Avalanche photodiode

- an overview or tutorial describing the basics of the of the avalanche photodiode and detailing how it differs from the basic p-n and p-i-n photodiodes.

There is a variety of types of photodiode that are available. The p-n and p-i-n photodiodes are the most widely used, but avalanche photodiodes are also available.

Avalanche photodiodes have advantages in some applications although their use may be more specialised.

Avalanche photodiode basics

The avalanche photodiode possesses a similar structure to that of the PIN or PN photodiode. A structure similar to that of a Schottky photodiode can also be used but this is less common. However the structure is optimised for avalanche operation.

The main difference of the avalanche photodiode operates under a slightly different scenario to that of the more standard photodiodes. It operates under a high reverse bias condition to enable avalanche multiplication of the holes and electrons created by the initial hole electron pairs created by the photon / light impact.

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

Avalanche photodiode advantages and disadvantages

The avalanche photodiode has a number of different characteristics to the normal p-n or p-i-n photodiodes, making them more suitable for use in some applications. In view of this it is worth summarising their advantages and disadvantages..

The main advantages of the avalanche photodiode include:

  • Greater level of sensitivity

The disadvantages of the avalanche photodiode include:

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

Circuit conditions

Avalanche photodiodes require a high reverse bias for their operation. For silicon, a diode will typically require between 100 and 200 volts, and with this voltage they will provide a current gain effect of around 100 resulting from the avalanche effect. Some diodes that utilise specialised manufacturing processes enable much higher bias voltages of up to 1500 volts to be applied. 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.

The avalanche photodiodes are not as widely used as their p-i-n 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.

By Ian Poole

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