What Is A Photodiode And Where It Used?

By Ashwini Kumar Sinha and Vinay Minj

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The photodiode is a special type of diode that converts light energy into electrical energy when incident light falls on it and the amount of current flow is directly proportional to the intensity of light and designed to work in reverse bias. This means if we increase the intensity of light at PN junction of photodiode the reverse current also increases in the photodiode.

The symbol of photodiode looks something like a diode but having an incident light falling on it as you can see in the pic below.

Photodiode Symbol
Photodiode Symbol

How does Photodiode work?

It has a P and N junction and is connected in reverse bias that results in a very wide depletion region at the PN Junction. In P-type the majority carriers are holes and n-type majority carriers are electrons. When we connect the photodiode in reverse bias and if there is no illumination or light on photodiode in that condition we get a very small amount of current in microampere we called that current as dark current.

Photodiode working

When a photon having energy greater than the bandgap energy strikes on diode covalent bond breaks and new electrons and hole pairs are generated. This makes a couple of electrons and holes called inner photoelectric effect and the holes move towards the anode and electrons move toward cathode this results in photocurrent. The total current through the diode is the sum of dark current and photocurrent. To maximize the sensitivity of photodiode we need to minimize the dark current

Hv > Eg

Energy of photon > Bandgap energy

The photodiodes can work in the following modes:

Photovoltaic Mode

In the photovoltaic mode, the diode is not connected to power source we do not bias the diode. You can say there is no biased source. When the light falls in such condition on the photodiode, it excites the electrons to higher energy state and results in electrons to move towards cathode terminal and holes towards anode terminal. This process creates a potential difference between two terminals.

Photoconductive Mode

In photoconductive mode, the diode is connected to the power source and we reverse biased the diode. When the light falls on the photodiode creates a pair of electrons and holes and moves towards the opposite direction due to biased voltage.

V-I Characteristic of Photodiode 

The photodiode works in reverse biased mode . The photocurrent is independent of applied reverse biased voltage.In absence of light or you can say in zero illuminance we get almost zero  photo current and a very little amount of dark current . While we increase the intensity of light we can experience the increase in photocurrent as well.So Photocurrent increases linearly with the increase in optical power.

E4>E3>E2>E1

(Image Courtesy: Wikimedia.org)
(Image Courtesy: Wikimedia.org)

Advantages Of Photodiodes

  • Works on light make it fast and fit for fastest communication like communication-based on fiber optics
  • Very low noise
  • Long Lifetime
  • Very low resistance
  • Can work in very small voltage

Application Of Photodiode

There is a wide range of use of photodiodes and found in most of the devices:

  • Photodiode used as a light sensor. As the current in it is directly proportional to the intensity of light thus also used to measure the intensity of light.
  • We can use the photodiode in smoke detectors to sense smoke and fire.
  • Photodiode coupled with led to make optoisolators and optocouplers
  • Used in a solar panel as solar cells
  • Used in barcode scanner, character recognition
  • Used in the obstacle detection system,
  • Can be used in printers as page presence and page counter
  • Used in proximity detection, oximeters
  • It is also used in optical encoders and decoders
  • Optical message transmission, Fibre optics-based communication
  • Position sensor

For reading other interesting Basics articles: click here

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