| Avalanche Breakdown |
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THE AVALANCHE PROCESS Avalanche breakdown is a current multiplication process that occurs only in strong / M is typical, while in a good insulator such as some Ceramic s, fields in excess of 40 MV/m can be needed. 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 Photodiode s, 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, but 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. 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 Impact Ionization for obvious reasons). Both the original electron and the one that has just been knocked free are then accelerated by the electric field and strike other atoms, in turn knocking additional electrons free. As this process continues, the number of free electrons moving through the material increases exponentially, often reaching a maximum in just Picosecond s. The avalanche can result in the flow of very large currents, limited only by the external circuitry. APPLICATIONS If the current is not externally limited, the process normally destroys the material, and in situations such as power line insulators, this can take the form of an explosive breakdown of the insulator. In situations where the current is externally limited, such as avalanche diodes or avalanche photodiodes, this effect can be used to multiply normally tiny currents. In avalanche photodiodes, current gains of over a million can be achieved. SEE ALSO REFERENCES
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