Hot carrier reliability of high-speed SiGe HBT’s under accelerated collector-base avalanche bias

As SiGe heterojunction bipolar transistor (HBT) based technologies reach f_T performance above 200 GHz the transistors are more likely to be operating in the avalanche region of collector-base junction. This avalanche effect imposes a reliability concern due to the damage induced by high-energy avalanche charges (hot carriers) on the Si-SiO₂ interfaces. The impact of accelerated avalanche bias stress on 200 GHz SiGe HBTs was investigated and found to cause an increase in base current with no corresponding change of collector current. The increase of the base current correlates with the injected avalanche charge and its corresponding kinetic energy. A new empirical avalanche degradation model has been developed to predict the shifts in DC characteristics. Further studies showed that the SiGe HBT avalanche damage can be partially recovered by normal forward operation and/or high temperature anneal. The AC impact of this degradation mechanism has also been studied and found not to be significant. According to the reliability results of this study, the avalanche induced hot carrier degradation will not have a significant impact on the lifetime of the 200 GHz SiGe HBTs in a typical high speed digital circuit application.