APD - ACRONYM

An avalanche photodiode (APD) is a semiconductor photodetector in which incident light generates a photocurrent, which is then multiplied by an avalanche process to give a stronger signal.

An avalanche photodiode is a single device that incorporates two distinct semiconductor stages. The first is a photodiode detector, in which light with energy above the bandgap of a semiconductor delivers enough energy to valence electrons for them to enter the conduction band. The electron leaves behind a hole in the valence band, which also functions as a current carrier. Application of a voltage across the device pulls the electrons and holes in opposite directions.

The second stage applies a strong reverse bias across the semiconductor to accelerate electrons. When electrons reach high enough velocities, they can ionize other atoms in the semiconductor, producing an avalanche of electrons. This multiples the original photocurrent and produces a much stronger response than a simple photodiode. Typically, silicon APDs are biased with about 100 V to multiply photocurrent by around a factor of 100. Further increasing the reverse voltage increases the dark current as it approaches the ionization threshold of the semiconductor. Breakdown occurs at a reverse bias of about 150 to 200 V for silicon, depending on device design, and at different voltages in other semiconductors.

You can think of APDs as solid-state counterparts of photomultiplier tubes (PMTs), but they are far from plug-in replacements. As you expect from solid-state devices, APDs are much smaller and their bias voltages are much lower--about a tenth of those in PMTs. However, PMTs are less subject to noise, their gain does not depend as strongly on bias voltage, and they can be engineered to respond to different wavelengths than APDs, so they are among the few survivors of the vacuum-tube era.

New designs and new materials are extending the range of APDs for applications ranging from single-element fiber-optic detectors to focal-plane arrays for imaging. Germanium/silicon ADPs have reached a gain-bandwidth product of 105 GHz, attractive for high-speed optical interconnects. APD elements designed for single-photon counting can be assembled in arrays to make a single-photon counting camera.

Gain or multiplication factor of an APD increases sharply as voltage approaches breakdown, but so does dark current, limiting usable gain. (From Jeff Hecht, Understanding Fiber Optics: 5th edition [Pearson Prentice-Hall, 2006])