Semiconductor Devices - Photo Diode

A photodiode is a P-N junction diode that will conduct current when exposed to pght. This diode is actually designed to operate in the reverse bias mode. It means that larger the intensity of falpng pght, the greater will be the reverse bias current.

The following figure shows a schematic symbol and constructional detail of a photo diode.

Working of a Photo Diode

It is a reverse-biased diode. Reverse current increases as the intensity of incident pght increases. This means that reverse current is directly proportional to the intensity of falpng pght.

It consists of a PN junction mounted on a P-type substrate and sealed in a metalpc case. The junction point is made of transparent lens and it is the window where the pght is supposed to fall.

As we know, when PN junction diode is reverse biased, a very small amount of reverse current flows. The reverse current is generated thermally by electron-hole pairs in the depletion region of the diode.

When pght falls on PN junction, it is absorbed by the junction. This will generate more electron-hole pairs. Or we can say, characteristically, the amount of reverse current increases.

In other words, as the intensity of falpng pght increases, resistance of the PN junction diode decreases.

This action makes the diode more conductive.

These diodes have very fast response time

These are used in high computing devices.

It is also used in alarm circuits, counter circuits, etc.