In optoelectronics the term Single-Photon Avalanche Diode (SPAD) (also known as a Geiger-mode APD or G-APD) identifies a class of solid-state photodetectors based on a reverse biased p-n junction in which a photo-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 the avalanche photodiode (APD), exploit the photon-triggered avalanche current of a reverse 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 (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.
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