Avalanche breakdown: Encyclopedia II - Avalanche breakdown - The avalanche process

Avalanche breakdown - 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 MV/m is typical, while in a good insulator such as some ceramics, 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 present, this always happens. In devices that exploit the avalanche effect, the electric field is normally kept just on 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 the avalanche breakdown begins, the 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 electrons free from them. As this process continues, the number of free electrons moving through the material increases exponentially, often reaching a maximum in just picoseconds. This results in very large currents if nothing external limits them.

Adapted from the Wikipedia article "The avalanche process", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki