Theoretical thermal runaway analysis of heterojunction bipolar transistors: Junction temperature rise threshold

The thermal runaway behavior of single- and multiple-emitter-element heterojunction bipolar transistors (HBTs) under both voltage and current modulated operations was studied. Under voltage modulation, thermal runaway causes device destruction due to an ever increasing junction temperature. Under current modulation, thermal runaway induces current instability concurrent with the formation of hot spots. It is found theoretically that thermal runaway occurs when the junction temperature rise reaches a threshold value. The dependence of this value for both current and voltage modulation modes of operation on device electrical and thermal parameters is numerically calculated. The electrical parameters include the negative temperature coefficient of the base-emitter turn-on voltage, base current ideality factor, apparent valence band discontinuity for npn HBT and emitter ballast resistance. The thermal parameters include the self-induced and coupled thermal resistances.