diode (a) Commercial picture (b) Circuit symbol V-I Characteristics of Zener diode The circuit to study the forward and reverse characteristics of a Zener diode is shown in Figure . (a) and Figure . (b). The V-I characteristics of a Zener diode is shown in Figure .
(c). The forward characteristic of a Zener diode is similar to that of an ordinary p-n junction diode. It starts conducting approximately around . V.
However, the reverse characteristics is highly significant in Zener diode. The increase in reverse voltage with semiconductor atoms while passing through the depletion region. This leads to the breaking up of covalent bonds and this results in the generation of electron - hole pairs. The newly generated charge carriers are also accelerated by the reverse voltage resulting in more collisions and further production of charge carriers.
This cumulative process leads to an avalanche (uncontrollably large number) of charge carriers across the junction. This causes diode current to rise abruptly and breakdown takes place. This breakdown is called avalanche breakdown. ii) Zener breakdown Heavily doped p-n junctions have narrow depletion layers whose width is of the order of < – m.
When reverse voltage across this junction is increased to the breakdown limit, a very strong electric field of strength × V m – is set up across the narrow layer. This electric field is strong enough to break or rupture the covalent bonds in the lattice and thereby generating electron-hole pairs. This effect is called Zener effect . Even a small further increase in reverse voltage produces a large number of charge carriers which move across the junction through the thin depletion region.
This process gives rise to a large amount reverse current or breakdown current and this breakdown is called Zener breakdown. In Avalanche breakdown, the minority charge carriers gain sufficient energy from excessive reverse bias voltage to break covalent bond in order to produce new charge carriers. But Zener breakdown occurs due to the direct rupture of covalent bonds because of the existence of the strong electric field. Since depletion region is thin, Zener breakdown