Photoelectric conversion device

Abstract

To provide a technique advantageous for ensuring uniformity of characteristics between pixels while reducing a circuit scale.SOLUTION: A photoelectric conversion device has a plurality of pixels arranged therein, and each of the plurality of pixels includes: an avalanche photodiode that is arranged between a first potential supply line and a second potential supply line; a first switch element that is arranged between the first potential supply line and the avalanche photodiode; a second switch element that is arranged between the first switch element and the avalanche photodiode; a capacitive element that includes a first terminal, and a second terminal connected with a first node to which the first switch element and the second switch element are connected; and a detection circuit for detecting avalanche breakdown of the avalanche photodiode according to a change in the potential of a second node to which the second switch element and the avalanche photodiode are connected.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a photoelectric conversion device. [Background technology]

[0002] There is known a photoelectric conversion device in which each pixel is provided with a single-photon avalanche photodiode (SPAD) element capable of detecting weak light at a single photon level. Patent Document 1 shows a light receiving device in which a SPAD element is arranged in each of a plurality of pixels. In the SPAD element, a voltage obtained by adding an excess voltage to the breakdown voltage of an avalanche photodiode (APD) is applied to the APD. When the breakdown voltage of the APD varies between pixels, if the same voltage is applied to the APD of each pixel, the value of the excess voltage supplied to the APD will differ between the pixels. When the excess voltage varies between pixels, it becomes impossible to maintain uniformity in the characteristics of each pixel. Patent Document 1 shows that a signal output from the APD is detected by a signal processing unit, and the value of the excess voltage is adjusted by feeding back the detection result to a bias adjustment unit. Specifically, the signal processing unit detects the characteristics of the APD, such as a dead time during which the APD cannot respond to photons, and the bias voltage is adjusted in a bias adjustment unit using a linear regulator or the like so that the characteristics of the APD are uniform. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-089962 Summary of the Invention [Problem to be solved by the invention]

[0004] In the configuration shown in Patent Document 1, the circuit scale of the signal processing unit and bias adjustment unit for adjusting the excess voltage that varies between pixels becomes large.

[0005] An object of the present invention is to provide a technique that is advantageous in ensuring uniformity of characteristics between pixels while suppressing the circuit scale. [Means for solving the problem]

[0006] In view of the above problems, a photoelectric conversion device according to an embodiment of the present invention is a photoelectric conversion device in which a plurality of pixels are arranged, each of the plurality of pixels including: an avalanche photodiode arranged between a first potential supply line and a second potential supply line; a first switch element arranged between the first potential supply line and the avalanche photodiode; a second switch element arranged between the first switch element and the avalanche photodiode; a capacitive element having a first terminal and a second terminal connected to a first node to which the first switch element and the second switch element are connected; and a detection circuit for detecting avalanche breakdown of the avalanche photodiode in response to a change in potential of a second node to which the second switch element and the avalanche photodiode are connected. Effect of the Invention

[0007] According to the present invention, it is possible to provide a technique that is advantageous in ensuring uniformity of characteristics between pixels while suppressing the circuit scale. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram showing an example of the configuration of a photoelectric conversion device according to an embodiment of the present invention. [Diagram 2] 2 is a diagram for explaining a configuration example of a pixel arranged in the photoelectric conversion device in FIG. 1; [Diagram 3] 3 is a diagram showing signal waveforms at nodes A and B of the pixel in FIG. 2; [Figure 4] FIG. 3 is an equivalent circuit diagram showing a configuration example of the pixel in FIG. 2. [Diagram 5] 5 is a timing chart showing an example of the operation of the pixel in FIG. 4. [Figure 6] FIG. 5 is an equivalent circuit diagram showing a modification of the pixel in FIG. [Figure 7]7 is a timing chart showing an example of the operation of the pixel in FIG. 6. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, the embodiments will be described in detail with reference to the attached drawings. Note that the following embodiments do not limit the invention according to the claims. Although the embodiments describe a number of features, not all of these features are essential to the invention, and the features may be combined in any manner. Furthermore, in the attached drawings, the same reference numbers are used for the same or similar configurations, and duplicated descriptions are omitted.

[0010] A photoelectric conversion device according to an embodiment of the present disclosure will be described with reference to Fig. 1 to Fig. 7. In the following embodiment, a single-photon avalanche photodiode (hereinafter, sometimes referred to as a SPAD) element will be described, in which electrons are signal carriers and an avalanche photodiode (hereinafter, sometimes referred to as an APD) detects a change in the potential of the cathode of the APD due to an avalanche current when the APD undergoes avalanche breakdown. However, holes may be the signal carriers, and a configuration may be adopted in which a change in the potential of the anode of the APD is detected.

[0011] In this disclosure, as will be described in detail later, a capacitive element is applied to the cathode of the APD via a switch element. The capacitance of the cathode of the APD is C0, and the capacitance of the capacitive element is C1. In addition, when the anode potential of the APD is taken as a reference, the cathode potential of the APD immediately after avalanche breakdown occurs due to signal electrons is the breakdown voltage Vbd of the APD. When a pulse of voltage Vp is applied to the capacitance C1 of this cathode, the cathode potential rises by C1 / (C0+C1)×V due to capacitive division. This voltage becomes the excess voltage Vex. Thereafter, when the switch element is turned off, the cathode is reset to the potential (Vbd+Vex).

[0012] In this disclosure, it is assumed that the variation of the excess voltage Vex is sufficiently smaller than the variation of the breakdown voltage Vbd. If the variation of the excess voltage Vex is not sufficiently smaller than the variation of the breakdown voltage Vbd, the problem of the variation of the breakdown voltage Vbd is simply replaced by the problem of the variation of the excess voltage Vex. The reason why this assumption is true will first be explained below.

