Physics of Hot Carrier Degradation Under Saturation Mode Operation in SiGe HBTs

We report, for the first time, base current degradation in SiGe HBTs when high base current stress is applied under saturation mode operation. In contrast to high collector current density stress, which has a known positive temperature dependence, the high base current stress in saturation investigated in the present work shows a negative temperature dependence. Measurements and simulations suggest that the physics of degradation under saturation mode is driven by Auger hot carrier generation. This new degradation mechanism only poses reliability challenges for circuits when base current density exceeds the bias required for peak fT operation by a factor of at least 60X. Saturation mode operation finds small duty cycle use in limited applications like switched power circuits.