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.