[0013] The capacitance C0 of the cathode and the capacitance C1 of the capacitive element vary by several percent depending on the conditions. The variation is set to 5%. Following the numerical example used in the embodiment described later, if C0=3fF, C1=2fF, and the design value of the excess voltage Vex applied to the APD is 2.0V, the pulse voltage Vp applied to the capacitance C1 is 5V. Due to the variation of only the capacitance C0, the capacitance division ratio C1 / (C0+C1) varies by about 3.0%. Similarly, the variation of the capacitance division ratio due to the variation of only the capacitance C1 is also about 3.0%. Due to the variation of both the capacitance C0 and the capacitance C1, the variation of the capacitance division ratio is 3.0%×√2, which is about 4.2%. On the other hand, the variation of the breakdown voltage Vbd of the APD varies by about 500mV depending on the conditions. Therefore, the variation of the excess voltage Vex is 1.5V to 2.5V × 4.2%, and is estimated to be about 63mV to 105mV. In this way, the maximum variation of the estimated excess voltage Vex, 105mV, is sufficiently smaller than the variation of the breakdown voltage Vbd of the APD (±500mV). In other words, the premise for the establishment of the present disclosure, that the variation of the excess voltage Vex is sufficiently smaller than the variation of the breakdown voltage Vbd of the APD, is sufficiently realistic.

[0014] First embodiment 1 is a block diagram showing a configuration example of a photoelectric conversion device 100 according to a first embodiment of the present disclosure. The photoelectric conversion device 100 includes a pixel unit 101, a control pulse generating circuit 115, a horizontal scanning circuit 111, a readout circuit 112, a signal line 113, and a control circuit 110. The pixel unit 101 includes a plurality of pixels 104 arranged in a matrix. Each pixel 104 includes a photoelectric conversion unit 102 including an APD and a signal processing circuit 103. The photoelectric conversion unit 102 converts light incident on the pixel 104 into an electrical signal. The signal processing circuit 103 outputs an electrical signal generated by the photoelectric conversion unit 102 in response to the incident light to the readout circuit 112.

[0015] The control circuit 110 supplies a control pulse to each pixel 104 in response to a pulse signal supplied from the control pulse generating circuit 115. Therefore, it can be said that the control circuit 110 controls the operation of each pixel 104. The control circuit 110 can use a logic circuit such as a shift register or an address decoder.

[0016] The signal output from the photoelectric conversion unit 102 of the pixel 104 is processed by the signal processing circuit 103. The signal processing circuit 103 may be provided with a counter, a memory, and the like, and the digital value counted by the counter may be held in the memory.

[0017] The horizontal scanning circuit 111 inputs a control pulse for sequentially selecting the pixels 104 column by column to the signal processing circuit 103 in order to read out the signal from the memory of the pixels 104 in which the digital signal is held. A signal is output from the signal processing circuit 103 of the pixel 104 selected by the control circuit 110 to a signal line 113. The signal output to the signal line 113 is output via the output circuit 114 to a signal processing device or the like disposed outside the photoelectric conversion device 100, and can be displayed as a captured image on a display device, for example.

[0018] 1, the pixels 104 may be arranged in an array, but are not limited thereto. For example, the pixels 104 may be arranged in a one-dimensional (linear) shape. Also, the function of the signal processing circuit 103 does not necessarily need to be provided for each pixel 104. For example, one signal processing circuit 103 may be shared by a plurality of pixels 104, and signal processing may be performed sequentially.

[0019] FIG. 2 is a block diagram for explaining a configuration example of a pixel 104 arranged in the photoelectric conversion device 100, and is a diagram for explaining a SPAD element including an APD. The pixel 104 is provided with an avalanche photodiode (APD) 201 arranged between a potential supply line 251 and a potential supply line 252. The APD 201 generates a pair of charges according to incident light by photoelectric conversion. A voltage VL is supplied to the anode of the APD 201 from the potential supply line 252. A voltage VH higher than the voltage VL supplied to the anode is supplied to the cathode of the APD 201 from the potential supply line 251. A reverse bias voltage is supplied between the anode and the cathode such that the APD 201 performs an avalanche breakdown operation. By supplying such a voltage, the charges generated by the incident light undergo avalanche multiplication, and an avalanche current is generated (avalanche breakdown).

[0020] The APD201 can operate in a Geiger mode where the potential difference (voltage) between the anode and cathode is greater than the breakdown voltage Vbd, and in a linear mode where the voltage between the anode and cathode is close to or less than the breakdown voltage. An APD that operates in the Geiger mode is called a SPAD element. If the breakdown voltage Vbd of the APD201 is 30V, for example, the voltage VL is set to -30V and the voltage VH is set to 3V.

[0021] The signal processing circuit 103 may include a waveform shaping circuit 210, a count circuit 211, and a selection circuit 212. The signal processing circuit 103 may also include a quench element 202. The quench element 202 is disposed between the APD 201 and a potential supply line 251 that supplies a voltage VH. The quench element 202 has a function of replacing a change in avalanche current occurring in the APD 201 with a voltage signal. The quench element 202 functions as a load circuit (quench circuit) during signal multiplication due to avalanche breakdown, and has a function of suppressing the avalanche breakdown by suppressing the voltage supplied to the APD 201 (quench operation). The quench element 202 may be, for example, a resistive element, or a transistor or the like may function as a load.

[0022] The waveform shaping circuit 210 is connected to a node connected to the cathode or anode of the APD 201, and outputs a signal based on the potential of the electrode (cathode or anode) of the APD 201. In this embodiment, the operation of the waveform shaping circuit 210 will be described later with reference to FIGS. 3(a) to 3(c), and the waveform shaping circuit 210 shapes the potential change of the cathode of the APD 201 obtained when a photon is incident on the APD 201, and outputs a pulse signal. That is, the waveform shaping circuit 210 has a function as a detection circuit for detecting that the APD 201 has undergone avalanche breakdown in response to a change in potential. For example, an inverter circuit is used as the waveform shaping circuit 210. In the configuration shown in FIG. 2, an example in which one inverter is used as the waveform shaping circuit 210 is shown, but for example, a circuit in which a plurality of inverters are connected in series may be used. Any circuit may be used as the waveform shaping circuit 210 as long as it can shape the potential change of the APD 201 into a desired waveform and detect that the APD has undergone avalanche breakdown.

[0023] The count circuit 211 may include a counter that counts the number of times the waveform shaping circuit 210 outputs a pulse signal, and a memory that holds the count value (number of times). When a control pulse is supplied from the control circuit 110 via the drive line 213, the count value held in the count circuit 211 is reset.

[0024] A control pulse is supplied from the control circuit 110 to the selection circuit 212 via a drive line 214, and the selection circuit 212 switches between electrical connection and non-connection between the count circuit 211 and the signal line 113. When the count circuit 211 and the signal line 113 are electrically connected, a count value is output from the count circuit 211 to the signal line 113. The selection circuit 212 may include, for example, a buffer circuit for outputting a signal.

