Stable hot spots and second breakdown in power transistors

The mechanism of hot spot formation in transistors is examined from both experimental and theoretical viewpoints. It is shown that after the device becomes thermally unstable the device may restabilize in a hot spot mode of operation. The IC, VCE thermal instability locus can accurately be predicted assuming the current density is uniform prior to hot spot formation. A new model is proposed which explains why the device may restabilize in a hot spot mode and why devices exhibit "thermal hysteresis". It is also shown using thermal mapping techniques that emitter current crowding exists in the stable hot spot mode. Finally, the experiments support the idea that second breakdown occurs when the current density within the hot spot reaches a critical value.