Zener Diode

 

Zener Diode - 

 A zener diode is a silicon semiconductor electronic device that allows the current to flow in either a forward or reverse direction. It is designed to conduct current in reverse direction when a certain specified voltage is reached(Zener voltage).

Electronic Symbol-

Zener diodes are also called "Breakdown diodes".

The zener diode behaves just like normal pn junction diodes when connected in forward bias.

Breakdown in zener diode-

There are two types of breakdown region in a zener diode- Avalanche and zener breakdown.


1. Avalanche Breakdown- This breakdown occurs in both normal diodes and zener diodes whenever a high voltage is supplied in reverse direction. The free electrons(minority carriers) gains large amount of energy and accelerated to greater velocities. These free electrons moving at high speed will collide with atoms and knock off other electrons. These electrons are again accelerated and they collide with other atoms. This causes a continuous collision, a large number of free electrons are produced. By this, the electric current rises rapidly. But this sudden rise in the current may destroy the normal diodes. Zener diodes are specially designed to operate in avalanche breakdown region.

2. Zener Breakdown- This breakdown occurs in heavily doped pn junction diodes due to small depletion region. A strong electric field is produced when the reverse voltage of the diode is increased. When the reverse biased voltage is equal to the rated zener voltage, then the electric field is strong enough to pull the electrons from the valence band. The freed electrons carries current through it. At zener breakdown region, a small increase in voltage will increase the current rapidly.

Difference between avalanche and zener breakdown- 

•   The  zener breakdown occurs at low reverse voltage while the avalanche breakdown occurs at high reverse voltage.
• Zener breakdown occurs due to small depletion region.
• Breakdown region is normal operation region for a zener diode.
• Zener breakdown usually occurs at voltage less than 6V.

V-I Characteristics for zener diode-

Advantages of zener diode- 

1. Power dissipation capacity is  very high.

2. High accuracy.

3. Very small in size.

4. Cheap.

Applications for zener diode-

1. It is widely used for voltage reference.

2. It is used in voltage stabilizer or  shunt regulators.

3. Surge suppression circuits

4. Over voltage protection.