The Single Photon Avalanche Diode (SPAD) is a photodiode with a unique physical structure allowing it to operate beyond its breakdown voltage, in what is called the Geiger-mode of operation. In this state, the SPAD is functioning in a semi-stable manner where it produces no output until it is triggered by an event, or the detection of a photon. Detection involves absorption of the photon resulting in creation of a mobile electron-hole pair.
Once a photon has been detected, the SPAD loses its stability and a charge buildup begins through an effect known as impact ionization: the mobile electron and/or hole may be accelerated to high enough speeds to knock other bound electrons free, creating more free-electron-hole pairs (i.e. more charge carriers), increasing the current and leading to an electron-hole avalanche. This rapid increase in current results in an almost instantaneous electrical signal that can be sensed or processed by electronic circuitry. After the detection of a photon, the SPAD recovers back to its initial point of operation with the assist of a simple electronic circuit known as the quench element, where the device is ready to detect the next arriving photon.