Photo diode, construction and working

What is photo diode?

Photo diode is a semiconductor device which converts light energy into electric current. When it is exposed to light , it absorbs the light and forms the electron hole pairs, thus the current is generatedIt is also called as photo detector or light detector and photo sensor or light sensor.

Symbol of photo diode:

Symbol of photo diode
Symbol of photo diode

The symbol of photo diode is similar to the PN junction diode. The difference is the arrow pointing towards the diode. 

Construction of photo diode:

Construction of photo diode
Construction of photo diode

Photodiodes are formed from single crystal silicon wafers. It consists of two layers. P type layer is formed above the N type layer. Depletion layer is formed at the junction of P region and N region. It has a small transparent window which allows the light to strike the PN Junction.  

The non active surface is coated with silicon di oxide SiO2 .The active region where the device is exposed to radiation is coated with anti reflection material so that maximum light energy is converted into current.

Working of photo diode:

When the photo diode is reversed biased the depletion layer near the PN junction increases. 

  • Photons create electron-hole pairs:

Photons create electron-hole pairs
Photons create electron-hole pairs

When the device is exposed to radiation or illuminated the photons are absorbed by the diode. The electrons in the valence band get energized and they jump to the conduction band leaving hole in the valence band. Thus electron hole pair is produced at the junction.

  • Holes and electrons attracted by reverse bias:

Holes and electrons attracted by reverse bias
Holes and electrons attracted by reverse bias

When the photo diode is reverse biased, the electrons are attracted towards the positive terminal and the holes are attracted towards negative terminal.  

  • Holes and electrons forms the photo electric current:

Holes and electrons forms the photo current
Holes and electrons forms the photo current

When the reverse biased voltage is increased the electron hole pair breaks and holes move toward the negative terminal and the electrons move towards positive terminal which results in high reverse current. This is called photo current.

Modes of operation of photo diode:

  • Photo conductive

When the reverse bias is applied the electron hole combination takes place thus the depletion layer is formed. Thus the diodes become conductive. The width of the depletion layer is varied by changing the reverse voltage.

  • Photo- voltaic mode: 

This is called Zero bias mode. No external voltage is applied to the device. When the device is exposed to the radiation current is produced due to minority carriers.

V-I characteristics of photo diode:

V-I Characteristics of photo diode
V-I Characteristics of photo diode

The dark current is due to minority carriers when the device is not exposed to light. In the above figure we can observe that between each curve there is equal spacing. This is because increase in current which is  directly proportional to the luminous flux.


  • Low noise
  • Highly sensitive to noise
  • It operates at high frequencies
  • Long lifetime
  • High voltage is not required
  • It operates very fast


  • The active area is very small
  • Temperature stability is less, when the temperature increases the dark current also increases
  • Change in current is very small
  • Should not exceed the working temperature limit

Application of Photodiode:

  • Used in counters and switching circuits.
  • Used widely in optical communication system.
  • Used in detection of both visible and invisible light rays.
  • Used in safety electronics like fire and smoke detector .
  • Used as photo sensors in camera.
  • Used in medical applications like pulsed oximeters, in instruments which analyze sample.

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