Junction Breakdown

In case of reverse bias, a point is reached where application of too much negative case voltage will result in a sharp in the characteristic.
     At this point junction breakdown and current increases at a very rapid rate in a direction opposite to voltage region The reverse bias voltage that results in this dramatic change in character is called "Breakdown Potential" or " Zener Potential" and this sharp change in characteristics at any level is called zener region. It is found that once breakdown has occurred, very little further increase in voltage is required to increase the current to relatively high value. The junction itself offers almost zero resistance at this point.
The breakdown voltage depends on the width of the depletion region which, in turn, depend on doping level. There are two Dypes of breakdown:
(1) Zener Breakdown
(2) Avalanche Breakdown.