Tech Trill

A Zener diode is a type of diode that permits current not only 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 kneevoltage" or "Zener voltage". The device was named after

Clarence Zener, who discovered this electrical property.

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 circuitry, the diode will be permanently damaged due to overheating.

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.

By contrast with the conventionaldevice, a reverse-biased Zener diode will exhibit a 

controlled breakdown and allow the current to keep the voltage across the Zener diode

 close to the Zener voltage. For example, a diode with a Zener breakdown voltage of 3.2 V

 will exhibit a voltage drop of very nearly 3.2 V across a wide range of reversecurrents. 

The Zener diode is therefore ideal for applications such as the generation of a reference 

voltageor as a voltage stabilizer for low-current applications.

The Zener diode is mainly made of heavely dooped with impurities.

Another mechanism that produces a similar effect is the avalanche effect as in the 

avalanche diode. Thetwo 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 andexhibits

a positive temperature coefficient^[1]. 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 choicein temperature-critical applications. Modern manufacturing

techniques have produced devices with voltageslower 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".