0066] The above description is only the preferred embodiment of the present application, and is not intended to limit the present application. For those skilled in the art, the present application may have various changes and variations. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application. It should be noted that similar numbers and letters represent similar items in the following drawings, so once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings. The above description is only the preferred embodiment of the present application, and is not intended to limit the present application. For those skilled in the art, the present application may have various changes and variations. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A pixel circuit, characterized in that: include: A photoelectric conversion unit, comprising at least two taps, for receiving an optical signal and converting the optical signal into an electrical signal; at least two transmission gates, respectively connected to the taps, for controlling the transmission of the electrical signal; at least two floating diffusion capacitors, respectively connected to the transmission gates, for storing the electrical signal; At least two reset transistors, respectively connected to the floating diffusion capacitors, for resetting the floating diffusion capacitors; wherein each floating diffusion capacitor is equal to the coupling capacitance of the transmission gate and the reset transistor; The pixel circuit further includes: at least two readout transistors, respectively connected to the floating diffusion capacitors, for reading the electrical signals of the floating diffusion capacitors; wherein each floating diffusion capacitor is equal to the coupling capacitance of the readout transistor.

2. The pixel circuit according to claim 1, wherein: The pixel circuit further includes: at least two source followers, respectively connected to the floating diffusion capacitors, for reading out the electrical signals of the floating diffusion capacitors; wherein each floating diffusion capacitor is equal to the coupling capacitance of the readout transistor.

3. An image sensor, characterized in that: include: pixel array and processing circuitry; The pixel array is composed of a plurality of pixel units, and the pixel units include: a photoelectric conversion unit, including at least two taps, for receiving an optical signal and converting the optical signal into an electrical signal; at least two transmission gates, respectively connected to the taps, for controlling the transmission of the electrical signal; at least two floating diffusion capacitors, respectively connected to the transmission gates, for storing the electrical signal; at least two reset transistors, respectively connected to the floating diffusion capacitors, for resetting the floating diffusion capacitors; wherein each floating diffusion capacitor is equal to the coupling capacitance of the transmission gate and the reset transistor; The pixel unit further includes: at least two readout transistors, respectively connected to the floating diffusion capacitors, for reading the electrical signals of the floating diffusion capacitors; wherein each floating diffusion capacitor is equal to the coupling capacitance of the readout transistor.

4. The image sensor according to claim 3, wherein: The pixel unit further includes: at least two source followers, respectively connected to the floating diffusion capacitors, for reading out the electrical signals of the floating diffusion capacitors; wherein each floating diffusion capacitor is equal to the coupling capacitance of the readout transistor.

5. A detection device, characterized in that: include: A transmitting module, used for transmitting a detection signal; A receiving module, used for receiving the detection signal; The receiving module includes an image sensor, and the image sensor includes: a pixel array, which is composed of a plurality of pixel units, and the pixel unit includes: a photoelectric conversion unit, which includes at least two taps, for receiving an optical signal and converting the optical signal into an electrical signal; at least two transmission gates, respectively connected to the taps, for controlling the transmission of the electrical signal; at least two floating diffusion capacitors, respectively connected to the transmission gates, for storing the electrical signal; at least two reset transistors, respectively connected to the floating diffusion capacitors, for resetting the floating diffusion capacitors; wherein each floating diffusion capacitor is equal to the coupling capacitance of the transmission gate and the reset transistor; The pixel unit further includes: at least two readout transistors, respectively connected to the floating diffusion capacitors, for reading the electrical signals of the floating diffusion capacitors; wherein each floating diffusion capacitor is equal to the coupling capacitance of the readout transistor.

6. The detection device according to claim 5, characterized in that The pixel unit further includes: at least two source followers, respectively connected to the floating diffusion capacitors, for reading out the electrical signals of the floating diffusion capacitors; wherein each floating diffusion capacitor is equal to the coupling capacitance of the readout transistor.

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