A new common-emitter hybrid-π 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_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 (τ _μ) can be properly handled, τ_μ 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 τ_μ is far larger than the forced-V_BE output resistance τ₀ does not apply to devices with significant NBR. In practice, τ_μ can be comparable to (and smaller than) τ₀ 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