What is secondary breakdown in transistor?

Secondary breakdown is a drawback of bipolar junction transistors (BJT's), particularly in power transistors. Simply put, when high current flow causes a build up of electric fields inside of the semiconductor crystal creating "hot spots" which cause the transistor to fail.

At the large collector current, the collector to emitter drops. due to drop in voltage,collector current increases in power dessipation. this power dessipation is liocalized in highly concentrated region. In this region temperature increases rapidly.

Semiconductor material, regardless if it is P-type or N-type has a negative temperature coefficient meaning that when temperature increases its resistance decreases. Ohm's Law states that Voltage equals Current times Resistance which shows that as resistance decreases current increases. Since Power equals Current times Voltage, if current increases then power increases. A bi-product of power is heat. So, in these hot spots increased current creates heat in a small area which causes resistance to decrease in that area causing higher current flow in that area causing a further increase of heat. The domino effect eventually causes the transistor to fail.