Photodetector

A photodetector, comprising:

• a semiconductor substrate that has a first main surface and a second main surface opposed to the first main surface;
• a first semiconductor layer that is of a first conductivity type, and is included in the semiconductor substrate and closer to the first main surface than to the second main surface;
• a second semiconductor layer that is of a second conductivity type different from the first conductivity type, and is included in the semiconductor substrate and interposed between the first semiconductor layer and the second main surface;
• a multiplication region that is included in the first semiconductor layer and the second semiconductor layer and causes avalanche multiplication to a charge generated in the semiconductor substrate through photoelectric conversion;
• a circuit region that is disposed alongside the first semiconductor layer in a direction parallel to the first main surface;
• an isolation transistor that is disposed in the circuit region, the isolation transistor comprising at least one isolation transistor;
• an isolation region that is interposed between the first semiconductor layer and the circuit region;
• a first contact that is disposed on the first semiconductor layer, the first contact comprising at least one first contact;
• a first line that connects the first contact to a source of the isolation transistor;
• and
• a readout circuit that is connected to a drain of the isolation transistor and reads a first charge included in the charge generated in the semiconductor substrate through photoelectric conversion.