[0025] The electrical connection may be switched by disposing a switching element such as a transistor between the quench element 202 and the APD 201 or between the photoelectric conversion unit 102 and the signal processing circuit 103. Similarly, the supply of the voltage VH or the voltage VL to the photoelectric conversion unit 102 may be electrically switched using a switching circuit such as a transistor.

[0026] In the present embodiment, the count circuit 211 is disposed in the signal processing circuit 103. However, the present invention is not limited to this. Instead of the count circuit 211, a time-to-digital converter (TDC) and a memory may be used to obtain the pulse detection timing of the photoelectric conversion device 100. In this case, the generation timing of the pulse signal output from the waveform shaping circuit 210 is converted into a digital signal by the TDC. A control pulse (reference signal) may be supplied to the TDC from the control circuit 110 via a drive line in order to measure the timing of the pulse signal. The TDC obtains, as a digital signal, a signal obtained when the input timing of the signal output from the APD 201 via the waveform shaping circuit 210 is converted into a relative time based on the control pulse.

[0027] 3(a) to 3(c) are diagrams that diagrammatically show the relationship between the operation of the APD 201 and the output signal of the waveform shaping circuit 210. As shown in Fig. 3(a), a node that represents the potential of the cathode of the electrodes of the APD 201 is referred to as node A. Also, a node that represents the potential of the output of the waveform shaping circuit 210 is referred to as node B. Figs. 3(b) and 3(c) show the waveform changes of the nodes A and B, respectively.

[0028] Between time t0 and time t1, a voltage (VH-VL) is applied to the APD 201. As shown in FIG. 3(b), when a photon is incident on the APD 201 at time t1, an avalanche current flows through the quench element 202, and the potential of the node A drops. When the amount of drop in the potential of the node A further increases and the voltage applied to the APD 201 decreases, the avalanche breakdown of the APD 201 stops, and the potential level of the node A does not drop below a predetermined value (time t2). After that, a current that compensates for the drop in potential flows from the potential supply line 251 to the node A, and at time t3, the node A is stabilized to the original potential level. As shown in FIG. 3(c), when the APD 201 undergoes avalanche breakdown, if the output waveform at the node A exceeds a predetermined threshold, the waveform shaping circuit 210 shapes the output waveform of the node A and outputs a signal (pulse signal) to the node B. The occurrence of avalanche breakdown in the APD 201 is detected by the pulse signal output by the waveform shaping circuit 210.

[0029] 1, the signal processing circuit 103 and the scanning circuit around the pixel unit 101 are depicted as being formed on the same semiconductor substrate as the APD 201. However, this is not limited thereto, and the signal processing circuit 103 and the scanning circuit around the pixel unit 101 may be formed on a semiconductor substrate separate from the pixel unit 101 in which the APD 201 is disposed. In this case, a substrate including the pixel unit 101 in which the pixels 104 including the APD 201 are disposed and a substrate in which the signal processing circuit 103 and the like are disposed may be stacked.

[0030] In the following embodiment, a switch element is provided instead of the quench element 202, and the operation of the switch element being turned on or off is the basis. The details will be described below, but when the switch element is turned off, the node A is in a floating state. In this case, when signal electrons are incident on the APD 201 and an avalanche occurs, the potential of the node A becomes a potential (VL+Vbd) lower than the voltage VH. This is the same whether one or multiple signal electrons are incident on the APD 201. When no signal electrons are incident, the potential of the node A is the voltage VH. When the node A becomes a potential (VL+Vbd), that is, when it is detected that an avalanche breakdown has occurred, the switch element is changed to an on state under the control of the control circuit 110, and the node A is reset by capacitive coupling.

[0031] From here, the configuration and operation of the pixel 104 of this embodiment will be described in detail. Fig. 4 is an equivalent circuit diagram showing a configuration example of the pixel 104 in this embodiment. The pixel 104 includes an APD 201 arranged between a potential supply line 351 and a potential supply line 352, a switch element 305 arranged between the potential supply line 351 and the APD 201, a switch element 303 arranged between the switch element 305 and the APD 201, and a capacitive element 301 having a first terminal 300 and a second terminal connected to a node C to which the switch element 305 and the switch element 303 are connected. As shown in Fig. 4, the switch element 305 and the switch element 303 can each be an N-type MOS transistor.

[0032] The pixel 104 also includes a detection circuit 331 for detecting that the APD 201 has undergone avalanche breakdown in response to a change in the potential of a node A to which the switch element 303 and the APD 201 are connected. The node A is a node that indicates the potential of the cathode of the electrodes of the APD 201. The detection circuit 331 includes the above-mentioned waveform shaping circuit 210 and count circuit 211. As shown in FIG. 4, the waveform shaping circuit 210 may be an inverter that is connected to the node A and outputs a signal (pulse signal) indicating that the avalanche breakdown has been detected in response to a change in the potential of the node A. The detection circuit 331 may further include a switch element 307 arranged between the waveform shaping circuit 210 and the count circuit 211, and a switch element 309 arranged between a node D to which the switch element 307 and the count circuit 211 are connected and a potential supply line 353. The switch element 307 and the switch element 309 may be N-type MOS transistors. The potential supply line 353 is supplied with the ground voltage GND.

[0033] 5 is a timing diagram for explaining an example of the operation of the pixel 104 of this embodiment. The control pulse 302 is a signal input to the first terminal 300 of the capacitance element 301. While the control pulse 302 is at H level, the voltage Vp is supplied to the first terminal 300 of the capacitance element 301. The control pulse 304 is a signal input to the control terminal of the switch element 303. The control pulse 306 is a signal input to the control terminal of the switch element 305. The control pulse 308 is a signal input to the control terminal of the switch element 307. The control pulse 310 is a signal input to the control terminal of the switch element 309. While the control pulses 304, 306, 308, and 310 are at H level, the switch elements 303, 305, 307, and 309 are respectively in the ON state, and while the control pulses 304, 306, 308, and 310 are at L level, the switch elements 303, 305, 307, and 309 are respectively in the OFF state. The control pulses 302, 304, 306, 308, and 310 can be supplied from the control circuit 110 via drive lines. Fig. 5 shows the control pulses 302, 304, 306, 308, and 310 and the potential changes of the nodes A, C, and D. In Fig. 5, the timing at which photons are incident on the APD 201 is indicated by "↓".

