Photoelectric conversion device
A photoelectric conversion device includes a photoelectric conversion unit including a first semiconductor region, a second semiconductor region, and a third semiconductor region forming a p-n junction with the second semiconductor region, and a control circuit for switching a voltage of a reverse bias voltage applied between the first and the second semiconductor regions between first and second voltages. The photoelectric conversion device performs a period in which the reverse bias voltage is set to the first voltage to accumulate signal charge in the third semiconductor region, and a period in which the reverse bias voltage is set to the second voltage to enable transfer of signal charge to the first semiconductor region, and avalanche multiplication at the p-n junction between the first and second semiconductor regions. The control circuit switches between the first voltage and the second voltage by changing the reverse bias voltage to a rectangular shape.
1 . A photoelectric conversion device comprising:
a photoelectric conversion unit including
a first semiconductor region of a first conductivity type having carriers of same polarity as a signal charge as majority carriers,
a second semiconductor region of a second conductivity type forming a p-n junction with the first semiconductor region, and
a third semiconductor region of the first conductivity type forming a p-n junction with the second semiconductor region; and
a control circuit configured to switch a reverse bias voltage, applied between the first semiconductor region and the second semiconductor region, between a first voltage and a second voltage larger than the first voltage,
wherein the photoelectric conversion device is configured to perform
a signal accumulation operation period of accumulating a signal charge generated by an incidence of a photon on the photoelectric conversion unit in the third semiconductor region by setting the reverse bias voltage to the first voltage less than a breakdown voltage of the p-n junction between the first semiconductor region and the second semiconductor region, and
a signal readout operation period of enabling a transfer of the signal charge from the third semiconductor region to the first semiconductor region and an avalanche multiplication at the p-n junction between the first semiconductor region and the second semiconductor region by setting the reverse bias voltage to the second voltage greater than the breakdown voltage, and
wherein the control circuit is configured to switch the reverse bias voltage between the first voltage and the second voltage by changing a waveform in a rectangular shape.
2 . The photoelectric conversion device according to claim 1 , further comprising:
an avalanche multiplication detection circuit configured to detect an avalanche multiplication occurred at the p-n junction between the first semiconductor region and the second semiconductor region,
wherein the photoelectric conversion device is configured to repeatedly perform the signal accumulation operation period and the signal readout operation period a plurality of times, and count the number of times of the avalanche multiplication detected by the avalanche multiplication detection circuit.
3 . The photoelectric conversion device according to claim 1 , wherein the control circuit is configured to repeatedly perform a plurality of unit periods each including the signal accumulation operation period and the signal readout operation period during one frame period.
4 . The photoelectric conversion device according to claim 2 , further comprising: a counter configured to count the number of times of avalanche multiplication detected by the avalanche multiplication detection circuit during one frame period.
5 . The photoelectric conversion device according to claim 1 , wherein a transition time required for switching between the first voltage and the second voltage is 100 ns or less.
6 . The photoelectric conversion device according to claim 1 , wherein a length of the signal readout operation period is equal to or less than 1 / 100 of a length of the signal accumulation operation period.
7 . The photoelectric conversion device according to claim 1 , wherein a length of the signal accumulation operation period is set such that the number of signal charge generated by an incident of a photon on the photoelectric conversion unit is at most one.
8 . The photoelectric conversion device according to claim 2 , wherein the avalanche multiplication detection circuit is configured to output one pulse signal indicating an occurrence of avalanche multiplication regardless of the number of occurrences of the avalanche multiplication, when one or more avalanche multiplications occur during the signal readout operation period.
9 . The photoelectric conversion device according to claim 2 , wherein the first voltage is a voltage that forms a potential barrier in the second semiconductor region that prevents the signal charge from being transferred from the third semiconductor region to the first semiconductor region.
10 . The photoelectric conversion device according to claim 1 , wherein, in a transitional state in which the reverse bias voltage is switched from the first voltage to the second voltage, the reverse bias voltage when a transfer of the signal charge accumulated in the third semiconductor region to the first semiconductor region is started is a voltage greater than the breakdown voltage by 1 V or more.
11 . The photoelectric conversion device according to claim 2 , wherein, in a transitional state in which the reverse bias voltage is switched from the first voltage to the second voltage, the avalanche multiplication detection circuit is configured to enable detection of avalanche multiplication when the reverse bias voltage reaches a voltage at which a transfer of the signal charge accumulated in the third semiconductor region to the first semiconductor region is started.
12 . The photoelectric conversion device according to claim 1 , wherein the control circuit is configured to control the reverse bias voltage by controlling a potential of the first semiconductor region.
13 . The photoelectric conversion device according to claim 2 , further comprising: a quenching element connected between an output node of the photoelectric conversion unit and the avalanche multiplication detection circuit.
14 . The photoelectric conversion device according to claim 2 , further comprising:
a first switch provided between an output node of the photoelectric conversion unit and an input node of the avalanche multiplication detection circuit,
wherein the first switch is configured to be controlled to be in a non-conductive state during the signal accumulation operation period and to be in a conductive state during the signal readout operation period.
15 . The photoelectric conversion device according to claim 14 , further comprising:
a second switch provided between the output node of the photoelectric conversion unit and a node to which a predetermined fixed voltage is supplied,
wherein the second switch is configured to be controlled to be in a non-conductive state during the signal readout operation period and to be in a conductive state during the signal accumulation operation period.
16 . The photoelectric conversion device according to claim 1 ,
wherein the photoelectric conversion device includes a plurality of pixels arranged to form a plurality of rows and a plurality of columns, and
wherein each of the plurality of pixels includes the photoelectric conversion unit, and a signal processing circuit unit including an avalanche multiplication detection circuit configured to detect avalanche multiplication occurred in the p-n junction between the first semiconductor region and the second semiconductor region.
17 . The photoelectric conversion device according to claim 16 , further comprising:
a first substrate provided with a semiconductor layer including the photoelectric conversion unit of each of the plurality of pixels; and
a second substrate provided with the signal processing circuit unit of each of the plurality of pixels.
18 . A photodetection system comprising:
a photoelectric conversion device according to claim 1 ; and
a signal processing device configured to process a signal output from the photoelectric conversion device.
19 . The photodetection system according to claim 18 , wherein the signal processing device generates a distance image representing distance information to an object based on the signal.
20 . A movable object comprising:
a photoelectric conversion device according to claim 1 ;
a distance information acquisition unit configured to acquire distance information to an object from a parallax image based on a signal output from the photoelectric conversion device; and
a control unit configured to control the movable object based on the distance information.
21 . The photoelectric conversion device according to claim 1 , further comprising:
a first switch connected between the first semiconductor region and a first power supply voltage node; and
a second switch connected between the first semiconductor region and a second power supply voltage node,
wherein the first voltage and the second voltage are switched by switching a state in which the first switch is on-state and the second switch is off-state and a state in which the first switch is off-state and the second switch is on-state.