Photodiode

A photodiode consists of an active P-N junction. When light falls on the junction it causes the photoelectric effect or photoemission. New free electrons are produced in the intrinsic region. This leads to a reverse current which flows and which is proportional to the illumination. The linear dependence of current to light makes photodiodes useful as photodetectors for some applications. It is also used as the active element in light-activated switches.

Some photodiodes are based on silicon. They are sensitive throughout the visible spectral region and in the near-infrared. For longer wavelengths, InGaAs diodes are available, although at higher prices. Germanium diodes can be a cheaper alternative to InGaAs diodes, but they have a much slower response.