How it works
Two mechanisms cause reverse breakdown in a zener diode. Zener breakdown occurs below 5 V: the high reverse electric field across the narrow depletion region directly ruptures covalent bonds and generates carriers — this happens at fields above 10⁶ V/cm. Avalanche breakdown dominates above 6 V: carriers gain enough energy from the field to ionise atoms on collision, triggering a multiplication chain. Between 5 V and 6 V both coexist. The zener voltage VZ is maintained because the diode current adjusts automatically to absorb changes in input voltage, governed by the circuit equation: Vin = VZ + IZ × RS, where RS is the series resistor chosen to keep IZ between IZ(min) and IZ(max).
Key points to remember
The series resistor RS = (Vin(min) − VZ)/IZ(max) must be calculated to ensure regulation across the full load range — this numerical appears in nearly every exam. Zener diodes below 5 V have a negative temperature coefficient (VZ decreases with temperature); those above 6 V have a positive TC. At around 5.6 V the two effects cancel, giving near-zero TC — this is the reference point for precision voltage references like the LM336-5.0. Dynamic zener resistance rz = ΔVZ/ΔIZ is typically 5–30 Ω and represents output impedance of the regulator. Maximum power dissipation PZ = VZ × IZ(max) must not be exceeded — common ratings are 400 mW, 500 mW, and 1 W.
Exam tip
The examiner always asks you to design a zener regulator by calculating RS — use RS = (Vin − VZ)/IZ and then verify the power rating using PZ = VZ × IZ(max) to complete your answer.