Zener Diodes

What is a Zener Diode?

A Zener diode is a special-purpose silicon diode optimized for operation in the breakdown region.

When sufficiently large reverse bias voltages are applied, the diode current increases very rapidly. This is known as a reverse breakdown. Small-signal and rectifier diodes are never used in the breakdown region because this may cause their destruction.

Once a breakdown occurs the voltage UZ across the zener diode becomes almost constant and independent of the current through the diode. By utilizing a constant voltage zener diodes often operate as voltage regulators - circuits that keep the output voltage constant despite the large changes in the input voltage.

Application

Every electronic equipment needs a dc power supply - circuit that converts the alternating voltages from the power line to direct voltage. Usually the power supply produces a dc voltage with ripple (small fluctoations). It is desirable that the power supply's output voltage remains constant independent of load current and the supply fluctuations.

For this reason the power supply's output is connected to a zener voltage regulator as shown in the figure. Ideally, the zener regulator reduces the ripple to zero since the load voltage is constant and equal to the zener's voltage UZ.

Zener Diode Structure

The zener diode has the same structure as an ordinary pn junction diode. It represents a single semiconductor crystal. One part of this semiconductor crystal is a p-type and the other is an n-type, as shown in the figure.

The border zone between the p-type and the n-type regions is called a pn junction. Processes concerning zener diode operation are performed in the layer, depleted of mobile charge carriers - known as depletion layer do. Ions in the depletion layer produce an electric field Eo and corresponding barrier potential Uo.

Zener Diode Schematic Symbol

The zener diode has two electrodes. The p-side is called anode, and the n-side - cathode.

The figure shows the schematic symbol of a zener diode and the relation between symbol electrodes and diode structure. When zener diode operates in breakdown mode, the cathode must be positive in respect to the anode.

Zener Diode Packages

The figure shows some of the existing packages (not relative in sizes) for zener diodes. They are available in glass, plastic or metal cases. A color band indicates the cathode.

Large power devices have metal cases. At higher power levels, the cooling can be supported by mounting the device on a heat sink (a mass of metal).

Zener Diode Mode of Operation

Zener diode operates in the breakdown region. Before breakdown the current is very small. It consists of thermally produced minority carriers moving across the junction. At breakdown voltage UZ, the current IZ increases rapidly for a small instance of voltage.

This is caused by carriers multiplication effects, which occur within the depletion layer when a critical value of electric field EZ, called a breakdown field, is exceeded. Once the breakdown voltage is reached, a large number of minority carriers suddenly appears in the depletion layer and the diode starts to conduct heavily. This results from two mechanisms known as avalanche and zener breakdown.

Avalanche Breakdown

The reverse current consists of minority carriers diffusing to the depletion layer and then accelerated by the build-in electric field. At higher reverse voltage, the accelerated carriers gain enough kinetic energy to ionize an atom of the crystal if they collide with it. This creates an additional free electron and a hole.

These carriers can now be accelerated by the field and generate another two electron- hole pairs on collisions with crystal atoms. The process may continue, causing the increase of the current. One can make analogy with an avalanche accreting snow as it slides down a mountainside. This process is called the avalanche effect or avalanche breakdown.

Zener Breakdown

Another mechanism of a reverse breakdown occurs when an electric field of the barrier potential is large enough to break the covalent bonds. This process creates additional free electrons and holes and is called Zener effect or Zener breakdown. This type of breakdown requires high electric fields on the order of 300 000 V/cm.

Such very intense electric fields are obtained in heavily doped (with high impurity concentrations) diodes. They have a very narrow depletion layer. For this reason the electric field across the depletion layer (voltage divided by distance) is very large even though the applied voltage is small. In such diodes Zener breakdown occurs with a reverse bias less than 5V.

Zener Diode VA Characteristic

The figure shows the volt ampere (VA) characteristic of a zener diode.

A zener diode can operate in any of the three regions: forward, reverse and breakdown.

In the forward region it starts conducting around 0.7 V, just like an ordinary Si diode. In the reverse region (between zero and breakdown) it has only a small reverse current.

In the breakdown region there is a very sharp knee, followed by an almost vertical rise in current. Notice that the voltage is almost constant, approximately equal to zener breakdown voltage UZ over most of the breakdown region.

Breakdown Region

A Zener diode is sometimes called a voltage regulator diode because it maintains a constant output voltage even though the current changes through it.

In the breakdown region a zener diode ideally acts like a battery and can be replaced by a voltage source of UZ. Diodes are called zener diodes even though the actual breakdown mechanism is often the avalanche one.

Load Line

The figure shows one of the simplest zener diode circuits. Applying Kirchoff's law to the circuit a linear relation between current and voltage is obtained: IZ = (US1 - UZ)/R. Here US1 is the source voltage, UZ and IZ are voltage and current across the zener diode. This equation represents a straight line called a load line. It has a slope of 1/R.

The load line can be plotted with its intercepts on the horizontal and vertical axes - (IZ=0, U=US1) and (UZ=0, IZ = US1 / R), respectively. The interception point of a load line with the zener diode VA characteristic gives operational point of Q1.

Load Line Shifting

If the source voltage changes the zener current will also change. With R fixed the load line slope is unchanged but the U intercept shifts. Thus as US1 is increased to US2, the load line shifts parallel to itself and moves to the left. The new intersection is at Q2. It is obvous that even though the source voltage has changed from US1 to US2, the zener voltage is still approximately equal to UZ.

This is the basic idea of voltage regulation - the output voltage remained almost constant even though the input voltage changed by a large amount.

The Zener Static Resistance

Zener static (or dc) resistance RZ is the ratio of total zener diode voltage to total diode current measured at given operating point. For operating point Q shown in the figure the zener diode static resistance is RZ = UzQ / IzQ.

