Avalanche Photodiode

Avalanche photodiodes are used in applications where very sensitive light detection is needed.

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Avalanche photodiodes can be used in a number of applications to provide performance that other types of photodiode may mot be able to attain.

As the name implies, the avalanche photodiode uses the avalanche process to provide additional performance, although the avalanche process does have some disadvantages.

In view of the advantage and disadvantages, avalanche photodiodes are used in a number of niche applications where their characteristics enable them to provide the additional sensitivity that may be required.

Avalanche photodiode basics

The avalanche photodiode possesses a similar structure to that of the PN or PIN photodiode. An avalanche diode structure similar to that of a Schottky photodiode may also be used but the use of this version is much less common.

The main difference of the avalanche photodiode to other forms of photodiode is that it operates under a high reverse bias condition. This enables avalanche multiplication of the holes and electrons created by the photon / light impact.

As a photon enters the depletion region and creates a hole electron pair, these charge carriers will be pulled by the very high electric field away from one another. Their velocity will increase to such an extent that when they collide with the lattice, they will create further hole electron pairs and the process will repeat.

The avalanche action enables the gain of the diode to be increased many times, providing a very much greater level of sensitivity.

Avalanche photodiode circuit conditions

Avalanche photodiodes require a high reverse bias for their operation. For silicon, this will typically be between 100 and 200 volts. With this level of reverse bias they see a current gain effect of around 100 as a result of the avalanche effect.

Some diodes that utilise specialised manufacturing processes enable much higher bias voltages of up to 1500 volts. As it is found that the gain levels increase when higher voltages are applied, the gain of these avalanche diodes can rise to the order of 1000. This can provide a distinct advantage where sensitivity is of paramount importance, but this is obviously at the expense of all the additional circuitry and safety features needed for the very high voltages.

Avalanche photodiode advantages and disadvantages

Avalanche photodiodes possess a number of advantages and disadvantages. These can be considered in the selection of a suitable photodetector device.

Avalanche photodiode advantages

  • High level of sensitivity as a result of avalanche gain

Avalanche photodiode disadvantages:

  • Much higher operating voltage may be required.
  • Avalanche photodiode produces a much higher level of noise than a PN photodiode
  • Avalanche process means that the output is not linear

The avalanche photodiodes are not as widely used as their PIN counterparts. They are used primarily where the level of gain is of paramount importance, because the high voltages required, combined with a lower reliability means that they are often less convenient to use.

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