Breakdown mechanisms
There are two mechanisms which
give rise to the breakdown of a PN junction under reverse bias condition. They
are (i) avalanche breakdown and (ii) zener breakdown.
(i) Avalanche breakdown : When both sides of the PN
junction are lightly doped and the depletion layer becomes large, avalanche
breakdown takes place. In this case, the electric field across the depletion
layer is not so strong. The minority carriers accelerated by the field, collide
with the semiconductor atoms in the crystal. Because of this collision with
valence electrons, covalent bonds are broken and electron hole pairs are
generated. These charge carriers, so produced acquire energy from the applied
potential and in turn produce more and more carriers. This cumulative process
is called avalanche multiplication and the breakdown is called avalanche breakdown.
(ii) Zener breakdown : When both sides of the PN
junction are heavily doped, consequently the depletion layer is narrow. Zener
breakdown takes place in such a thin narrow junction. When a small reverse bias
is applied, a very strong electric field is produced across the thin depletion
layer. This field breaks the covalent bonds, extremely large number of
electrons and holes are produced, which give rise to the reverse saturation
current (Zener current). Zener current is independent of applied voltage.
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