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 Va higher than VB (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 VD down to or below VB: 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."

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