What is breakdown in P-N junction diode? What is Zener & Avalanche breakdown?

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When a P-N junction is reversed biased, it allows very small current to flow through it. This current is due to the movement of minority charge carriers & it is almost independent of the voltage applied.
If reverse bias is made too high, the current through P-N junction increases abruptly & the voltage at which this phenomenon occurs is called breakdown voltage. At this breakdown voltage,the crystal structure breaksdown.
There are two processes which causes junction breakdown.
(1)Zener breakdown- When reverse bias is increased, the electric field across the junction also increases. At some stage the electric field becomes so high that it breaks the covalent bonds creating free electrons. These free electrons, in turn , give rise to high value of reverse saturation current.This mechanism known as zener breakdown. The increase in temperature decreases the breakdown voltage.

V-I Characteristic of Zener & avalanche breakdown.

(2) Avalanche breakdown- At high reverse voltage, due to high electric field, the minority charge carriers, while crossing the junction acquires very high velocities. These by collision breakdown the covalent bonds, generating more carriers. A chain reaction is established, giving rise to high current. This mechanism is called avalanche breakdown. The increase in temperature increases the breakdown voltage.

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