A Geiger-avalanche photodiode (G-APD), aka, single-photon avalanche diode (SPAD) is a type of solid state photon sensor, i.e., functioning like a photomultiplier tube (PMT) but not involving a vacuum tube. It is a type of avalanche photodiode (APD) operating in what is termed Geiger-mode. Like a PMT or Geiger-Müller tube, it internally amplifies an electrical signal resulting from a sensed photon. APDs and G-APDs are considered to have much future potential and there is current interest and activity developing them. Goals of the development include creating them small enough to be used in arrays for imaging, and devising types that can pass along a signal of an arrive photon quickly as well as prepare itself to sense another photon very rapidly, so as to miss as few photons as possible.