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. It is a type of
electron avalanche. The Avalanche process occurs when the carriers in the transition region are accelerated by the electric field to energies sufficient to free electron-hole pairs via collisions with bond electrons.
Explanation
Materials conduct electricity if they contain mobile charge carriers. There are two types of charge carrier in a semiconductor:
free electrons and
electron holes. A fixed electron in a reverse-biased
diode may break free due to its thermal energy, creating an electron-hole pair. If there is a voltage gradient in the semiconductor, the electron will move towards the positive voltage while the hole will "move" towards the negative voltage. Most of the time, the electron and hole will just move to opposite ends of the crystal and stop. Under the right circumstances, however, (ie. when the voltage is high enough) the free electron may move fast enough to knock other electrons free, creating more free-electron-hole pairs (ie. more charge carriers), increasing the current. Fast-"moving" holes may also result in more electron-hole pairs being formed. In a fraction of a nanosecond, the whole crystal begins to conduct.
Avalanche breakdown usually destroys regular diodes, but avalanche diodes are designed to break down this way at low voltages and can survive the reverse current.
The voltage at which the breakdown occurs is called the breakdown voltage. There is a hysteresis effect; once avalanche breakdown has occurred, the material will continue to conduct if the voltage across it drops below the breakdown voltage. This is different from a Zener diode, which will stop conducting once the reverse voltage drops below the breakdown voltage.
See also
QBD (electronics)
Spark gap
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
Category:Electrical breakdown
