A type of breakdown observed in a reverse-biased p-n junction that has very high doping concentration on both sides of the junction. At sufficiently high doping levels, the tunnel effect is the dominant mechanism under reverse bias. The built-in field (see p-n junction) is high and the depletion layer narrow as a result of the high level of doping. The application of a small reverse voltage (of up to about six volts) is sufficient to cause electrons to tunnel directly from the valence band to the conduction band (see energy bands). At the Zener breakdown voltage a sharp increase in the reverse current is obtained. Above the breakdown value the voltage across the diode remains substantially constant. This process is reversible since the dielectric properties of the material remain unchanged. Unlike avalanche breakdown no multiplication of charge carriers occurs.

Avalanche breakdown is the dominant breakdown mechanism in most semiconductor devices except those with high doping concentrations. Zener breakdown is utilized in Zener diodes.