Gourt Home::Gourt :: Avalanche breakdown
Avalanche breakdown is a phenomenon that can occur in both insulating and 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. Avalanche breakdown can occur within solids, liquids, or gases when the voltage applied across the insulating material is great enough to accelerate free electrons to the point that, when they strike a atoms in the material, they can knock other electrons free. This phenomenon can be quite useful in semiconductor diodes such as the avalanche diode and avalanche photodiode, but in other situations, such as in MOSFET transistors, it can destroy the device. When avalanche breakdown occurs within a solid insulating material it is almost always destructive. When an avalanche-like effect occurs without connecting two electrodes, it is referred to as an electron avalanche. Although there are some superficial similarities to Zener breakdown, this is a very different effect.
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 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, 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 picoseconds. 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
- 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
"Avalanche breakdown"