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Avalanche breakdown

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Avalanche breakdown

Avalanche breakdown is a phenomenon that can occurs in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents 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 bound 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, (i.e. when the voltage is high enough) the free electron may move fast enough to knock other electrons free, creating more free-electron-hole pairs (i.e. 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.

The large voltage drop and possibly large current during breakdown necessarily leads to the generation of heat. Therefore, a diode placed into a reverse blocking power application will usually be destroyed by breakdown, as the external circuit will be able to sustain a large current and dump excessive amounts of heat. In principle, however, avalanche breakdown only involves the passage of electrons, and intrinsically need not cause damage to the crystal. Avalanche diodes (commonly encountered as high voltage Zener diodes) are constructed to have a uniform junction that breaks down at a uniform voltage, to avoid current crowding during breakdown. These diodes can indefinitely sustain a moderate level of current while on the edge of breakdown.

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 even 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

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
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