PN Junction Breakdown Processes

We have seen that a PN junction allows a very small current to flow when reversed biased. This current is due to the movement of minority carriers. It is almost independent of the voltage applied. However if the reverse bias is made too high, the current through the PN junction increases abruptly. The voltage at which this phenomenon occurs is called breakdown voltage.

 

There are two processes which can cause junction breakdown.

1. Zener breakdown

2. Avalanche breakdown

 

Zener breakdown: 

When reverse bias is increased, the electric field(how electric field is created that explained above link1) at the junction also increases. High electric field causes covalent bonds to break. Thus a large number of carriers are generated. This causes a large current to flow. This mechanism of breakdown is called zener breakdown.

 

Avalanche breakdown: 

When reverse bias is increased, the electric field(how electric field is created that explained above link1) at the junction also increases. The increased electric field causes increase in the velocity of the minority carriers. That means minority carriers become more energetic. These high energy carrier heat an atom during their movement and break covalent bonds, thereby generating more carriers. Again these generated carriers are accelerated by the electric field. They break more covalent bonds during their travel. A chain reaction is thus established, creating a large number of carriers. This gives rise to a high reverse current. This mechanism of breakdown is called avalanche breakdown.