A conventional solid-state diode will not let current flow if reverse-biased (up to a breakdown voltage). By exceeding the breakdown voltage a conventional diode is destroyed in the breakdown due to excess current and overheating. In case of forward-bias (in the direction of the arrow) the diode exhibits a voltage drop of roughly 0.7 volt. The voltage drop depends on the type of the diode.

A Zener diode exhibits almost the same properties, except the device is especially designed so as to have a greatly reduced breakdown voltage, the so-called Zener voltage. A Zener diode contains a heavily doped p-n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material. A reverse-biased Zener diode will exhibit a controlled breakdown and let the current flow to keep the voltage across the Zener diode at the Zener voltage. For example, a 6.2 volt Zener diode will exhibit a voltage drop of 6.2 volt if reverse biased. However, the current is not unlimited, so the Zener diode is typically used to generate a reference voltage for an amplifier stage.

The breakdown voltage can be controlled quite accurately in the doping process. Tolerances up to 0.05% are available though the most widely used tolerances are 5% and 10%.

The effect was discovered by the American physicist Clarence Melvin Zener.

Another type of diode designed to break down under reverse bias, by a different mechanism to the Zener diode, is the avalanche diode.