A Novel Low-Bias Charge Concept for HBT/BJT Models Including Heterobandgap and Temperature Effects—Part I: Theory

A new compact model approach is suggested for the low-bias base charge of homo- and heterojunction bipolar transistors. It is shown that the junction-related (Early) components of this charge depend on an effective doping density. This concept leads to an accurate description with the conventional capacitance-charge model formulas but using different parameters directly extracted from direct-current measurements instead of the classical parameter values derived from alternating-current measurements. The temperature dependence of the low-bias base charge has also been revised, resulting in a new term added to the currently adopted temperature law. The applied novel analytic technique made it possible to determine the influence of bandgap narrowing (BGN) effects and Ge doping on the temperature behavior of SiGe transistors.