What's a SPAD?

SPADsWhen every photon counts...

The term Single-Photon Avalanche Diode (SPAD) defines a class of photodetectors able to detect low intensity signals (down to the single photon) and to signal the time of the photon arrival with high temporal resolution (few tens of picoseconds).

SPADs, like the Avalanche photodiode (APD), exploits the photon-triggered avalanche current of a reverse biased p-n junction to detect an incident radiation. The fundamental difference between SPADs and APDs is that SPADs are specifically designed to operate with a reverse bias voltage well above the breakdown voltage (on the contrary APDs operate at a bias lesser than the breakdown voltage). This kind of operation is also called Geiger-mode in literature, for the analogy with the Geiger counter.

SPADs are semiconductor devices based on a p-n junction reversed biased at a voltage higher than the breakdown voltage. At this bias, the electric field is so high (higher than 3E5 V/cm) that a single charge carrier injected in the depletion layer can trigger a self-sustaining avalanche. The current rises swiftly (sub nanosecond rise-time) to a macroscopic steady level, in the milliampere range. If the primary carrier is photo-generated, the leading edge of the avalanche pulse marks (with picosecond time jitter) the arrival time of the detected photon. The current continues to flow until the avalanche is quenched by lowering the bias voltage down to or below VB: the lower electric field is not able any more to accelerate the carriers to impact-ionize with lattice atoms, therefore current ceases. In order to be able to detect another photon, the bias voltage must be raised again above breakdown.

These operations require a suitable circuit, which has to i) sense the leading edge of the avalanche current; ii) generate a standard output pulse synchronous with the avalanche build-up; iii) quench the avalanche by lowering the bias down to the breakdown voltage; iv) restore the photodiode to the operative level. This circuit is usually referred to as a quenching circuit.