Impact ionization is the process in a material by which one energetic charge carrier can lose energy by the creation of other charge carriers. If this occurs in a region of high electrical field then it can result in avalanche breakdown, this process is exploited in avalanche type devices to provide gain.

Charge carriers are typically electrons, but in a semiconductor they could also be holes. This is the process occurring in avalanche diodes, by which a small optical signal is amplified before entering an external electronic circuit. In an avalanche photodiode the original charge carrier is created by the absorption of a photon.

In some senses, it is the reverse process to Auger recombination.

Semiconductors | Stoßionisation