Draw V-I characterictics of a p-n junction diode. Why does the reverse current show a sudden increase at the critical voltage?
Name any semiconductor device which operates under the reverse bias in the breakdown region.

Explanation

For V-I characteristics of a p-n junction, see Fig.and Fig.
When p-n junction is operated under a reverse bias condition a very small reverse current (about few muA) flows due to drifting of minirity charge carriers, which is independent of the applied potential upto a critical voltage.
When the reverse voltage across the p-n junction reaches a critical voltage, the reverse current suddenly rises to a large value. It is due to sudden increase in the number of minority charge carriers, resulting breakdown of the diode. If the p-n junction is lightly doped, the avalanche breakdown occurs due to ionisation by collosion. If the p-n junction is heavily doped, the zener breakdown occurs at even low oltages due to field emission.
Asemiconductor device which operates under the reverse bias in the breakdown region is the zener diode.