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 within materials which are otherwise good insulators. It is a type of electron avalanche. The avalanche process occurs when carriers in the transition region are accelerated by the electric field to energies sufficient to create mobile or free electron-hole pairs via collisions with bound electrons.

Materials conduct electricity if they contain mobile charge carriers. There are two types of charge carrier in a semiconductor: free electrons (mobile electrons) and electron holes (mobile holes which are missing electrons from the normally occupied electron states). A normally bound electron (e.g., in a bond) in a reverse-biased diode may break loose due to a thermal fluctuation or excitation, creating a mobile electron-hole pair. If there is a voltage gradient (electric field) in the semiconductor, the electron will move towards the positive voltage while the hole will move towards the negative voltage. Usually, the electron and hole will simply move to opposite ends of the crystal and enter the appropriate electrodes. Under the right circumstances, however, (i.e., when the electric field is strong enough) the mobile electron and/or hole may be accelerated to high enough speeds to knock other bound electrons free, creating more free-electron-hole pairs (i.e. more charge carriers), increasing the current and leading to further "knocking out" processes and creating an avalanche. In this way, large portions of a normally insulating crystal can begin 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.

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