Question

What is mean by secondary breakdown in bjt?

Answer 1

Certain areas of silicon inside the BJT become disproportionately hotter than the other areas beacuse of large current flow or due to other defects. As a region becomes hot, the number of current carriers increases(current increases). This current causes more Joule heat (\[Heat= I ^{2}*R*t \] where I is current,R is resistance and t is time for which current flows) to get generated and again result in increase in current and it continues till the BJT gets spoiled. This usually happens in power BJT's as they usually carry large current

Answer 2

There's basically to types of breakdowns with BJ's one is called avalanche the other is called zener. In diodes the junction voltage associated between the types usually occurs around 6 volts.