Single-photon Avalanche Diode - SPAD Operating Principle

SPAD Operating Principle

SPADs are semiconductor devices based on a p-n junction reverse-biased at a voltage V_a higher than V_B (Figure 1). "At this bias, the electric field is so high that a single charge carrier injected into the depletion layer can trigger a self-sustaining avalanche. The current rises swiftly to a macroscopic steady level in the milliampere range. If the primary carrier is photo-generated, the leading edge of the avalanche pulse marks the arrival time of the detected photon." The current continues until the avalanche is quenched by lowering the bias voltage V_D down to or below V_B: the lower electric field is no longer able to accelerate 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.

"This operation requires a suitable circuit, which has to:

1. sense the leading edge of the avalanche current;
2. generate a standard output pulse synchronous with the avalanche build-up;
3. quench the avalanche by lowering the bias down to the breakdown voltage;
4. restore the photodiode to the operative level. This circuit is usually referred to as a quenching circuit."