Physics of Hot Carrier Degradation Under Off-State Mode Operation in High Performance NPN SiGe HBTs

We report on the hot carrier generation and device degradation mechanisms in high performance NPN SiGe HBTs under off-state mode operation. In contrast to the forward-active mode, where the emitter current has a central role in the device performance degradation processes, the low emitter electron injection under off-state is not sufficient to initiate impact ionization and is found to have no impact on the induced damage. Measurements and simulations suggest that the physics of hot carrier generation under off-state mode is driven by the band to band tunneling (BTBT) mechanism in the collector-base (CB) junction which has a positive temperature dependence. Despite the positive temperature dependence of the hot carrier generation rate, off-state hot carrier stress experiments covering a wide range of bias and temperature conditions induce base current shifts that exhibit a negative temperature dependence, indicating that device damage at high temperatures is limited by the reduction of carrier mean free path due to increased phonon scattering. Furthermore, device degradation is shown to correlate with the BTBT stress current and the CB breakdown voltage measured with the emitter open (BVCBO), highlighting the importance of a judicious optimization of the BVCBO parameter to enable a robust design for reliability methodology.