Zener diodes

Zener diodes

The reverse voltage characteristics of a semiconductor diode including the breakdown region is shown below.

Zener diodes are the diodes which are designed to operate in the breakdown region. They are also called as Breakdown diode or Avalanche diodes.
The breakdown in the Zener diode at the voltage Vz may be due to any of the following mechanisms.

Avalanche breakdown

• We know that when the diode is reverse biased a small reverse saturation current I0 flows across the junction because of the minority cariers in the depletion region
• The velocity of the minority charge carriers is directly proportional to the applied voltage. Hence when the reverse bias voltage is increased, the velocity of minority charge carriers will also increase and consequently their energy content will also increase. 
• When these high energy charge carriers strikes the atom within the depletion region they cause other charge carriers to break away from their atoms and join the flow of current across the junction as shown above. The additional charge carriers generated in this way strikes other atoms and generate new carriers by making them to break away from their atoms.
• This cumulative process is referred to as avalanche multiplication which results in the flow of large reverse current and this breakdown of the diode is called avalanche breakdown.

Zener breakdown

                          electric field strength = Reverse voltage/Depletion region

• From the above relation we see that the reverse voltage is directly proportional to the  electric field hence, a small increase in reverse voltage produces a very high intensity electric field with ina narrow Depletion region.
• Therefore when the reverse voltage to a diode is increased, under the influence of high intensity electric filed large numbr of electrons within the depletion region break the covalent bonds with their atoms as shown above and thus a large reverse current flows through the diode. This breakdown is referred to as Zener breakdown.