Photodiodes:
A p-n junction diode which converts an optical signal into electric current is known as photodiode. Thus, the operation of photodiode is exactly opposite to that of an LED. Photo diode words in reverse bias. Its circuit symbol is shown in figure (a). The direction of arrows indicates that the light is incident on the photo diode.
The device consists of a p – n junction semiconductor made of photosensitive material kept safely inside a plastic case. It has a small transparent window that allows light to be incident on the p – n junction. Photodiodes can generate current when the p – n junction is exposed to light and hence are called as light sensors.
When a photon of sufficient energy (hυ) strikes the depletion region of the diode, some of the valence band electrons are elevated into conduction band, in turn holes are developed in the valence band. This creates electron – hole pairs. The amount of electron – hole pairs generated depends on the intensity of light incident on the p-n junction. These electrons and holes are swept across the p-n junction by the electric field created by reverse voltage before recombination takes place. Thus, holes move towards the n-side and electrons towards the p-side.
When the external circuit is made, the electrons flow through the external circuit and constitute the photocurrent. When the incident light is zero, there exists a reverse current which is negligible. This reverse current in the absence of any incident light is called dark current and is due to the thermally generated minority carriers.
