Avalanche Photodiode, SPAD, and SiPM

Multiplication by impact ionization is a very welcome mechanism in photodetectors. Avalanche multiplication by impact ionization is the gain mechanism used for amplification of the photogenerated charge internal to a photodiode junction. The avalanche photodiode (APD) has been the first device to take advantage of the gain of impact ionization. Following the development and the technical maturity of APD, the Single-Photon Avalanche Detector (SPAD) has been invented, the first semiconductor device able to detect single photons but with a kind of Geiger-mode limitation. From it, recently the Silicon Photomultiplier (SiPM) emerged as a truly solid-state counterpart of the vacuum-tube technology PMT. The SiPM is an example of SPAD array, as it incorporates a large number of individual sensors each with its own resistance for passive quenching.