AvalanchePhotodiode

An avalanche photodiode (APD) is a highly sensitive photodetector that uses internal gain from avalanche multiplication to convert light into an amplified electrical signal. The device is typically operated in reverse bias, with a narrow multiplication region where a strong electric field accelerates photo-generated carriers, inducing impact ionization and a self-sustaining chain reaction that multiplies the current.

APDs are usually fabricated as p-n junction or p-i-n structures. In linear-mode operation, they provide moderate gain, typically from about 10 to several hundred, enabling detection of weak optical signals with improved signal-to-noise compared with ordinary photodiodes. In Geiger mode, the bias is raised above the breakdown voltage to trigger a single, self-quenched avalanche, allowing photon counting with high sensitivity. Geiger-mode APDs are often referred to as single-photon avalanche diodes (SPADs).

Material choices determine spectral response. Silicon APDs cover visible to near-infrared light up to roughly 1 micrometer. InGaAs APDs extend sensitivity to the near-infrared around 1.0 to 1.7 micrometers, widely used for telecommunications. Ge and other materials can extend infrared response further in specialized devices. A key feature of APDs is the excess noise factor, which increases with gain due to the stochastic nature of avalanche multiplication and degrades signal quality at higher gains.

Applications include fiber-optic communication, LIDAR, time-resolved spectroscopy, and photon-counting detectors in research and industry. APDs offer high sensitivity and fast response but require high reverse bias, exhibit dark current, and, in SPAD form, may suffer from afterpulsing and dead time.