In the active region the ratio of collector current to base current (DC current gain ($\beta$)) remains fairly constant up to certain value of the collector current after which it falls of rapidly.

At still higher levels of collector currents the allowable active region is further restricted by a potential failure mode call “the second breakdown”.

It appears on the o/p characteristics of the BJT as a precipitous drop in the collector emitter voltage at large collection currents. The collector voltage drop is often accompanied by signif‌icant rise in the collector current & a substantial increase in the power dissipation. Most importantly this dissipation is not uniformly spread over the entire volume of the device but is concentrated in highly localized regions. This localized heating is a combined effect of the intrinsic non uniformity of the collector current density distribution across the cross section of the device & the negative temperature. Coeff‌icient of resistively of minority carrier devices which leads to the formation of “current f‌ilaments” (localized across of very high current density) by a positive feedback mechanism.

Once current f‌ilaments are formed localized “thermal runaway” quickly takes the junction temperature beyond the safe limit & the device is destroyed.

How to avoid secondary breakdown

Secondary breakdown can be avoided by using power transistor in safe operating area.

The safe operating area (SOA) of a power transistor specifies the safe operating limit of collector current IC verses collector emitter voltage VCE. For reliable operation of the transistor the collector current & voltage must always lie within this area.

FBSOA is for its dc a well as single pulse operation & for temperature of 250 C. Boundary AB is the maximum limit for dc & continuous current for VCE less than about 80 V. For VCE more than 80V, collector current has to be reduced to BC so as to limit the junction temperature to safe values. For still higher VCE current should further be reduced so as to avoid second breakdown limit.