Question

Give reason for the following: A photodiode , when used as a detector of optical signal, operates under reverse bias.

Answer 1

Before going through the question let us first know about semiconductors. A semiconductor material's electrical conductivity lies somewhere between that of a conductor, such as metallic copper, and that of an insulator, such as glass. When the temperature rises, its resistivity decreases, while metals have the opposite effect.

Complete answer:
Photodiode: A photodiode is a semiconductor device with a p-n junction that converts light into electricity. When photons are absorbed in the photodiode, current is produced. Optical filters, built-in lenses, and wide or small surface areas are all possible features of photodiodes.

Working principle: A photodiode's operating theory is that when a photon with sufficient energy hits the diode, it creates a pair of electron-holes. As a result, holes in the area pass toward the anode and electrons toward the cathode, generating a photocurrent.
Now, coming to the question, A photodiode , when used as a detector of optical signal, operates under reverse bias the potential is applied across this photo resistor in such a way that it can conduct as soon as a light ray or photons drop on its surface.

Note: An LED can be used as a photodiode light sensor / detector in addition to emitting light. An LED is sensitive to wavelengths that are equal to or shorter than the prevailing wavelength it emits as a photodiode. Blue and some green light would be vulnerable to a green LED, but yellow and red light would not.