Load-Pull Based RF Reliability Investigations of SiGe HBTs
In process design kits (PDKs), the open-base collector-emitter (CE) breakdown voltage is often specified as the upper usable CE voltage limit and thus interpreted by many circuit designers as the upper limit of what the transistor can withstand in terms of dynamic CE swings. Large-signal RF reliability studies with signal swings beyond conventionally DC-defined operating limits enable to explore the short-term ruggedness as well as the long-term robustness and degradation of a specific transistor technology. This tutorial addresses two main goals. First, latest results of large-signal RF stress tests with different dynamic operating conditions are presented that prove SiGe HBTs to be reliably operable far beyond conventionally defined static operating limits typically found in PDKs. This will include a review of the degradation of the most important transistor parameters due to large-signal stress and reveal a previously unknown degradation mechanism occurring only under extreme stress conditions. Second, a HICUM/L2 model card has been carefully calibrated within and beyond conventional extraction regions, including specifically the regions of increased avalanche and self-heating. With quiescent bias points as well as dynamic signal swings beyond the conventional operating region, the model is shown to be in excellent agreement with large-signal measurements across various load reflections coefficients of a 50GHz impedance tuner system and available powers from the source and thus serves as a basis for detailed investigations of the origin of the observed degradation.