Specification :

A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation.[ 1]
Photodiodes are similar to regular semiconductor diodes except that they may be either exposed ( to detect vacuum UV or X-rays) 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 will also use a PIN junction rather than the typical PN junction.