Zener diode

Zener Diode

Definition: A heavily doped semiconductor diode which is designed to operate in reverse direction is known as the Zener diode.

Diodes generally are known as a device that allows the flow of current in one direction (forward biased) and offers resistance to the flow of current when used in reverse bias. Zener Diode  on the other hand, not only allow the flow of current when used in forward bias, but they also allow the flow of current when used in the reversed bias so far the applied voltage is above the breakdown voltage known as the Zener Breakdown Voltage. Or in other words Breakdown voltage is the voltage, on which Zener Diode starts conducting in reverse direction.

VI characteristics of zener diode :

The VI characteristics of a zener diode is shown in the below figure. When forward biased voltage is applied to the zener diode, it works like a normal diode. However, when reverse biased voltage is applied to the zener diode, it works in different manner.

When reverse biased voltage is applied to a zener diode, it allows only a small amount of leakage current until the voltage is less than zener voltage. When reverse biased voltage applied to the zener diode reaches zener voltage, it starts allowing large amount of electric current. At this point, a small increase in reverse voltage will rapidly increases the electric current. Because of this sudden rise in electric current, breakdown occurs called zener breakdown. However, zener diode exhibits a controlled breakdown that does damage the device.

The zener breakdown voltage of the zener diode is depends on the amount of doping applied. If the diode is heavily doped, zener breakdown occurs at low reverse voltages. On the other hand, if the diode is lightly doped, the zener breakdown occurs at high reverse voltages. Zener diodes are available with zener voltages in the range of 1.8V to 400V.

The Avalanche Breakdown and Zener Breakdown are two different mechanisms by which a PN junction breaks. The Zener and Avalanche breakdown both occur in diode under reverse bias. The avalanche breakdown occurs because of the ionisation of electrons and hole pairs whereas the Zener diode occurs because of heavy doping.

Advantages of zener diode:

• Power dissipation capacity is very high
• High accuracy
• Small size
• Low cost

Applications of zener diode:

• It is normally used as voltage reference
• Zener diodes are used in voltage stabilizers or shunt regulators.
• Zener diodes are used in switching operations
• Zener diodes are used in clipping and clamping circuits.
• Zener diodes are used in various protection circuits