A p-n junction diode that has sufficiently high doping concentrations on each side of the junction for Zener breakdown to occur. The diode therefore has a well-defined reverse breakdown voltage (of the order of a few volts only) and can be used as a voltage regulator. Unlike tunnel diodes, the doping level is not high enough for the semiconductor to become degenerate and the diode behaves like a normal p-n junction in the forward direction.
The term is also applied to less highly doped p-n junction diodes that have higher breakdown voltages (up to 200 volts) and undergo avalanche breakdown. Most so-called Zener diodes are in fact avalanche breakdown diodes. True Zener diodes have a low value of reverse breakdown voltage.