Optics 101

What is a Photodiode?

A photodiode can be described as a photo detector that is capable of converting incident light into voltage or current. The conversion depends on the mode in which the diode is operated.

Photodiodes work on the same principle as semiconductor diodes, the only difference being the exposure to light. Light is incident on the sensitive region of these diodes either through a window or an optical fiber connection. Some types of photodiodes incorporate a PIN junction instead of the p-n junction in semiconductor diodes.

This datasheet will elaborate further on the working principle, construction and applications of photodiodes.

Photodiode optical detector from PHOTONIS, as used in the CERN Large Hadron Collider

Working Principle

Photodiodes consist of either a p-n junction or a PIN junction. When the incident light of sufficient energy strikes the junction, an electron is excited. This electron is called a free electron; along with this electron a positively charged electron hole is also created. This mechanism is referred to as inner photoelectric effect.

When light absorption occurs in the depletion region of the junction, these carriers are swept from the junction by the built-in electric field of the region. This results in the production of a photocurrent due to the movement of holes towards anode and electrons towards cathode.

Apart from the photocurrent, some amount of dark current is generated from the flow of electrons with or without light. The total current produced by the device is the sum of photocurrent and the dark current.

In order to attain maximum sensitivity from the device it is essential to minimize the dark current. There are mainly two modes of operation of the photodiode, namely, photovoltaic and photoconductive modes.

In the photovoltaic mode, the diode is operated under zero bias, while in the photoconductive mode it is operated in reverse bias. Another type of diode operation is the avalanche photodiode, which is nothing but a stronger reverse bias operation.

Construction

Silicon photodiodes are made of single crystal silicon wafers of high purity, the resistivity of the diode is proportional to its purity. The n-type is the starting material, upon the n-layer a p-type layer is deposited by ion implantation or thermal diffusion. This forms the p-n junction.

Small metal contacts are planted on the front surface (anode) and the back surface (cathode) is coated with a contact metal. The active area is coated with silicon nitride or silicon dioxide for the purpose of protection and to prevent reflection. The thickness of the p-layer depends on the wavelength of the incident radiation.

Applications

The principal areas of application of photodiodes are given below:

  • Consumer electronics – in compact discs, smoke detectors, remote control devices
  • Medical diagnostics – computer aided tomography, immunoassay and pulse oximeters
  • Optical communication
  • Astronomy, spectroscopy, laser range finding and night vision equipment.

References

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