[0034] In explaining the operation, first, the relationship between the voltages (potentials) of the respective components of the pixel 104 will be described. The designed breakdown voltage Vbd of the APD 201 is 30V, varying within a range of ±0.5V. The designed value of the excess voltage Vex applied to the APD 201 is 2.0V. The voltage VL supplied to the potential supply line 352 is −29.5V. If the potential of the node A immediately after the APD 201 causes an avalanche breakdown is Vb, then Vb=(VL+Vbd). When the breakdown voltage Vbd of the APD is 30.5V, Vb=1.0V, and when the breakdown voltage Vbd is 29.5V, Vb=0.0V.

[0035] Next, the operation of the pixel 104 will be described. In this embodiment, the node A to which the cathode of the APD 201 is connected basically operates in a floating state. As will be described later, the node A is reset to a potential Vh, and when the APD 201 captures signal electrons and causes avalanche breakdown, the node A drops to a potential Vb. The switch element 307 periodically connects the output of the waveform shaping circuit 210 using an inverter to a node D, which is an input of the count circuit 211, by a control pulse 308, which is periodically supplied with an H level. The potential of the node D is reset to the ground voltage GND, that is, the L level, by a control pulse 310 slightly preceding the control pulse 308. When the control pulse 308 is at an H level, the count of the count circuit 211 advances when the potential of the node D changes from an L level to an H level. In other words, if the node A is at a potential Vb when the control pulse 308 becomes an H level, the count of the count circuit 211 advances by one. This detects the avalanche breakdown of the APD 201. In the case of the above-mentioned voltage (potential) relationship, the threshold voltage at which the inverter-based waveform shaping circuit 210 outputs a signal indicating that the avalanche breakdown has been detected may be, for example, 1.5 V.

[0036] Next, the reset operation of node A will be described. The reset operation is performed by control pulses 302, 304, and 306 supplied from the control circuit 110. However, basically, the reset operation is performed only when node A is at potential Vb. The control circuit 110 performs a reset operation to reset the potential of node A in response to the detection circuit 331 detecting an avalanche breakdown. That is, when node A is at potential Vh, the reset operation does not have to be performed. Specifically, when node A is at potential Vh, the control pulses 302 and 304 do not have to be at H level. For this determination, a logical AND (AND operation) of a control pulse that periodically becomes H level, which is the source of the control pulses 302 and 304, and the level of node D may be performed, and the control pulses 302 and 304 may be supplied. In the timing diagram shown in FIG. 5, the H level pulses indicated by dotted lines of the control pulses 302 and 304 indicate the control pulses that periodically become H level, which are the source of the control pulses 302 and 304 before the AND operation is performed. In the pixel 104 where the node D is at the L level, that is, the node A is at the potential Vh, the control pulses 302 and 304 remain at the L level as indicated by the solid lines.

[0037] In this way, the detection circuit 331 performs an operation for detecting an avalanche breakdown of the APD 201 at a predetermined period. The reset operation of the node A is configured to be executable at the same period as the period in which the detection circuit 331 detects an avalanche breakdown. If the detection circuit 331 does not detect an avalanche breakdown, the control circuit 110 does not perform a reset operation of the node A, regardless of the timing at which the reset operation can be performed, for example, by using the above-mentioned AND operation. On the other hand, if the detection circuit 331 detects an avalanche breakdown of the APD 201, the control circuit 110 performs the reset operation shown below. The control circuit 110 can perform the reset operation at the timing immediately after the detection circuit 331 detects an avalanche breakdown among the timings at which the reset operation can be performed.

[0038] Next, consider the case where, when the node D becomes H level, a photon is incident on the APD 201, causing an avalanche breakdown, and the node A becomes the potential Vb. Here, the capacitance of the node A is the above-mentioned capacitance C0 of the cathode, and the capacitance of the node C is approximately the capacitance C1 of the capacitive element 301. As described above, the values ​​of the capacitances C0 and C1 are, for example, C0=3fF and C1=2fF. Also, the voltage VR supplied to the potential supply line 351 is set to, for example, 0.5V, which is the same as the average value of the potential Vb (the potential Vb when the breakdown voltage Vbd of the APD 201 is 30V). In the following description, the breakdown voltage Vbd of the APD 201 varies, and the case where Vbd=29.5V will be mainly described.

[0039] When the control pulse 306 goes to H level, the potential of the node C becomes 0.5 V in accordance with the voltage VR of the potential supply line 351. At this time, the potential Vb of the node A is Vb=(VL+Vbd)=0.0 V. Next, when the control pulse 304 goes to H level, the nodes A and C become conductive, and the potentials thereof become (Vb×C0+VR×C1) / (C0+C1)···(1) In the above-mentioned voltage (potential) relationship, it is 0.2 V. In this state, the control pulse 302 becomes H level, and the voltage Vp is applied to the first terminal 300 of the capacitance element 301. At this time, with respect to the design value of the excess voltage Vex, Excess voltage Vex = {C1 / (C0+C1)} × Vp (2) The value of the voltage Vp is set so that: In the above-mentioned voltage (potential) relationship, the design value of the excess voltage Vex is 2.0V, so the voltage Vp is 5.0V. Therefore, the voltage Vp applied to the first terminal 300 of the capacitance element 301 pushes up the potentials of the nodes A and C by the excess voltage Vex due to the capacitive coupling between the capacitance C0 of the cathode of the APD 201 and the capacitance C1 of the capacitance element 301. In the above-mentioned voltage (potential) relationship, the potentials are pushed up by 2.0V from 0.2V to 2.2V. In this state, when the control pulse 304 goes to the L level and the switch element 303 goes to the OFF state, the reset of the node A is completed. In the above-mentioned voltage (potential) relationship, the potential Vh of the node A is reset to 2.2V.

