Zener diode
A reverse-biased Zener diode conducts more strongly near its breakdown region. With suitable current limiting, it can establish a reference or limit a voltage. Its voltage is not perfectly fixed: current, temperature, tolerance and dynamic resistance matter.
Tunnelling dominates in lower-voltage Zener diodes, while avalanche breakdown dominates at higher voltages. These mechanisms can overlap. The specified Zener voltage is measured under stated current and temperature conditions, not guaranteed at every operating point.
In plain terms
Like a pressure-relief valve, it allows more flow once a threshold is reached. The surrounding circuit still has to limit the flow and handle the heat.
Why designers use it
- Creating a simple voltage reference with suitable bias and loading.
- Limiting a voltage within a designed current and power budget.
- Setting bias or clipping signal amplitudes.
Best for
- Voltage clamping
- Cheap reference
- Bias setting
Key specifications
- Zener voltage: Nominal value and tolerance at the stated test current
- Dynamic resistance: Voltage change with current near the operating point
- Temperature behaviour: Depends on voltage grade and operating current
- Power and pulses: Check thermal derating and documented pulse conditions
When not to use it
- When the chosen part cannot meet accuracy, noise, current or thermal requirements.
- When the expected transient exceeds its documented pulse capability. Select and validate a suitable protection network instead.
Common mistakes
- Connecting a reverse-breakdown diode to a stiff source without suitable current limiting.
- Treating nominal Zener voltage as the exact voltage at every current and temperature.
- Assuming a 5.6 V part always has zero temperature coefficient.
- Ignoring load-current variation and resistor dissipation in a shunt reference.
Where you will find it
- A resistor-fed Zener can create a simple shunt reference; evaluate minimum bias current and maximum dissipation over the full supply and load range.
- A suitably designed gate-clamp circuit can limit a MOSFET's gate voltage. The clamp level, current and transient behaviour must remain compatible with the MOSFET.
- Two oppositely connected Zener diodes can limit both polarities of an AC signal, with the forward drop also contributing to the clipping level.
A short history
The devices take their name from Clarence Zener's 1934 explanation of the tunnelling mechanism. In everyday component terminology, 'Zener diode' also includes devices whose breakdown is dominated by avalanche. The name does not imply that every voltage grade uses one mechanism exclusively.
Good to know
- The commonly used name covers both tunnelling-dominated and avalanche-dominated devices.
- Changing the bias current can change both the voltage and its temperature behaviour.