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.

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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 electrons to the point that, when they strike atoms in the material, they can knock other electrons free: the number of free electrons is thus increased rapidly as newly generated particles become part of the process. This phenomenon is usefully employed in special purpose semiconductor devices such as the avalanche diode, the avalanche photodiode and the avalanche transistor, as well as in some gas filled tubes. In general purpose semiconductor devices such as common diodes, MOSFETs, transistors, it poses an upper limit on the operating voltages since the associated electric fields can start the process and cause excessive (if not unlimited) current flow and destruction of 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, 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 MV/m is typical, while in a good insulator such as some 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 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. When all electrons reach the anode, the process stops, unless of course 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 photodiodes, this effect is used to multiply normally tiny currents, thus increasing the gain of the devices: in avalanche photodiodes, current gains of over a million can be achieved. Also, the phenomena is very fast, meaning that avalanche current quickly follows avalanche voltage variations or starting charge (number of free electrons available to start the process) variations, and this gives to avalanche transistors and avalanche photodiodes the capability of working in the microwave frequency range and in pulse circuits. In avalanche diodes, this effect is mainly used to construct over voltage protection circuits and voltage reference circuits: as a matter of fact, avalanche breakdown and Zener breakdown are jointly present in each avalanche diode, depending on breakdown voltage, which is the leading contributing process to the avalanche current.

See also

• Avalanche noise
• Gas filled tube
• Electron avalanche
• Zener diode

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