Avalanche photodiode

APDs use materials and a structure optimised for operating with high reverse bias, approaching the reverse breakdown voltage, such that charge carriers generated by the photoelectric effect are multiplied by an avalanche breakdown; thus they can be used to detect relatively small amounts of light.

From a functional standpoint, they can be regarded as the semiconductor analog of photomultiplier tubes; unlike solar cells, they are not optimised for generating electricity from light but rather for detection of incoming photons.

Typical applications for APDs are laser rangefinders, long-range fiber-optic telecommunication, positron emission tomography, and particle physics.

[1] However, study of avalanche breakdown, micro-plasma defects in silicon and germanium and the investigation of optical detection using p-n junctions predate this patent.

A standard silicon APD typically can sustain 100–200 V of reverse bias before breakdown, leading to a gain factor of around 100.

However, by employing alternative doping and bevelling (structural) techniques compared to traditional APDs, a it is possible to create designs where greater voltage can be applied (> 1500 V) before breakdown is reached, and hence a greater operating gain (> 1000) is achieved.

This coefficient has a strong dependence on the applied electric field strength, temperature, and doping profile.

This mode is particularly useful for single-photon detection, provided that the dark count event rate and afterpulsing probability are sufficiently low.

In principle, any semiconductor material can be used as a multiplication region: APDs are often not constructed as simple p-n junctions but have more complex designs such as p+-i-p-n+.

Two of the larger factors are: quantum efficiency, which indicates how well incident optical photons are absorbed and then used to generate primary charge carriers; and total leakage current, which is the sum of the dark current, photocurrent and noise.