A Photodiode is a p – n junction diode that can absorb photons and generate either a Photo Voltage or Free Carriers that can produce Photocurrent. They are used for detection of Optical signals and for conversion of Optical Power to Electrical Power.

A Photodiode is an example of an optoelectronic junction device, which implies that it is used as an electrical to optical or an optical to electrical transducer. It works on the effect of light falling onto a diode which leads to the generation of current through it. It is implemented over a special p-n junction diode by fabricating a transparent window on top of it which allows light to be incident on the diode.

Photodiode Symbol: The symbol of photodiode is similar to the normal p-n junction diode except that it contains arrows striking the diode. The arrows striking the diode represent light or photons.

A photodiode has two terminals: a cathode and an anode.

Photodiode Symbol

A photodiode is subjected to photos in the form of light which affects the generation of electron–hole pairs. If the energy of the falling photons (hv) is greater than the energy gap (Eg) of the semiconductor material, electron-hole pairs are created near the depletion region of the diode. The electron-hole pairs created are separated from each other before recombining due to the electric field of the junction. The direction of electric field in the diode forces the electrons move towards the n – side and consequently the holes move towards the p – side, a rise in the electromotive force is observed. Now when an external load is connected to the system, a current flow is observed through it.

The more the electromotive force created, the greater is the current flow. The magnitude of the electromotive force created depends directly upon the intensity of the incident light. This effect of proportional change in photocurrent with change in light intensity can be easily observed by applying a reverse bias.

Since photodiodes generate current flow directly depending upon the light intensity received, they can be used as photo detectors to detect optical signals. Built-in lenses and optical filters may be used to enhance the power and productivity of a photodiode.

Types of photodiodes: The working operation of all types of photodiodes is same. Different types of photodiodes are developed based on specific application. For example, PIN photodiodes are developed to increase the response speed. PIN photodiodes are used where high response speed is needed.

The different types of photodiodes are:

  • PN junction photodiode
  • PIN photodiode
  • Avalanche photodiode

Among all the three photodiodes, PN junction and PIN photodiodes are most widely used.