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A Zener diode is a type of diode that is frequently employed as a shunt regulator to hold a voltage constant at a pre-determined level. Zener diodes may be constructed as discrete components or embedded into Integrated circuits.

History


Zener diodes were proceeded by Voltage regulator tubes. By comparison to modern zener diodes, the VR tube is considerably larger, delicate, and requires high operating voltages. The accompanying picture illustrates the size difference between a type 5651 voltage regulator tube and a type 1N4735A zener diode.

The zener diode which bears the name of its inventor was developed in the late 1950 by Dr. Clarence Zener while working at Westinghouse Electrical Company

The zener diode was further refined in the mid 1970s when National Semiconductor developed the buried-zener diode as featured in the LM199 precision voltage reference. The buried-zener diode has the advantage of being less electrically noisy than the conventional zener diode.

Zener diode as a shunt regulator
The Zener diode may be used as simple shunt regulator. With an appropriate series connected current limiting resistor (R1), the circuit will provide an accurate voltage level independent of changes in either the input power supply (VS) or changes in the load (R2).

This type of circuit is highly inefficient. For a given (VS) the current is always constant since the Zener diode and load effectively form a current divider i.e., the circuit consumes power even when the load is disconnected. This attribute must be considered when selecting a Zener diode to guard against possible destruction when the load is disconnected.

The resistor R1 supplies the Zener current IZ as well as the load current IR2 (R2 is the load). R1 can be calculated as -

$$R1 = \frac{V_{S} - V_{Z}}{I_{Z} + I_{R2}}$$ where, VZ is the zener voltage, and IR2 is the required load current.

Zener diode as a voltage reference



 * Make distinction between shunt regulation and VR - the Zener may be low power as the external pass element handles the current

Regulated power supplies often contain a Zener Diode as a voltage reference. The output of the power supply (feedback) is compared to the voltage reference in a comparator stage. The comparator then adjusts the output of the power supply to maintain the feedback at the same level as the voltage reference thereby providing regulation. These devices are also encountered, typically in series with a base-emitter junction, in transistor stages where selective choice of a device centered around the avalanche/Zener point can be used to introduce compensating temperature co-efficient balancing of the transistor PN junction. An example of this kind of use would be a DC error amplifier used in a stabilized power supply circuit feedback loop system.

Zener diode as a clamp
The voltage to a to a sensitive amplifier stage may be limited by using a Zener diode as shown in the accompanying figure.

Zener diode as a noise generator
A Zener diode may be used as a wideband white noise generator. This white noise is caused when a Zener diode operates in the reverse breakdown mode. A low noise amplifier may be required to boost the signal to a usable level.

Zener diode as a level shifter


An AC signal may passed through a Zener diode.

Specification
The specification for an individual Zener diode are available in the device's data sheet. While each data sheet contains many unique specification, this article will focus on breakdown voltage, power, and the temperature coefficient.

Breakdown voltage

 * Voltages available (~1 - 100V)
 * Doping (semiconductor)

Power dissipation
The power dissipated by a Zener diode is a product of the voltage drop and amount of current passing through the Zener diode. Consequently, the power rating is largely determined by case style (physical size and the heatsink). A stud mounted device such as the XXXX may dissipate XXX watts while attached to a the specified minimum heatsink. The smaller 2N4735A may only disipate XXX watts.
 * power (~0.25 to 50 W)

The maximum power dissipation is also dependent on the temperature of the device. The Zener diode data sheet will normally present a chart derating the device.

Temperature coefficient

 * Not applicable to all data sheets
 * sweet spot at ~6.2VDC

Lead identification


The Zener diode is a two terminal device consisting of a cathode and anode. The cathode is normally identified by a small band on the body of the device. Larger devices may have the schematic symbol of the Zener diode printed on the case. Refer to the manufactures data sheet if there are no identifiable markings.

Theory of operation
Because it is almost always the reverse-breakdown property of the Zener diode that is useful, in circuit schematics the Zener diode symbol (i.e., a diode arrow) typically points in the opposite direction of the circuit's positive current flow. This use is opposite that of a typical diode, which would be aligned with the current flow.



A Zener diode is a type of diode that permits current in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage known as "Zener knee voltage" or "Zener voltage".

A conventional solid-state diode will not allow significant current if it is reverse-biased below its reverse breakdown voltage. When the reverse bias breakdown voltage is exceeded, a conventional diode is subject to high current due to avalanche breakdown. Unless this current is limited by external circuitry, the diode will be permanently damaged. In case of large forward bias (current in the direction of the arrow), the diode exhibits a voltage drop due to its junction built-in voltage and internal resistance. The amount of the voltage drop depends on the semiconductor material and the doping concentrations.

A Zener diode exhibits almost the same properties, except the device is specially designed so as to have a greatly reduced breakdown voltage, the so-called Zener voltage. A Zener diode contains a heavily doped p-n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material. In the atomic scale, this tunneling corresponds to the transport of valence band electrons into the empty conduction band states; as a result of the reduced barrier between these bands and high electric fields that are induced due to the relatively high levels of dopings on both sides. A reverse-biased Zener diode will exhibit a controlled breakdown and allow the current to keep the voltage across the Zener diode at the Zener voltage. For example, a diode with a Zener breakdown voltage of 3.2 V will exhibit a voltage drop of 3.2 V if reverse bias voltage applied across it is more than its Zener voltage. However, the current is not unlimited, so the Zener diode is typically used to generate a reference voltage for an amplifier stage, or as a voltage stabilizer for low-current applications.

The breakdown voltage can be controlled quite accurately in the doping process. While tolerances within 0.05% are available, the most widely used tolerances are 5% and 10%.

Another mechanism that produces a similar effect is the avalanche effect as in the avalanche diode. The two types of diode are in fact constructed the same way and both effects are present in diodes of this type. In silicon diodes up to about 5.6 volts, the Zener effect is the predominant effect and shows a marked negative temperature coefficient. Above 5.6 volts, the avalanche effect becomes predominant and exhibits a positive temperature coefficient.

In a 5.6 V diode, the two effects occur together and their temperature coefficients neatly cancel each other out, thus the 5.6 V diode is the component of choice in temperature-critical applications.

Modern manufacturing techniques have produced devices with voltages lower than 5.6 V with negligible temperature coefficients, but as higher voltage devices are encountered, the temperature coefficient rises dramatically. A 75 V diode has 10 times the coefficient of a 12 V diode.

All such diodes, regardless of breakdown voltage, are usually marketed under the umbrella term of "Zener diode".