Avalanche breakdown



Avalanche breakdown is a phenomenon that can occur in both semiconducting materials. It is a form of electric current multiplication that can allow very large currents to flow within materials which are otherwise good insulators.

Explanation

Avalanche breakdown can occur within insulating or semiconducting solids, liquids, or gases when the electric field in the material is great enough to accelerate free electron avalanche. Although there are some superficial similarities to Zener breakdown, the physical origins of the two phenomena are very different.

The avalanche process

Avalanche breakdown is a current multiplication process that occurs only in strong electric fields, which can be caused either by the presence of very high voltages, such as in electrical transmission systems, or by more moderate voltages which occur over very short distances, such as within semiconductor devices. The electric field strength necessary to achieve avalanche breakdown varies greatly between different materials: in air, 3 ceramics, fields in excess of 40 MV/m are required. Field strengths used in semiconductor devices that exploit the avalanche effect are often in the 20–40 MV/m range, but vary greatly according the details of the device.

Once the necessary field strength has been achieved, all that is necessary to start the avalanche effect is a free electron, and since even in the best insulators a tiny number of free electrons are always present, an avalanche will always occur. In devices that exploit the avalanche effect, the electric field is normally kept just below the threshold at which avalanche breakdown is possible, resulting in a current that is highly dependent on the generation of free electrons. In avalanche photodiodes, for example, incoming light is used to generate these free electrons.

As avalanche breakdown begins, free electrons are accelerated by the electric field to very high speeds. As these high-speed electrons move through the material they inevitably strike atoms. If their velocity is not sufficient for avalanche breakdown (because the electric field is not strong enough) they are absorbed by the atoms and the process halts. However, if their velocity is high enough, when they strike an atom, they knock an electron free from it, ionizing it (and this is referred to as holes are created also.

For a bipolar junction transistor the strength of the base drive has an important impact on the avalanche voltage. If a low impedance is connected to the base then charge is quickly removed from the base which helps hold back the avalanche process, but if the base is driven by a high impedance, such as a current source, then the excess charges stay in the base and avalanche occurs at a lower electric field.

Applications

If the current is not externally limited, the process normally destroys the device where it has started, and in situations such as power line insulators, this can take the form of an explosive breakdown of the insulator. When avalanche current is externally limited, avalanche breakdown can successfully serve to several purposes. In avalanche transistors and avalanche diode, depending on breakdown voltage, which is the leading contributing process to the avalanche current.

See also

References

  • Microelectronic Circuit Design — Richard C Jaeger — ISBN 0-07-114386-6
  • The Art of Electronics — Horowitz & Hill — ISBN 0-521-37095-7
  • University of Colorado guide to Advance MOSFET design
 
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