A new common-emitter hybrid-/spl pi/ small-signal equivalent circuit for bipolar transistors with significant neutral base recombination

The linear superposition approach to the modeling of small-signal parameters in the presence of substantial base recombination, which involves a virtual transistor without base recombination, is identified to cause incorrect emitter current modeling. All of the terminal current changes can be correctly modeled by using the measured forced-V/sub BE/ Early voltage in a new equivalent circuit, which properly accounts for NBR and Early effect in a physically consistent manner. As a result, practical situations of small collector-base resistance (/spl tau//sub /spl mu//) can be properly handled, /spl tau//sub /spl mu// is related to the ac current-drive and ac voltage-drive Early voltages, which facilitates parameter extraction and circuit modeling. Measurements on state-of-the-art UHV/CVD SiGe HBT's show that the conventional assumption that /spl tau//sub /spl mu// is far larger than the forced-V/sub BE/ output resistance /spl tau//sub 0/ does not apply to devices with significant NBR. In practice, /spl tau//sub /spl mu// can be comparable to (and smaller than) /spl tau//sub 0/ depending on the device processing, profiles and operating temperature. Temperature dependent data are presented, and circuit implications are discussed based on the new equivalent circuit.