Avalanche noise is a form of noise that does not occur in most circuit, but can be experienced with PN junctions that are operated at the point of avalanche breakdown or close to it.
Diodes like Zener diodes exhibit this and the effects can be filtered at source to remove it and prevent it entering other circuits. Also it is possible to use the diodes that exhibit avalanche breakdown as noise generators which sometimes needed for some test applications.
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.
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.
Avalanche noise is relatively limited in its generation. Zener diodes and other forms of microwave diode that use avalanche breakdown as part of the mechanism of their operation will generate this form of noise. For Zener diodes it can normally be removed quite easily by filtering. For microwave diodes using breakdown techniques to generate the signal, there is an inherent level of noise that must be tolerated.
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