photodiodes

A photodiode is a semiconductor device that converts light into an electrical current or voltage. When photons with energy above the semiconductor bandgap are absorbed, electron-hole pairs are generated. In a p-n junction or PIN photodiode, an electric field across the depletion region separates the carriers, producing a photocurrent that is proportional to the incident light intensity.

Photodiodes come in several structures, with p-n junctions and PIN configurations being the most common. A PIN photodiode includes an intrinsic layer between the p-type and n-type regions, enlarging the depletion region to improve quantum absorption and speed. Avalanche photodiodes operate under high reverse bias to achieve internal gain through impact ionization, trading some added noise for greater sensitivity.

Operation can be in photovoltaic mode (zero bias) or photoconductive mode (reverse bias). Photovoltaic mode minimizes dark current and noise, suitable for low-light applications, while photoconductive mode offers higher speed and greater responsivity at the expense of increased dark current.

Key performance metrics include responsivity (A per watt of incident light), spectral response, dark current, gain, bandwidth, and noise. Quantum efficiency, transit-time limitations, and RC time constants influence speed. Dark current and noise affect detectivity and noise-equivalent power.

Materials determine spectral range. Silicon photodiodes cover visible to near-infrared up to about 1100 nm. Germanium and GaAs extend into longer wavelengths, while InGaAs photodiodes are common for 900–1700 nm. Applications span fiber-optic communications, imaging sensors, spectroscopy, and environmental and safety monitoring.