[0040] In this way, in the reset operation, the control circuit 110 turns the switch element 303 to an ON state and applies a predetermined voltage Vp to the first terminal 300 of the capacitance element 301. After applying the predetermined voltage Vp to the first terminal 300 of the capacitance element 301, the control circuit 110 turns the switch element 303 from an ON state to an OFF state, and then ends the application of the voltage Vp to the first terminal 300 of the capacitance element 301. Before performing the reset operation, the control circuit 110 turns the switch element 305 to an ON state and resets the node C to the voltage VR supplied from the potential supply line 351. As a result, the node A is reset to the potential Vh. After performing the reset operation, the control circuit 110 controls the switch element 303 to an OFF state until the detection circuit 331 detects the avalanche breakdown of the APD 201. As a result, as described above, the node A basically operates in a floating state while the APD 201 is performing the operation of detecting photons.

[0041] When the control pulse 302 returns to the L level in the reset operation, the potential of the node C tends to drop by the voltage Vp due to the capacitive coupling between the capacitors C0 and C1. In the above-mentioned voltage (potential) relationship, the potential tends to drop from 2.2V to 5V. When the potential of the node C becomes negative, it becomes forward biased with respect to the GND potential setting well of the switch element 303, which is an N-type MOS transistor, and there is a possibility that the forward bias current of the switch element 303 flows into the node A. However, by appropriately setting the threshold values ​​Vth of the switch elements 303 and 305, it is possible to prevent the forward bias current of the switch element 303 from flowing into the node A. For example, the threshold value Vth of the switch element 303 is set to 0.6V, and the threshold value Vth of the switch element 305 is set to 0.0V. In other words, the threshold value of the switch element 305 is set to be lower than the threshold value of the switch element 303. With these settings, the potential of the node C is clipped to 0.0V. This is because at 0.0 V or less, the switch element 305, which is an N-type MOS transistor, is turned on, and electrical continuity is established between the node C and the potential supply line 351. This prevents leakage current to the node A via the switch element 303.

[0042] The operation of setting the node A to the potential Vh after the avalanche breakdown of the APD 201 described above is a reset operation of the node A using the capacitive coupling between the capacitance C0 of the node A (cathode of the APD 201) and the capacitance C1 of the node C (capacitive element 301). In the example of the timing diagram shown in FIG. 5, a case is shown in which no photons are incident on the APD 201 by the time of the next reset timing, and the node A is not reset, and further, the node A is reset again at the next reset timing. In the above, a case has been described in which the control pulse 302 does not become H level when the node D is at L level, but this is not limited to this. Regardless of the state of the node D, the control pulse 302 may be periodically supplied with an H level, similarly to the control pulses 306, 308, and 310. This is because, if the switch element 303 is in an off state, even if the voltage Vp is supplied to the first terminal 300 of the capacitive element 301 by the control pulse 302, it does not affect the node A.

[0043] As described above, the variation in the breakdown voltage Vbd is reflected to some extent in the excess voltage Vex applied to the pixel 104. For example, if the variation in the breakdown voltage Vbd is ΔVbd and the variation in the excess voltage Vex is ΔVex, then ΔVex=C0 / (C0+C1)×ΔVbd (3) In the above-mentioned voltage (potential) relationship, ΔVbd=0.5V, ΔVex=0.2V. More specifically, when the breakdown voltage Vbd is 29.5V, the excess voltage Vex is 2.2V, and when the breakdown voltage Vbd is 30.5V, the excess voltage Vex is 1.8V. That is, the variation (±0.5V) of the excess voltage Vex when a constant voltage is applied to the APD 201 without performing a reset operation becomes ±0.2V. In this way, the variation of the excess voltage Vex is greatly reduced. As a result, the excess voltage Vex changes only ±0.2V with respect to the variation of the breakdown voltage Vbd of ±0.5V. In order to reduce ΔVex, it is understood from the formula (3) that the capacitance C1 should be set smaller than the capacitance C0. In this case, as can be seen from the formula (2), the voltage Vp supplied to the first terminal 300 of the capacitance element 301 becomes large. The values ​​of the capacitances C0 and C1 may be appropriately designed depending on the characteristics required for the pixel 104 including the APD 201 and the performance and specifications required for the photoelectric conversion device 100 including the pixel 104.

[0044] When the photoelectric conversion device 100 is operated, it is necessary to set the excess voltage Vex so that the APD 201 arranged in all the pixels 104 undergoes avalanche breakdown in response to the incidence of photons. In this case, if the voltage applied to the APD 201 is constant for each pixel 104, an excessive excess voltage Vex is applied to the APD 201 having a small breakdown voltage Vbd due to variations in the breakdown voltage Vbd of the APD 201. When an excessive excess voltage Vex is applied, the energy required for signal detection increases, so that the power consumption of the photoelectric conversion device 100 as a whole increases. Furthermore, when an excessive excess voltage Vex is applied, the amount of light emitted at the time of avalanche breakdown increases, and crosstalk increases. Furthermore, when an excessive excess voltage Vex is applied, the characteristics of the APD 201 deteriorate quickly, thereby reducing reliability, and the dark current of the APD 201 increases, which may increase noise.

[0045] On the other hand, the reset operation of node A using the above-mentioned capacitive coupling can suppress variations in the excess voltage Vex applied to the APD 201 of each pixel 104. This can suppress variations in the probability of avalanche breakdown occurring in response to incident photons on the APD 201, which would otherwise occur due to variations in the excess voltage Vex, and can suppress variations in sensitivity for each pixel 104. Furthermore, suppressing variations in the excess voltage Vex can realize a photoelectric conversion device 100 including a SPAD element having excellent characteristics, such as reduced power consumption, small crosstalk, high reliability, and small dark current.

[0046] Second embodiment Next, a second embodiment of the present disclosure will be described. In this embodiment, the following events are assumed when performing the operations described in the first embodiment, and the second embodiment deals with these events.

