Zener Effect

The Zener effect is a type of electrical breakdown that occurs in a reverse-biased PN junction when the electric field enables tunnelling of electrons from the valence to the conduction band of a semiconductor, leading to a large number of free minority carriers which suddenly increase the reverse current. The Zener effect is best-known for its use in the appropriately-named Zener diode.

A normal PN junction diode freely allows current flow if forward biased, but blocks current if it is reverse biased – a useful attribute in rectifier circuits, for example. However, if the reverse voltage applied across such a diode becomes too high, the device will break down and probably suffer permanent damage.

A Zener diode behaves similarly to the extent that it allows forward current, but blocks it if reverse-biased. The difference comes if the reverse voltage reaches the device’s critical break-down value, because it begins to conduct in the reverse direction. This is due to an effect known as avalanche breakdown, which occurs in the semiconductor depletion layer; a current starts to flow through the diode to limit the voltage increase.

The voltage value at which the Zener breakdown occurs can be very accurately set by the semiconductor manufacturer’s doping stage; this allows Zener diodes to be widely used as voltage reference devices in power supply circuits. The devices are also valuable as voltage limiters, preventing spikes from reaching and damaging electronic equipment.