Explain construction and working of photodiode.

Explanation

A Photodiode is a special purpose p-n junction diode.
If is fabricated with a transparent window to allow light to fall on the diode.
When the photodiode is illuminated with light (photons) with energy (hv) greater than the energy gap `(E_(g))` of the semiconductor then electron hole (e-h) pairs are generated due to the absorption of photons.
The illuminated photo diode in reverse bias is shown in the figure (a).

The diode is fabricated such that the generation of (e - h) pairs takes place in or near the depletion region of the diode.
Due to the electric field of the junction, electron and holes are separated before they recombine.
The direction of the electric field is such that electrons reach n-side and holes reach p-side. Electrons are collected on n-side and holes are collected on p-side giving rise to an emf. When an external load is connected, current flows.
The magnitude of the photocurrent depends on the intensity of incident light, really photocurrent is proportional to incident light intensity, which we have seen in chapter 11.
If a reverse bias is applied, the change in the current with change in the light intensity. Thus, photodiode can be used as a photodetector to detect optical signals.
A typical I-V characteristics of photodiode is shown in figure (b).

This diode is always used in reverse bias mode.
Photodiode are used in CD player, computer as well as security system.