[0047] When the node A is reset by the above-mentioned reset operation and the period until the next photon is incident on the APD 201 is long, the node A may not be able to maintain the potential Vh. Although a large reverse bias voltage is applied to the APD 201, a leakage current that does not cause avalanche breakdown may flow between the anode and cathode of the APD 201. It is assumed that the potential of the cathode of the APD 201, that is, the node A, decreases little by little due to this leakage current. It is assumed that the potential of the node A continues to decrease due to the leakage current of the APD 201, and the potential of the node A becomes 1.4V, which is lower than the threshold value (for example, 1.5V) of the waveform shaping circuit 210 using an inverter, even though the APD 201 does not cause avalanche breakdown. At this time, when the control pulse 308 is supplied, the node D transitions to the H level. In this case, the reset operation is performed as described above, and the node A is reset. In the above-mentioned numerical example, when the breakdown voltage Vbd of the APD 201 is 29.5V, the potential Vb at the node A when the APD 201 undergoes avalanche breakdown is 0V, and the potential Vh at the node A after the reset operation should be 2.2V. However, when the potential at the node A drops as described above, the node A is reset in a state in which the potential Vb is 1.4V. In this case, in the numerical example given in the above-mentioned voltage (potential) relationship, the H level is applied to the control pulse 304 from the formula (1), and the potential at the node A when the node A and the node C are conductive becomes 1.04V. Next, the voltage Vp is applied to the first terminal 300 of the capacitance element 301, and the potential at the node A after the reset operation is completed becomes 3.04V. In other words, the node A is reset to a state in which an excessive excess voltage Vex is applied. In this embodiment, a configuration and operation for suppressing such an operation will be described.

[0048] 6 is an equivalent circuit diagram showing a configuration example of the pixel 104 in this embodiment. In this embodiment, compared to the above-mentioned first embodiment, the detection circuit 331 further includes a change detection circuit 341 that is connected to the node A and detects a change in the potential of the node A that is smaller than the change in the potential of the inverter-based waveform shaping circuit 210 that outputs a signal indicating that an avalanche breakdown has been detected, and a reset circuit 342 that resets the potential of the node A to a predetermined potential in response to the change detection circuit 341 detecting the change in the potential of the node A.

[0049] For example, the change detection circuit 341 includes an inverter 313 that is connected to node A and has a threshold value different from that of the inverter-based waveform shaping circuit 210 that outputs a signal. The inverter 313 has a threshold value set so as to output a signal when the change in potential of node A is smaller than that of the inverter used as the waveform shaping circuit 210. For example, if the threshold value of the inverter used as the waveform shaping circuit 210 is 1.5V, the threshold value of the inverter 313 may be 1.8V. The following description will be given using the above threshold values ​​as numerical examples.

[0050] Moreover, the change detection circuit 341 may include a switch element 314 and a switch element 315 in addition to the inverter 313. The switch element 314 is disposed between a node E, which is an output node of the change detection circuit 341, and the inverter 313. The node E is a node that temporarily holds the output state of the inverter 313, and may be a simple capacitance. The switch element 315 is disposed between the node E and a potential supply line 353 in order to reset the node E to a voltage GND supplied to the potential supply line 353. The switch element 314 may be controlled by a control pulse 308 in the same manner as the switch element 307. The switch element 315 may be controlled by a control pulse 310 in the same manner as the switch element 309. By controlling the switch element 307 and the switch element 314 (the switch element 309 and the switch element 315) using the same control pulse 308 (the control pulse 310), the number of output terminals and the number of drive lines of the control circuit 110 can be suppressed. The switch element 314 and the switch element 315 may be N-type MOS transistors.

[0051] The reset circuit 342 includes a switch element 311 disposed between the potential supply line 251 and the node A. As shown in FIG. 6, the switch element 311 may be a P-type MOS transistor. A voltage VH is supplied to the potential supply line 251. The voltage VH is a power supply potential that directly resets the node A, and may be set to a value obtained by adding a design value of the excess voltage Vex to an average value of the potential Vb (a potential Vb when the breakdown voltage Vbd of the APD 201 is 30V (0.5V in the above-mentioned voltage (potential) relationship)). The voltage VH may also be a value obtained by adding a design value of the excess voltage Vex to a maximum value of the potential Vb that varies according to the variation of the breakdown voltage Vbd of the APD 201. Here, Vb=0.5V, Vex=2.0V, and VH=2.5V.

[0052] FIG. 7 is a timing diagram for explaining an example of the operation of the pixel 104 of this embodiment. The control pulse 312 is a signal input to the control terminal of the switch element 311. While the control pulse 312 is at L level, the switch element 311 is in an ON state, and while the control pulse 312 is at H level, the switch element 311 is in an OFF state. The control pulse 312 can be supplied from the control circuit 110 via a drive line. The control pulses 302, 304, 306, 308, and 310 are the same as those in the first embodiment described above. FIG. 7 shows the control pulses 302, 304, 306, 308, 310, and 312 and the potential changes of the nodes A, D, and E. Also in FIG. 7, the timing at which photons are incident on the APD 201 is indicated by "↓".

[0053] In the timing diagram shown in FIG. 7, photons are incident on the APD 201, and the node A is reset by the control pulses 302, 304, and 306, which is the same as in the first embodiment. After that, no photons are incident on the APD 201, and the potential of the node A gradually decreases during that time. Here, it is assumed that the potential of the node A has decreased below the threshold voltage 1.8V of the inverter 313 by the time the third H level of the control pulse 308 shown in FIG. 7 is supplied. When the third H level of the control pulse 308 is supplied, the potential of the node A becomes higher than the threshold voltage 1.5V of the waveform shaping circuit 210 using an inverter and lower than the threshold voltage 1.8V of the inverter 313. Therefore, a signal is output from the inverter 313, and the node E changes to the H level, but the waveform shaping circuit 210 does not output a signal, so the node D remains at the L level.

[0054] Since the potential change at node A due to avalanche breakdown is rapid, when avalanche breakdown occurs in the APD 201, the potential at node A exceeds the thresholds of both the waveform shaping circuit 210 and the inverter 313. Therefore, both the waveform shaping circuit 210 and the inverter 313 output signals, and the nodes D and E become H level. On the other hand, the potential change at node A due to leakage current of the APD 201 is gradual. Therefore, by the control pulse 308 supplying H level at an appropriate period, the inverter 313 can capture the drop in the potential at node A due to leakage current before the output change of the waveform shaping circuit 210 using an inverter occurs. This allows the reset circuit 342 to reset node A.

[0055] When node D is at L level and node E is at H level, L level is supplied to control pulse 312, and node A is reset to voltage VH. This control pulse 312 can be realized by, for example, a logical AND operation of three inputs: a drive pulse that becomes L level at a constant cycle (a drive pulse including an L level indicated by a dotted line in control pulse 312) that is the source of control pulse 312 shown in Fig. 7, an inverted level of node D, and the level of node E. For example, control circuit 110 may be provided with a three-input AND gate to which the drive pulse that becomes L level at a constant cycle that is the source of control pulse 312, the inverted level of node D, and the level of node E are respectively supplied.

