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The photodiode is a p-n junction diode that converts light energy into electrical energy. It is also known as a photodetector, photosensor or light detector.

photodiode circuit symbol
Photodiode circuit symbol


A photodiode always works in reverse biased condition. That is the n region of the semiconductor is connected to the battery’s positive terminal and the p region of the semiconductor is connected to the negative terminal of the battery.

photodiode working

When the photodiode is exposed to light, the electrons from n region moves towards the positive terminal and holes from the p region move towards the negative terminal. Thus a photocurrent is produced which is proportional to the intensity of light. It is called the photoelectric effect.


  1. PN Junction Diode

It is formed with a p-type and an n-type semiconductor and creates a depletion region where the incidence of photons generates free electrons that can conduct electricity.

  1. PIN Diode

It has a thin layer of intrinsic(pure form of semiconductor)material between two extrinsic(n-type and p-type) materials. The added layer has high resistivity and electric field strength. It also increases the depletion region thereby increasing the possibility of a larger amount of photons hitting the region.

  1. Avalanche Photodiode(APD)

High reverse biased voltage is applied to create an avalanche effect, which is the rapid increase of current when enough electric force is applied to the material. This happens because photoexcited electrons have high energy to excite other atoms and free more electrons. 

Modes of Operation

  1. Photovoltaic modes

Also known as a zero-bias mode. It is forward biased, i.e., the positive terminal is connected to the p-region and the negative terminal to the n-region. This arrangement decreases the width of the depletion region and the voltage produced when photon hits will have a small dynamic range.

  1. Photoconductive mode

It works on reverse biased condition, increases the width of the depletion region and therefore is fast by having a low response time.

  1. Avalanche Mode

Under the high reverse biased condition, the avalanche effect is initiated which produces internal gain which in turn increases device response by producing a larger current per photon.


  • Optical communication system
  • Used in solar cell panels
  • Smoke detectors, Remote controls in VCR , Street lights
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