What is a Photodiode?
The photodiode is a two terminal PN junction device which is operated in reverse bias.
It has a small transparent window which allows light to strike the PN junction.
The photodiode is a semiconductor device which is used to convert light which is fall on it to current.
A photodiode is a semiconductor device with a PN junction and an intrinsic layer between p layer and n layer.
It consists of built in lenses and optical filters with small or large surface areas.
Construction of Photodiode
The silicon photodiodes are constructed from single crystal silicon wafers. A thin p layer is formed on the front surface of the device of thermal diffusion. N type silicon is a starting material. The interface between the p layer and n layer is known as PN junction. To the front of the surface of the device small metal contacts are applied and the entire back is coated with a metal contact. Anode is the front contact and the back contact is the cathode.
Construction of Photodiode
Operating principle
Reverse current increases with increases in temperature in a normal PN junction but in photodiode reverse current increases with light intensity at the PN junction. Light energy contains photons. When a photons strike at the diode it creates an electron-hole pair. This mechanism is also called as the inner photoelectric effect. If the absorption arises in the depletion region will pass their energy to the ions present there and generate electron hole pairs. The number of electron hole pairs will be dependent on the intensity. With increase in light intensity number of electron hole pair increases