When p-n junction is operated under a reverse bias condition a very small reverse current (about few μA) flows due to drifting of minority charge carriers, which is independent of the applied potential up to 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 from breakdown of the diode.
If the p-n junction is lightly doped, the avalanche breakdown occurs due to ionisation by collision.
If the p-n junction is heavily doped, the zener breakdown occurs at even low voltages due to field emission.
