Working Principle

Silicon Photomultipliers are solid state low level light detectors comprising an array of parallel connected micro avalanche photodiodes with a quenching resistor.

Figure 1: Top view and schematic of a Silicon Photomultiplier. Each photodiode has an individual quenching resistor. Photodiode and quenching resistor are indicated as microcell or pixel.

The microcells are operated at reverse bias above breakdown (Geiger-mode). In this supercritical state a single photon can trigger the avalanche breakdown with high internal gain.

The discharge of one fired microcell is always equal at a fixed bias voltage and independent of the incoming light intensity.

After discharge the microcell is recovered by passive quenching: A high-value ballast resistance in series to the micro APD is causing a voltage drop and with it a quenching of the avalanche current.

The light intensity information is generated by the internal superposition of the signal of all fired microcells.

The Avalanche discharge of a microcell is a very fast process. The rise time of a single pulse is below one nsec. The quenching resistor recovers the microcell after Geiger discharge. Typical resistor values are in the range of 100 kΩ up to several MΩ. The cell recovery time is limited by the microcell capacitance and the quenching resistivity. Typical time constants are in the range of 30 nsec to 300 nsec.

The high photon detection efficiency of Silicon Potomultipliers arises from the very high quantum efficiency of silicon compared to photocathode metals.