Zener Diode
A Zener diode is a silicon diode designed to be operated in reverse breakdown, where its voltage stays almost constant over a wide range of current, so it can serve as a voltage reference, a simple regulator or a clamp. Instead of treating reverse breakdown as a failure, the manufacturer controls it through doping so that it occurs sharply at a chosen voltage, from a couple of volts to a few hundred. In the forward direction it behaves like any other diode, with a drop of about 0.6 to 0.7 volts.
The name honours Clarence Zener, an American physicist who published a theory of the electrical breakdown of solid dielectrics in 1934. Zener proposed that in a sufficiently strong electric field, electrons can be pulled directly from the valence band into the conduction band by quantum tunnelling, without any need for collisions. That theory was written about insulators, decades before semiconductor diodes existed, and the diode was named after the physicist later, once breakdown in silicon junctions was understood. The history is a reminder that the device name credits an idea about the mechanism, not the invention of the component.
Two different physical mechanisms hide under the one name, and this is the diode's most instructive subtlety. In heavily doped junctions with breakdown voltages below roughly 5 volts, the depletion layer is thin enough that true Zener tunnelling dominates, and the breakdown voltage falls slightly as temperature rises, giving a negative temperature coefficient. In more lightly doped junctions with breakdown above about 7 volts the layer is wider, and avalanche multiplication, the impact ionization mechanism studied in silicon junctions in the 1950s, takes over, with a positive temperature coefficient because lattice scattering at higher temperature makes carriers lose energy and require more field. Between those, near 5 to 6 volts, a mixture of both gives the coefficient close to zero, which is why diodes of that voltage were chosen for the most stable references. Nearly every part sold as a Zener diode above 7 volts is in fact an avalanche diode, but the old name stuck.
The standard circuit is a Zener in reverse across the load, fed from a higher supply through a series resistor. The resistor limits the current, and the diode takes up whatever current the load does not, holding the output at its breakdown voltage. For a 5.1 volt part, widely sold in 1 watt and 500 milliwatt types such as the 1N4733 and 1N5231, the maximum current is the power rating divided by the voltage. Because the breakdown curve is not perfectly vertical, a diode has a dynamic impedance of a few ohms to tens of ohms, so output voltage shifts a little with load and supply, and it produces noise, in some cases considerable. The same property serves as a clamp: placed across a sensitive input, it conducts only if the voltage exceeds its rating, protecting the circuit from surges and electrostatic discharge, in which role related transient-voltage-suppression diodes are optimized for large pulses.
Refinements made Zener-type references far more stable than a bare diode. Temperature-compensated references stack a Zener diode of about 5.6 to 6.2 volts with a forward-biased diode whose opposing temperature coefficient cancels the drift, and integrated devices use a buried Zener structure under the oxide surface, away from surface-state noise and drift, to give references stable to a few parts per million per degree. A related approach, the bandgap reference, derives a voltage from the base-emitter junctions of transistors and now dominates in integrated circuits, particularly for supply voltages below 5 volts, but buried Zener references remain in precision instruments and laboratory voltage standards.
Common pitfalls involve misreading the datasheet and the application. The quoted Zener voltage is specified at a test current, and it will differ at another current, and its tolerance, commonly 5 percent, may be too loose for a reference without trimming. Zener diodes are not efficient regulators: they waste power continuously, so they suit small reference tasks, not power supply regulation, where linear regulators and switching converters have replaced them. Another mistake is to assume breakdown itself is harmful; the real limit is power dissipation, and a Zener connected without a series resistor will conduct until it burns out.
Read Zener diode as a controlled reverse breakdown: a junction doped so that it conducts at a chosen voltage, tunnelling below about 5 volts and avalanching above, so that it holds a nearly constant voltage as the current varies.