[0056] In the above-mentioned voltage (potential) relationship, the potential after the node A is reset by the reset circuit 342 is 2.5V (the voltage VH of the potential supply line 251), and the potential Vb is 0.0V, so the excess voltage Vex is 2.5V. Therefore, the excess voltage Vex after being reset by the reset circuit 342 is greater than the design value 2.0V of the excess voltage Vex. In addition, the potential (2.5V) of the node A after being reset by the reset circuit 342 is higher than the potential Vh (2.2V) of the node A when it is reset by the above-mentioned reset operation after the avalanche breakdown of the APD 201. Therefore, the excess voltage Vex after being reset using the reset circuit 342 is greater than the excess voltage Vex applied by the reset operation after the avalanche breakdown. However, the potential (2.5 V) of the node A after being reset by the reset circuit 342 is lower than the potential (3.04 V) applied to the APD 201 in the reset operation performed by the control pulses 302, 304, and 306 when there is a leakage current. In other words, it can be seen that the excessively applied excess voltage Vex is significantly alleviated.

[0057] Also, the reset of node A caused by the leakage current of the APD 201 is considered to occur less frequently than the reset operation performed after the above-mentioned avalanche breakdown of the APD 201. In other words, the reset of node A caused by the leakage current of the APD 201 occurs only very rarely, and most resets of node A are reset operations corresponding to avalanche breakdown. Therefore, it is possible to suppress variations in the excess voltage Vex applied to the APD 201 of each pixel 104.

[0058] In this embodiment, the reset operation after the avalanche breakdown of the APD 201 is performed in the same manner as in the first embodiment. Furthermore, according to this embodiment, it is possible to suppress the reset operation accompanied by an excessive excess voltage Vex caused by the leakage current of the APD 201. As a result, it is possible to realize a photoelectric conversion device 100 including a SPAD element with reduced power consumption, reduced crosstalk, high reliability, small dark current, and excellent characteristics.

[0059] The disclosure of this specification includes the following photoelectric conversion device.

[0060] (Item 1) A photoelectric conversion device having a plurality of pixels, Each of the plurality of pixels is an avalanche photodiode disposed between the first potential supply line and the second potential supply line; a first switch element disposed between the first potential supply line and the avalanche photodiode; a second switch element disposed between the first switch element and the avalanche photodiode; a capacitance element including a first terminal and a second terminal connected to a first node to which the first switch element and the second switch element are connected; a detection circuit for detecting avalanche breakdown of the avalanche photodiode in response to a change in potential of a second node to which the second switch element and the avalanche photodiode are connected; A photoelectric conversion device comprising:

[0061] (Item 2) Further comprising a control circuit; the control circuit performs a reset operation of resetting the potential of the second node in response to the detection circuit detecting an avalanche breakdown; 2. The photoelectric conversion device according to item 1, wherein in the reset operation, the control circuit turns on the second switch element and applies a predetermined voltage to the first terminal.

[0062] (Item 3) 3. The photoelectric conversion device according to item 2, wherein in the reset operation, the control circuit changes the second switch element from an on state to an off state after applying a predetermined voltage to the first terminal, and then terminates application of the predetermined voltage to the first terminal.

[0063] (Item 4) 4. The photoelectric conversion device according to item 2 or 3, wherein after the reset operation is performed, the control circuit controls the second switch element to an off state until the detection circuit detects an avalanche breakdown.

[0064] (Item 5) The photoelectric conversion device according to any one of items 2 to 4, characterized in that before performing the reset operation, the control circuit turns on the first switch element and resets the first node to a voltage supplied from the first potential supply line.

[0065] (Item 6) 6. The photoelectric conversion device according to any one of items 1 to 5, wherein the detection circuit performs an operation for detecting an avalanche breakdown at a predetermined period.

[0066] (Item 7) the detection circuit performs an operation for detecting an avalanche breakdown at a predetermined period; The reset operation is configured to be executable in the same cycle as the predetermined cycle, the control circuit performs the reset operation at a timing immediately after the detection circuit detects an avalanche breakdown among the timings at which the reset operation can be performed; The photoelectric conversion device described in any one of items 2 to 5, characterized in that if the detection circuit does not detect an avalanche breakdown, the control circuit does not perform the reset operation regardless of the timing at which the reset operation can be performed.

[0067] (Item 8) The photoelectric conversion device described in any one of items 1 to 7, characterized in that the detection circuit includes an inverter connected to the second node and outputting a signal indicating that an avalanche breakdown has been detected in response to a change in the potential of the second node.

[0068] (Item 9) The detection circuit, a change detection circuit connected to the second node and configured to detect a change in potential of the second node that is smaller than the change in potential at which the inverter outputs the signal; a reset circuit that resets the potential of the second node to a predetermined potential in response to the change detection circuit detecting a change in the potential of the second node; 9. The photoelectric conversion device according to item 8, further comprising:

[0069] (Item 10) 10. The photoelectric conversion device according to item 9, wherein the reset circuit includes a third switch element disposed between a third potential supply line and the second node.

[0070] (Item 11) The detection circuit, an inverter connected to the second node and outputting a signal indicating that an avalanche breakdown has been detected in response to a change in potential of the second node; a change detection circuit connected to the second node and configured to detect a change in potential of the second node that is smaller than the change in potential at which the inverter outputs the signal; a reset circuit that resets the potential of the second node to a predetermined potential in response to the change detection circuit detecting a change in the potential of the second node; Including, the reset circuit includes a third switch element disposed between a third potential supply line and the second node, 8. The photoelectric conversion device according to any one of items 2 to 5 and 7, wherein the predetermined potential is higher than the potential of the second node when reset by the reset operation.

[0071] (Item 12) 12. The photoelectric conversion device according to item 10 or 11, wherein the third switch element is a P-type MOS transistor.

[0072] (Item 13) A photoelectric conversion device described in any one of items 9 to 12, characterized in that the change detection circuit includes an inverter other than the inverter that is connected to the second node and has a different threshold for outputting a signal from the inverter.

