Explain the working principle of a photodiode.
Or
Explain briefly how a photodiode operates.

Explanation

A photodiode is a special purpose p-n junction diode designed to detect and measure the intensity of light. It is fabricated with a transparent window so that light is allowed to fall on it. A photodiode operates under reverse bias and hence its own current is negligible. When light photons having energy greater than the energy gap `E_(g)` of semiconductor are incident at the junction, the electron - hole pairs are generated due to absorption of these photons. The diode is fabricated such that electron-hole pairs are generated in or near the depletion region. Due to strong electric field of the junction, electrons and holes are separated before recombining. Due to reverse bias electrons reach n-side of junction and holes reach p-side. This leads to an emf. When an extermal load is connected, a weak current flows. Value of photocurrent is directly proportional to intensity of incident light.