Zener diode

A Zener diode is based on a p–n junction. Like an ordinary diode, it conducts readily when forward-biased. Under reverse bias, a small leakage current flows until the junction reaches its designed breakdown region. There, reverse current rises sharply while the voltage across the junction remains comparatively stable over a specified operating range.

Two physical processes can contribute. In a heavily doped junction, the depletion region is narrow and a strong electric field can let electrons tunnel across the junction; this is called Zener breakdown. In avalanche breakdown, carriers accelerated by the field collide with atoms and generate additional electron–hole pairs, multiplying the current. Practical devices may involve both mechanisms, with their relative importance shaped by doping and breakdown voltage.

A basic shunt regulator places a Zener diode in reverse bias across a load and uses a series resistor between the supply and the parallel diode–load combination. The resistor limits current. When the supply and load allow the diode current to remain within its specified operating range, changes in supply voltage or load current are partly absorbed as changes in Zener current, helping keep the load voltage near the diode's breakdown voltage.

This arrangement is simple, but it is not an ideal voltage source. Regulation fails if current falls below the required breakdown-region operating level, while excessive current can overheat and damage the diode. Designers therefore select the series resistance and check the diode's current and power ratings across expected supply and load conditions. A Zener diode can also provide a voltage reference or limit a signal's excursion.

Breakdown voltage varies with temperature. The balance between tunnelling and avalanche mechanisms influences the sign of the temperature coefficient: tunnelling-dominated breakdown generally shifts toward lower voltage as temperature rises, whereas avalanche-dominated breakdown generally shifts toward higher voltage. The crossover is not a universal exact voltage; it depends on device construction and conditions.

This temperature dependence matters in precision references and in circuits exposed to changing temperatures. Datasheets specify device characteristics at stated test conditions, and the nominal breakdown voltage should not be treated as perfectly constant under all currents and temperatures.

The name comes from physicist Clarence Zener, whose theoretical work described electron tunnelling in solids. In electronics, “Zener diode” is commonly used for diodes designed to exploit reverse breakdown, including devices whose breakdown at a particular voltage is substantially avalanche-driven. Thus the everyday device name does not imply that tunnelling is the only physical mechanism involved.