[0073] (Item 14) 14. The photoelectric conversion device according to any one of items 1 to 13, wherein the first switch element and the second switch element are N-type MOS transistors.

[0074] (Item 15) 15. The photoelectric conversion device according to item 14, wherein the threshold value of the first switch element is lower than the threshold value of the second switch element.

[0075] The invention is not limited to the above-described embodiments, and various modifications and variations are possible without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0076] 100: photoelectric conversion device, 104: pixel, 201: avalanche photodiode, 300: first terminal, 301: capacitance element, 303, 305: switch elements, 331: detection circuit, 351, 352: potential supply lines, A, C: nodes

Claims

1. A photoelectric conversion device in which a plurality of pixels and a control circuit are arranged, each of the plurality of pixels includes: an avalanche photodiode arranged between a first potential supply line and a second potential supply line; a first switch element arranged between the first potential supply line and the avalanche photodiode; a second switch element arranged between the first switch element and the avalanche photodiode; a capacitor element including a first terminal and a second terminal connected to a first node to which the first switch element and the second switch element are connected; a detection circuit for detecting that the avalanche photodiode has undergone avalanche breakdown in response to a change in the potential of a second node to which the second switch element and the avalanche photodiode are connected; and includes the control circuit performs a reset operation of resetting the potential of the second node in response to the detection circuit detecting avalanche breakdown, in the reset operation, the control circuit turns on the second switch element and applies a predetermined voltage to the first terminal, after applying the predetermined voltage to the first terminal, turns off the second switch element from the on state, and then ends the application of the predetermined voltage to the first terminal. A photoelectric conversion device characterized by this.

2. A photoelectric conversion device in which a plurality of pixels and a control circuit are arranged, each of the plurality of pixels includes: an avalanche photodiode arranged between a first potential supply line and a second potential supply line; a first switch element arranged between the first potential supply line and the avalanche photodiode; a second switch element arranged between the first switch element and the avalanche photodiode; a capacitor element including a first terminal and a second terminal connected to a first node to which the first switch element and the second switch element are connected; a detection circuit for detecting that the avalanche photodiode has undergone avalanche breakdown in response to a change in the potential of a second node to which the second switch element and the avalanche photodiode are connected; and includes the control circuit performs a reset operation of resetting the potential of the second node in response to the detection circuit detecting avalanche breakdown, in the reset operation, the control circuit turns on the second switch element and applies a predetermined voltage to the first terminal, After performing the reset operation, the control circuit controls the second switching element to be in an off state until the detection circuit detects avalanche breakdown. A photoelectric conversion device characterized by this.

3. In the reset operation, the control circuit applies a predetermined voltage to the first terminal, then changes the second switching element from an on state to an off state, and then ends the application of the predetermined voltage to the first terminal. The photoelectric conversion device according to claim 2, characterized by this.

4. Before performing the reset operation, the control circuit turns on the first switching element and resets the first node to the voltage supplied from the first potential supply line. The photoelectric conversion device according to claim 1 or 2, characterized by this.

5. The detection circuit performs an operation for detecting avalanche breakdown at a predetermined period. The photoelectric conversion device according to claim 1 or 2, characterized by this.

6. The detection circuit performs an operation for detecting avalanche breakdown at a predetermined period, the reset operation is configured to be executable at the same period as the predetermined period, the control circuit performs the reset operation at the timing immediately after the detection circuit detects avalanche breakdown among the timings at which the reset operation is executable, when the detection circuit does not detect avalanche breakdown, the control circuit does not perform the reset operation regardless of the timing at which the reset operation is executable. The photoelectric conversion device according to claim 1 or 2, characterized by this.

7. A photoelectric conversion device in which a plurality of pixels are arranged, each of the plurality of pixels an avalanche photodiode arranged between a first potential supply line and a second potential supply line, a first switching element arranged between the first potential supply line and the avalanche photodiode, a second switching element arranged between the first switching element and the avalanche photodiode, a capacitor element including a first terminal and a second terminal connected to a first node to which the first switching element and the second switching element are connected, a detection circuit for detecting that the avalanche photodiode has undergone avalanche breakdown in response to a change in the potential of a second node to which the second switching element and the avalanche photodiode are connected, including the detection circuit An inverter connected to the second node and outputting a signal indicating that an avalanche breakdown has been detected in response to a change in the potential of the second node; A change detection circuit connected to the second node and detecting a change in the potential of the second node that is smaller than the change in the potential of the second node when the inverter outputs the signal; A reset circuit that resets the potential of the second node to a predetermined potential in response to the change detection circuit detecting a change in the potential of the second node; A photoelectric conversion device, characterized by including the above.

8. The photoelectric conversion device according to claim 7, characterized in that the reset circuit includes a third switch element disposed between a third potential supply line and the second node.

9. The detection circuit is An inverter connected to the second node and outputting a signal indicating that an avalanche breakdown has been detected in response to a change in the potential of the second node; A change detection circuit connected to the second node and detecting a change in the potential of the second node that is smaller than the change in the potential of the second node when the inverter outputs the signal; A reset circuit that resets the potential of the second node to a predetermined potential in response to the change detection circuit detecting a change in the potential of the second node; Including The reset circuit includes a third switch element disposed between a third potential supply line and the second node, The photoelectric conversion device according to claim 1 or 2, characterized in that the predetermined potential is higher than the potential of the second node when reset by the reset operation.

10. The photoelectric conversion device according to claim 8, characterized in that the third switch element is a P-type MOS transistor.

11. The photoelectric conversion device according to claim 7, characterized in that the change detection circuit includes another inverter connected to the second node and having a different signal output threshold from that of the inverter.

12. The photoelectric conversion device according to any one of claims 1, 2, and 7, characterized in that the first switch element and the second switch element are N-type MOS transistors.

13. The photoelectric conversion device according to claim 12, characterized in that the threshold value of the first switch element is lower than the threshold value of the second switch element.

14. Further including a control circuit, The control circuit performs a reset operation to reset the potential of the second node in response to the detection circuit detecting an avalanche breakdown. In the reset operation, the control circuit turns on the second switching element and applies a predetermined voltage to the first terminal, The photoelectric conversion device according to claim 7, wherein after applying the predetermined voltage to the first terminal, the second switching element is turned off from the on state, and then the application of the predetermined voltage to the first terminal is terminated.

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