Question

Distinguish between avalanche breakdown and Zener breakdown.

Answer 1

Hint : The zener breakdown occurs due to the high electric field. Avalanche breakdown occurs due to acceleration of minority carriers. A “forceful” crossing of the carriers across the p-n junction occurs for zener breakdown.

Complete step by step answer
Generally, in a diode, reverse current is often taken to be zero. Since the diode allows current to move only in a particular direction and block current from the other direction, the current in that other disallowed direction is ideally supposed to be zero. Usually there is tiny minority current called leakage current due to minority carriers which flow in that direction. Also, beyond some certain voltage a breakdown can occur in a diode, and hence allow current to flow in the disallowed direction. This breakdown can be caused by two different mechanisms. The zener effect or the avalanche effect. When the zener effect causes the breakdown, it is called zener breakdown and avalanche breakdown when it is caused by the avalanche effect.
The avalanche breakdown – when a high reverse voltage is applied, the number of reverse carriers entering the depletion region, though small, becomes accelerated hence gain a very high kinetic energy and cause collisions with atoms and hence, break the covalent bond bonding the electrons. These electrons hence get accelerated and break down other electrons which also accelerate and break down others which in total increases the number of free carriers in the diode.
Zener breakdown – in a highly doped diode, when a reverse electric field is applied it may breakdown the bond of the electrons bound to the atoms (from p type semiconductor) which hence gains kinetic energy and then gets swept to the other side (n-type semiconductor).
The major difference is that avalanche breakdown is caused by acceleration of minority carriers due to breakdown of the covalent bond of electrons from atoms while zener is caused by the high electric field which breaks down the covalent bond.

Note
Some other differences are that
-They have opposite temperature coefficient – zener as a negative coefficient and avalanche has positive
-Zener occurs in highly doped diodes and avalanche in lightly doped diode
-In zener, the electric field must be strong whereas the electric field is weaker for avalanches.