Avalanche breakdown

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.

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 (exciton).

Usually, the electron and hole will simply move to opposite ends of the crystal and enter the appropriate electrodes.

When the electric field is strong enough, the mobile electron or hole may be accelerated to speeds high enough to knock other bound electrons free, creating more free charge carriers, increasing the current and leading to further "knocking out" processes and creating an avalanche.

Therefore, a diode placed into a reverse blocking power application will usually be destroyed by breakdown if the external circuit allows a large current.