Photodiodes

What is Photodiode?

A photodiode is a PN-junction diode that consumes light energy to produce an electric current. They are also called a photo-detector, a light detector, and a photo-sensor. Photodiodes are designed to work in reverse bias condition. Typical photodiode materials are Silicon, Germanium and Indium gallium arsenide.

Symbol of Photodiode

The following image shows the symbol of the photodiode:

The symbol of the photodiode is similar to that of an LED, but here the arrow points inwards.

Photodiode Working

A photodiode is subjected to photons 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 (E_g) 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 the electric field in the diode forces the electrons to move towards the n – side and consequently the holes move towards the p-side. As a result of the increase in the number of electrons on the n – side and holes on the p-side, a rise in the electromotive force are 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 the proportional change in photocurrent with the 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 photodetectors to detect optical signals. Built-in lenses and optical filters may be used to enhance the power and productivity of a photodiode.

Applications of Photodiode

• Photodiodes are used in simple day-to-day applications. The reason for their prominent use is their linear response of photodiode to light illumination.
• Photodiodes with the help of optocouplers provide electric isolation. When two isolated circuits are illuminated by light, optocouplers are used to couple the circuit optically. Optocouplers are faster compared to conventional devices.
• Photodiodes are used in safety electronics such as fire and smoke detectors.
• Photodiodes are used in numerous medical applications. They are used in instruments that analyze samples, detectors for computed tomography and also used in blood gas monitors.
• Photodiodes are used in solar cell panels.
• Photodiodes are used in logic circuits.
• Photodiodes are used in the detection circuits.
• Photodiodes are used in character recognition circuits.
• Photodiodes are used for the exact measurement of the intensity of light in science and industry.
• Photodiodes are faster and more complex than normal PN junction diode and hence are frequently used for lighting regulation and optical communication.

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