Basically, a SPAD detector is a semiconductor junction diode that can sustain an avalanche multiplication process when reverse biased (see Fig. 1). As opposed to a standard avalanche photodiode (APD), which is operated below the breakdown voltage in the analog multiplication regime, a SPAD is biased above its breakdown voltage and forced to operate in Geiger mode.
At such overvoltage, the electric field within the depleted region of the device is so high that a single carrier generated in that region is highly accelerated and collides with a bound electron to create an electron-hole pair. These electrons are then accelerated to generate additional electron-hole pairs leading to an exponential increase of current.
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