Reliability of SiGe HBTs for Power Amplifiers—Part II: Underlying Physics and Damage Modeling

This paper presents the underlying physics and modeling of aggressively biased cascode SiGe heterojunction bipolar transistor power amplifier (PA) cores under large-signal operating conditions. The damage characteristics observed during RF operation, particularly the base leakage and collector-base (CB) junction failure, are investigated in detail using dc stress methods. Base leakage was characterized across geometry, voltage, and current conditions, and a damage model is purposed based on Shockley-Read-Hall theory and the reaction-diffusion equation. This model is used to predict damage under aggressive RF operations, in order to extract the operational lifetime of SiGe PAs. The onset of CB junction failure was modeled using the current-gain collapse model, and it accurately captures the failure threshold current I _Fail observed during RF stress.