A common application is protecting electronic circuits against damaging high voltages. The avalanche diode is connected to the circuit so that it is reverse-biased. In other words, its cathode is positive with respect to its anode. In this configuration, the diode is non-conducting and does not interfere with the circuit. If the voltage increases beyond the design limit, the diode suffers avalanche breakdown, causing the harmful voltage to be conducted to earth.
Avalanche breakdown is due to impact ionization. Under a small reverse bias, the diode is almost non-conducting, although a very small current still flows. When the reverse electric field across the p-n junction is large enough, the energy of the few electrons flowing is enough to ionize atoms in the silicon. The ionization process is self-reinforcing, causing a rapid increase in current (theoretically taking only picoseconds to reach its peak). Avalanche breakdown is not destructive, as long as the diode is not allowed to overheat.
The Zener diode behaves in a similar way, but its operation is caused by a different mechanism, called Zener breakdown. Some diodes operate with a mixture of Zener and avalanche breakdown.
See also: Avalanche photodiode