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Difference between Avalanche breakdown and Zener breakdown – Definition and working

Real life examples of Avalanche breakdown and Zener breakdown

Avalanche breakdown in simple words is a large mass of prisoners gradually coming together and revolting by running and breaking the gates of prison.

Zener breakdown on other hand is large group of prisoners escaping adruptly and secretly by digging holes through the walls of prison.

Difference between Avalanche breakdown and Zener breakdown

Avalanche breakdown Zener breakdown

Both sides of PN junction are lightly doped

Both sides of PN junction are heavily doped

Depletion layer is large Depletion layer is narrow
Electric field is not so strong A strong electric field is produced
Electron hole pairs are generated Large number of holes and electrons are produced
Charge carriers acquire energy from the applied potential Zener current is independent of applied voltage

How Avalanche breakdown and Zener breakdown happen – Reason behind Avalanche breakdown and Zener breakdown phenomenon

A diode that is reverse biased does not conduct.  This is because of the presence of a depletion layer at the junction.  The depletion layer does not contain any free charge carriers, which is why current flow cannot occur across a reverse biased diode.

If there was some way to generate free charge carriers in the depletion region, current would be able to flow across the junction.

Both the phenomena of zener and avalanche breakdown achieve this.  They both introduce a large number of free charge carriers in the depletion region.

The difference between the two is in the manner in which free charge carriers are generated in the depletion region.

Zener breakdown Theory : It occurs because the reverse bias voltage causes an electric field which is strong enough to pull bound electrons from atoms in the depletion region.  When an electron becomes unbound, it becomes a free charge carrier, and it also leaves behind a hole in the atom from which it is pulled off.  When the zener voltage is reached, a large number of free charge carriers are generated by this freeing up of electrons and holes and a large current is able to flow through the diode.

Avalanche breakdown Theory : It occurs because of collisions.  The p region contains a small number of minority electrons and the n region contains a small number of minority holes.

When the reverse voltage is applied, the majority charge carriers move away from the junction, while the minority charge carriers move toward the junction.  At reverse breakdown voltage, the minority charge carriers acquire sufficient kinetic energy that if they collide with atoms in the depletion region, they are able to release the bound electrons.  Whenever a bound electron is released, a hole is also created in the atom.  This newly freed electron is again accelerated by the electric field and collides with another atom in the depletion region, releasing more free charge carriers.  This happens in an avalanche fashion (or domino effect if you like) and a large number of free charge carriers are released in the depletion region, allowing current to conduct across.

So, in summary, both breakdown mechanisms release free charge carriers in the depletion region, which allows the diode to conduct when reverse biased.