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What is a Zener Diode?

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Zener diodes are semiconductor devices that allow current to flow in both directions but specialize in current flowing in reverse. Also known as breakdown diodes, Zener diodes are the most common electronic components used as stable voltage references for electronic circuits.

American physicist, Dr. Clarence Melvin Zener, first described the electronic properties of Zener diodes while researching the breakdown mechanisms of semiconductor materials. As a theoretical physicist at Bell Labs in New Jersey, he wrote a research paper later published in 1934, identifying a type of electrical breakdown later known as the Zener effect. These specific processes were named after him in the years afterward in his honor.

Definitions, terms & abbreviations are located at bottom of the page.

Zener Diode Infographic

Zener Diodes Functionality

Current going across the terminals in reverse bias (backward) is called the Zener effect. When voltage potential is met, this causes the Zener voltage (Vz)/breakdown voltage. Zener diodes uniquely consist of a heavily doped P-N junction that allows current to flow in reverse when reaching Vz. A well-defined Vz can conduct current continuously in reverse bias without getting damaged. Current then increases to a maximum level determined by a series resistor and stabilizes, remaining constant over a range of applied voltages. Therefore, Zener diodes are applicable for use as voltage regulators.

A Zener diode operates within the normal range of forward bias, with a turn-on voltage between 0.3V and 0.7V. When connected in reverse bias, the current flows backward, thus causing a small leakage of current to flow. As the reverse voltage increases to the arranged Vz, current flows throughout the diode. Current increases to a maximum decided by the series resistor and then stabilizes to remain constant over a range of applied voltages.

Zener diodes have two breakdown effects. Knowing these two effects help with understanding their functionality. First is the Zener effect, prevalent in voltages below 5.5V and involves a tunneling effect to cause the breakdown. The second is the avalanche breakdown or impact ionization which occurs in voltages above 5.5V. These breakdowns both result in the same behavior, and they do not require different circuitry, but each does have a different temperature coefficient. The Zener effect has an anode (negative) terminal temperature coefficient, while the avalanche has a cathode (positive) terminal temperature coefficient. Both temperature effects are almost equal to 5.5V. They cancel each other out, making Zener diodes rated at around 5.5V and stable over a wide range of temperature requirements.

Think of a Zener diode like two diodes in parallel, facing opposite directions. The voltage that's forward bias in a Zener diode has a voltage drop of 1V needed for the diode to turn on for the current to flow. The forward voltage is forward biased. The current flowing backward is considered the Zener voltage or reversed biased due to its properties. An example of the Zener voltage is 3.3V. For the current to flow across the diode, the current must maintain at least a minimum of this voltage. Having a predictable voltage drop makes Zener diodes not only useful as voltage regulators, but a correctly set Zener diode may limit the voltage of other devices.

Zener Diode Examples & Applications 

  • Surge Suppressor: An electrical device, also known as a surge protector, that is used to protect equipment against power surges and voltage spikes while blocking voltage over a safe threshold of approximately 120V.
  • Switching Signals: An output signal switching device (or OSSD) is an electronic device in a machine's safety system that provides a coded signal that, when interrupted due to a safety event, signals the machine to shut down. This helps avoid a stray signal operating a machine during unsafe conditions.

Zener diodes applied to an electronic circuit board

  • Voltage Reference: An electrical device that produces a fixed (constant) voltage regardless of the loading on the device, power supply variations, temperature changes, and the passage of time.
  • Voltage Regulator: When the load voltage equals the Zener voltage of the diode, the series-connected resistors limit the current through the diode. This serves to regulate voltage when there is an excess amount of voltage while the diode is conducting. The diode generates noise which can be suppressed by adding a large value decoupling capacitor across the diode.

Power strips are used as surge protectors

  • Voltage Shifter: When applied to a circuit with a resistor, Zener diodes act as voltage shifters. The circuit lowers the output voltage equal to the diodes’ Zener voltage.
  • Waveform Clipper: Reshapes a signal and prevents voltage spikes from affecting circuits connected to a power supply.

Smartphones contain voltage regulators

Application examples that utilize Zener diodes are smartphones which contain voltage, voltage reference devices like digital multimeters, and power strips which are surge suppressors. Electronic circuit boards contain Zener diodes and are used to shift voltage, be a waveform clipper, and switch signals in a variety of devices involving Bluetooth technology, televisions, transmitters that produce radio waves, and more.

In Conclusion

Some notable manufacturers of Zener diodes include Nexperia, Vishay, ROHM, and Toshiba. These manufacturers produce Zener diodes for use in a variety of electronic components specializing in industries like consumer electronics, automotive, telecommunications, computing, industrial, and more. Zener diodes are small components that play a big role in the devices that serve industry professionals and everyday consumers and they will continue to do so for years to come.