Avalanche And Zener Breakdown Effect

In forward bias condition when the applied voltage is more than this threshold voltage or the barrier potential of this PN Junction diode, then the current start flowing from P side through the N side.
In reverse bias condition there is hardly any flow of current from P side to N side. But small amount of reverse saturation current flow from the N side to P side.
This current flows due to the minority charge carrier. And then if we increase the reverse bias voltage there is hardly any change in the reverse saturation current. But there is maximum limit on the voltage which can be applied in this reverse bias condition, and if apply voltage is beyond this limit then the suddenly lot of current starts flowing in this reverse direction.
So this region of operation of the diode is known as the breakdown region of operation.
This effect due to which it occurs is known as the Avalanche effect.

The avalanche breakdown is the breakdown mechanism of the PN – junction diodes which have thinner region. In this breakdown, when the electric field is applied across the diode, the velocity of the charge carrier increases. This charge carrier collides with the other atoms and creates the pairs of hole and electrons.
The free charge carrier further collides with other atoms and creates more pairs of electron and hole. These free electrons start moving across the junction and develop the reverse bias current. The reverse bias current completely destroys the junction. And once the junction breakdown occurs, it cannot regain its original position.

The junction breakdown mechanism which occurs in the heavily doped thin region is called Zener breakdown. In this mechanism when the high electric field is applied across the junction, the charge carrier starts jumping across the junction.
These electrons constitute the heavy current in the reversed direction. In Zener breakdown, the temporary breakdown of junction occurs. The junction regains its original position when the reverse voltage removes.