The current through zener resistance produces voltage drop in addition to the zener breakdown voltage. It is very small, typically a few tends of a volt. For more precise calculation a zener diode can be replaced by an ideal battery in series with a small zener resistance RZ.

The Zener Dynamic Resistance

The figure shows the volt ampere (VA) characteristic of a zener diode.

A zener diode can operate in any of the three regions: forward, reverse and breakdown.

The dynamic (or ac) zener resistance RZ is defined as voltage difference divided by current difference at given operating point. It can be found from the VA characteristic as shown in the figure. In the breakdown region dynamic resistance is very small.

The less the dynamic resistance the better the zener diode as a voltage regulator.

Zener Voltage for Avalanche & Zener Effect

The VA characteristics of a zener diode for both breakdown mechanisms are shown in the figure. The zener effect occurs in heavily doped diodes for reverse voltages less than 5V. Avalanche effect requires reverse voltages above 6V.

The onset of avalanche breakdown is usually much sharper than in the zener case and the conduction is generally higher. This allows this type of zener diodes to be used more effectively in voltage regulator circuits.

Temperature Coefficient

Raising the ambient (surrounding) temperature changes the zener voltage slightly. On data sheets, the effect of temperature is listed under the temperature coefficient, which is the percentage change per degree change.

For zener diode with breakdown voltages of less than 5V, the temperature coefficient is negative.

Thermo Compensation

For zener diode with UZ > 6V, the temperature coefficient is positive.

Positive temperature coefficient can be compensated by connecting a zener diode in series with forward based pn junction diode. A forward biased pn diode has negative temperature coefficient and its forward voltage decreases -2 mV/oC.

This way a total temperature coefficient of less than 0.001% / oC can be obtained. This way the zener voltage will not change over a large temperature range.

Zener Diode Ratings

Minimum zener current IZmin is the minimum reverse current where the breakdown becomes stable. If a zener diode has to remain in the breakdown region the current through it has to be more than IZmin.

Interesting feature of the zener diodes working with currents near to but less than IZmin is that they produce noise. This feature is used in noise generators.

Maximum Power

The power dissipation of a zener diode equals the product of its zener voltage and current PZ = Uz.Iz. When power is dissipated within a device, its junction temperature Tj tends to rise. However, the process is not necessarily destructive.

The large currents generate heat and it is the dissipation of this heat, which is critical to the survival of the device. As long as PZ is less than the maximum power PZmax rating the zener diode can operate in the breakdown region without being destroyed.

Maximum Zener Current

Commercially available zener diodes have power ratings from 1/4 to more than 50 W. The maximum current of a zener diode IZmax is related to the power rating PZmax as follows: IZmax = PZmax / UZ, where UZ is the zener voltage.

This parameter gives the max current a zener diode can handle without exceeding its power rating.

Current-Limiting Resistor

In the figure, the series resistor R is referred to as a current-limiting resistor. It's purpouse is to keep the zener current less than its maximum current rating IZmax. The zener diode will otherwise be destroyed like any device exposed to power overload.

The current through the resistor is I = (US - UZ) / R, where US is the source voltage and UZ is the zener breakdown voltage.

This equation is Ohm's law applied to the current- limiting resistor. The zener current has the same value as the current through resistor R, since this is a serial circuit.

Zener Cicuits - Voltage Regulator

The figure shows the output of a power supply connected to a series resistor and a zener diode. This type of circuit is called a zener voltage regulator as it maintains a constant output voltage even though the source voltage US changes.

For normal operation the zener diode should be reverse biased. as shown. To reach breakdown mode, the source voltage US must be greater than the zener breakdown voltage. In this case even if the current changes, the output voltage remains constant and equal to UZ. Resistor R is used to limit the zener current to less than its maximum current rating.

Loaded Voltage Regulator

The figure shows a loaded voltage regulator. The zener diode holds the load voltage constant despite large changes in source voltage or in load resistance.

The battery US represents a filtered but unregulated power supply voltage. When the zener is operating in the breakdown region the current I through resistor R is given by expression above. Since the load resistor RL and the zener diode are in parallel, the load voltage UL= UZ. Then the load current UL is calculated according Ohm's law. Now the zener current IZ is determined with Kirchhoff's current low.

Stabilization Coefficient

Variation in supply voltage or in load resistor will change the zener current, but they have almost no effect on the load voltage. It remains constant and equal to UZ.

The level of stabilization is measured by the stabilization coefficient S. It is defined as percentage of input voltage variation divided by output voltage changes. Since the load voltage is equal to zener voltage, the stabilization coefficient is determined by variation in zener voltage. The higher the S, the better the voltage regulation.

Operational Conditions

For a zener regulator to hold the output voltage constant, the zener diode must remain in the breakdown region under all operational conditions. The worst case occurs for minimum source voltage and maximum load current because the zener current drops to minimum IZmin.

Maximum allowable series resistance Rmax has to keep IZ value higher than IZmin. Otherwise, the breakdown operation is lost and the regulator stops working. Minimum allowable series resistance Rmin has to keep IZ less than IZmax. Otherwise the zener diode will burn out and the regulator will be destroyed.

Voltage Limiter

Voltage limiter removes signal voltages above or below a specified level. It is useful not only for signal shaping but also for protecting the circuits that receive the signal.

Figure shows a zener diode limiter. On the positive half cycle, the upper diode Z1 breaks down and the lower diode Z2 is forward biased. The output is then clipped. The clipping level equals the zener voltage UZ of broken-down diode plus 0.7V of forward-biased diode.

On the negative half cycle, the action is reversed. The upper diode is forward biased, and the lower diode breaks down. In this case, the output is almost a square wave.