A photodiode is a semiconductor device that converts light into an electrical current. The current is generated when photons are absorbed in the photodiode.
The photodiodes are similar to regular semiconductor diodes except that they may be either exposed or packaged with a window or optical fiber connection to allow light to reach the sensitive part of the device. Many diodes designed for use specifically as a photodiode use a PIN junction rather than a p–n junction, to increase the speed of response.
Photodiodes are used in a variety of applications including CD/DVD drives, Optical communications, Smoke detectors, Medical equipment, burglar alarms and general light sensing.