Avalanche Noise

- the basics of avalanche noise - how it is created, how it can be used or removed..

Avalanche noise is a form of noise that occurs in pn junctions that are operated in a region at or close to the point of avalanche breakdown.

Diodes that operate in this region are renowned for generating large amounts of avalanche noise.

These diodes are often used as noise generators for a variety of applications.

In other circuits, the avalanche noise must be removed to prevent other circuits being affected by it.

Avalanche breakdown

In order to look at avalanche noise, it is necessary first to look at the phenomenon that gives rise to it.

Avalanche breakdown occurs in semiconductors where a very high potential gradient exists. When this occurs electrons rapidly gain momentum and may hit the crystal lattice through which they travel with such energy that they can dislodge other charge carriers creating hole electron pairs. In turn these carriers are accelerated and may similarly hit the lattice and dislodge further carriers.

This process can lead to an avalanche of new carriers, and the breakdown of the pn- junction.

The way that breakdown occurs results in a very uneven or ragged current flow. This means that high levels of noise - avalanche noise are generated.

Avalanche noise generator

Avalanche diodes generate large quantities of radio frequency noise. As a result they are widely used noise sources for RF measurements, e.g. for RF for antenna analyser bridges and also in other items such as random number generators.

In these applications either avalanche diodes or even voltage regulator (Zener) diodes may be used. When Zener diodes are used, the diodes must have breakdown voltages above about 5.5 volts because diodes with voltages above this value chiefly use avalanche breakdown, below this value Zener breakdown is the chief mode.

A simple circuit using an avalanche diode or zener diode and amplier to create avalanche noise
Simple avalanche noise generator circuit

Avalanche noise removal

When using circuits that utilise diodes like voltage reference diodes that may operate using avalanche breakdown, it may be necessary to remove the avalanche noise.

The avalanche noise can be removed using simple capacitor based filter or smoothing networks.

Any filters designed to remove the avalanche noise should be designed so that their operation does not affect the normal functioning of the circuit.

By Ian Poole

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