A photodiode is used preferably in ?

Hint:Photodiode is a p-n junction diode which is an optoelectronic device in which current carriers are generated by photons through photo excitation i.e. photoconduction by light. A photodiode is fabricated with a transparent window to allow light to fall on it.

Complete answer:
A photodiode is a special p-n junction diode made of photosensitive semiconducting material, whose function is controlled by the light allowed to fall on it. Photodiode is operated under reverse bias below the breakdown voltage.

The conductivity of p-n junction photodiode is modulated by the absorption of incident light in or near the depletion layer which exists at the p-n junction. The conductivity of the p-n junction of photodiode increases with the increase in intensity of light falling on it.

Hence, a photodiode is used preferably in reverse bias condition as the change in reverse current through the photodiode due to change in light flux or light intensity can be measured easily as the reverse saturation current is directly proportional to light flux or light intensity but it is not so when photodiode is forward bias.

Note:Photodiode is an electrical device used to detect and convert light into an energy signal through the use of photodetector. It is also used for various purposes such as demodulation for optical signals, in optical communication equipment, in reading computers, punched cards and tapes etc.