What is a Zener Diode - voltage reference diode

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Zener / Reference Diode Tutorial Includes:
Zener diode     Zener diode theory of operation     Zener diode datasheet specs     Zener diode circuits    

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The Zener diode is a form of semiconductor diode that is widely used in electronics circuit designs as a voltage reference.

The Zener diode or voltage reference diode is an electronic component that provides a stable and defined voltage. As a result Zener diode circuits are often used in power supplies and other circuit designs where defined or regulated voltage sources are needed.

These diodes are also used for many other applications where stable defined voltage references are needed. For example, they can be used for electronic circuits where the voltage is limited or clipped to a certain value for many reasons including for removing transient spikes from signal lines, as well as a host of other situations.

Zener diodes / voltage reference diodes are cheap and they are also easy to use and these electronic components are easily available for use in a variety of electronic circuit designs with a variety of voltages and with a variety of power ratings, etc.

The Zener diode operates like a normal PN junction diode in the forward direction, but provides a very sharp breakdown in the reverse direction at a defined voltage. It is this reverse breakdown voltage that is used for voltage references or in clipping applications.

Zener diode history

Zener diode history has its foundations in the development of the first semiconductor diodes. Although early detectors such as cat's whisker, point contact diodes had been available since around 1905, much work on semiconductors and semiconductor diodes was undertaken during and after the Second World War.

The first person to describe the electrical properties utilised by the Zener diode was Clarence Melvin Zener (born 1 Dec 1905, died 15 July 1993).

Clarence Zener was a theoretical physicist who worked at Bell Labs and as a result of his work, Bell named the Zener diode after him. He first postulated the breakdown effect that bears his name in a paper that was published in 1934.

Nowadays, Zener diodes and other voltage reference diodes are widely available and are used within many electronic circuit designs.

Zener diode basics

Zener diodes are sometimes referred to as reference diodes as they are able to provide a stable reference voltage for many electronics circuits.

Zener diodes have many of the same basic properties of ordinary semiconductor diodes. They conduct in the forward direction and have the same turn on voltage as ordinary semiconductor diodes. For silicon this is about 0.6 volts.

In the reverse direction, the operation of a Zener diode or voltage reference diode is quite different to that of an ordinary semiconductor diode.

For low voltages the diodes do not conduct as would be expected. However, once a certain voltage is reached the diode "breaks down" and current flows.

Looking at the curves for a Zener diode, it can be seen that the voltage is almost constant regardless of the current carried. This means that a Zener diode provides a stable and known reference voltage for a wide range of current levels.

The remarkable stability of the breakdown voltage over a wide range of current throughput levels is what makes the Zener or voltage reference diode so useful.

These electronic components can be used in a wide variety of circuits to provide a stable reference voltage as well as being used in a variety of other circuits where its reverse breakdown characteristic can be utilised.

It is worth noting that there are two different forms of breakdown effect that occur in these semiconductor diodes:

• Zener breakdown:   Zener breakdown is the effect that dominates below about 5.5 volts.

• Impact ionisation:   Above about 5.5 volts impact ionisation becomes the major effect

in terms of their circuit operation, the effect is the same as both exhibit a very stable voltage reference.

However, it is worth noting that the temerpature coefficient of both effects are different, and this means that voltage reference diodes having a reverse voltage around 5.5 volts give the most stable reference with respect to temperature.

Zener diode circuit symbol

There are many styles of package for a Zener diode. Some are used for high levels of power dissipation and others are contained within surface mount formats. For home construction, the most common type is contained within a small glass encapsulation. It has a band around one end and this marks the cathode.

It can be seen that the band around the package corresponds to the line on the diode circuit symbol and this can be an easy way of remembering which end is which. For a Zener diode operating in its reverse bias condition the band is the more positive terminal in the circuit.

To distinguish the Zener or reference diode from other forms of diode within a circuit diagram, the Zener diode circuit symbol places two tags at the end of the bar - one in the upward direction and the other in the lower direction as shown in the diagram.

Zener diode type numbering

In terms of type numbering, Zener diodes, or voltage reference diodes pose a small problem for type numbering them. There can be a generic series of diodes all in the same family, but with different breakdown or reference voltages.

As a result it possible to reserve a consecutive series of diode numbers within the system, or add a suffix to the basic type number to indicate the voltage.

One method for numbering Zener diodes in the same family but with different voltages is to use a series within the standard numbering system. One example is the series1N4728A to 1N4764A with one part number allocated for each voltage. These diodes are 400 mW Zeners with voltages ranging from 3.3 to 100 V with a 5% tolerance and in E24 range.

The other method used is to have a number for the family and then suffix the part number with the voltage, e.g. BZY88 C5V6 where 5V6 is the voltage, 5.6 volts.

Zener diodes or voltage reference diode values are normally spaced using the E12 series, although some are available in the E24 series, e.g 5V1 used for a number of logic chips where is very simple Zener regulator is used. If a transistor emitter follower is used for greater current, then the 5V6 Zener is better as the transistor will drop 0.6 volts and this makes it ideal.

Although it is best to stick with the more commonly used E12 series, or better still E6, or even E3, often this is not feasible and Zener diode voltage values from the E24 series are available.

NB: E12 values marked in bold.

Zener diodes are not normally supplied in series higher than the E24 range The reason for this is that the manufacturing tolerances are not sufficient and the usage does not normally require this.

Zener diode technology

Zener diodes operate under reverse bias and rely on two forms of reverse breakdown. One form of reverse breakdown is referred to as Zener breakdown, and this gives the name is often used to describe all forms of voltage reference diode. The other type of reverse breakdown can be referred to as impact ionisation breakdown.

It is found that of the two effects the Zener effect predominates below about 5.5 volts whereas impact ionisation is the major effect above this voltage as mentioned before.

Zener diodes / voltage reference diode specifications

When selecting a Zener diode or voltage reference diode for use in a circuit, there are several specifications that need to be considered to ensure that the optimum diode for the application is selected.

The obvious Zener diode specification is the reverse voltage, but other specifications like the power dissipation, reverse current and the like are also important for any circuit design that might include the diode.

Zener diode circuits

There are many ways in which Zener diodes or voltage reference diodes can be used. The most widely known is as a voltage reference in some form of voltage regulator, but they can also be used as waveform limiters for circuits where it may be necessary to limit the excursion of a waveform to prevent overload, etc. They can also be used in voltage shifters.

Accordingly Zener diodes are often found in electronic circuit designs, and vast quantities of them re used in manufacturing, both as leaded devices and in surface mount formats.

A Zener diode is a particularly useful electronic component for the design of electronics circuits. As a result many millions of Zener diodes / voltage reference diodes are used each year in the construction of electronics equipment both as discrete components, as leaded versions and surface mount devices as well as components contained within large integrated circuits.

Although voltage reference integrated circuits are available that provide very high degrees of accuracy and temperature stability, for most uses a straightforward Zener diode of more than satisfactory and will provide a much cheaper solution. Accordingly it helps to know what is a Zener diode, how one works, and the basics of a Zener diode circuit.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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