single photon avalanche diodes
SPAD, Geiger-mode avalanche photodiode, GmAPD
Single photon avalanche diodes (SPAD) are solid-state photodetectors and belong to the same family as photodiodes and avalanche photodiodes (APDs) but require no vacuum. While APDs show their strengths in the undistorted amplification of low-intensity optical signals, the behavior of SPADs is more closely related to the basic diode behavior. SPADS enable the direct counting of pulses and thus provide an indication of the optical intensity of the input signal, while the pulses can simultaneously trigger timing circuits to enable precise measurements of the time of arrival. These sensors are used in particular in time-of-flight 3D imaging, LIDAR technology, PET scanning, TCSPC, fluorescence lifetime microscopy and optical communication, especially in quantum key distribution. SPADs are particularly useful for single photon detection, e.g. using SPAD arrays a full FLIM image can be captured as even single photons produce a clear output signal which can be detected. SPADs have a low dead time (less than 1 ns) and consequently a high count rate, with a delay spread comparable to MCPs. A distinctive feature of SPADs is their dark noise. Since dark noise increases with area, this effect is an important criterion when increasing the light-sensitive area in SPADs.
The name Geiger-mode avalanche photodiode (GmAPD) refers to the fact that the diode is operated slightly above the breakdown threshold voltage. This leads to the fact that already a single electron-hole pair (generated by absorption of a photon or by a thermal fluctuation) can trigger a strong avalanche.
Hint: You might also be interested in HPDs.