Single-photon avalanche diode

A single-photon avalanche diode (SPAD) (also known as a Geiger-mode APD or G-APD) is a solid-state photodetector in which a photon-generated carrier can trigger an avalanche current due to the impact ionization mechanism. This device is able to detect low-intensity signals (down to the single photon) and to signal the arrival times of the photons with a jitter of a few tens of picoseconds.

SPADs, like avalanche photodiodes (APDs), exploit the photon-triggered avalanche current of a reversely biased p–n junction to detect an incident radiation. The fundamental difference between SPAD and APD is that SPADs are specifically designed to operate with a reverse-bias voltage well above the breakdown voltage. This kind of operation is also called Geiger mode in literature (as opposed to linear mode in an APD), in analogy with the Geiger counter.

SPADs are semiconductor devices based on a p-n junction reverse-biased at a voltage V_a that exceeds breakdown voltage V_B of the junction (Figure 1). "At this bias, the electric field is so high [higher than 3×10⁵ V/cm] that a single charge carrier injected into 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 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.

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