For a device that makes use of the second breakdown effect see Avalanche transistor
The second breakdown is an irreversible failure mode in bipolar power transistors. In a power transistor with a large junction area, under certain conditions of current and voltage, the current concentrates in a single area of the base-emitter junction. This current contraction effect causes local heating and destruction of the transistor. Second breakdown can occur both with forward and reverse base drive. Except at low collector-emitter voltages, the second breakdown limit restricts the collector current more than the steady-state power dissipation of the device. Power MOSFETs do not exhibit secondary breakdown, and their safe operating area is limited only by maximum current (the capacity of the bonding wires), maximum power dissipation and maximum voltage. However, Power MOSFETs have parasitic PN and BJT elements within the structure, which can cause more complex localized failure modes resembling Secondary Breakdown.
Read more about this topic: Safe Operating Area
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