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

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4 Author(s)
Guofu Niu ; Dept. of Electr. Eng., Auburn Univ., AL, USA ; J. D. Cressler ; U. Gogineni ; A. J. Joseph

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-VBE 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, τμ 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 τμ is far larger than the forced-VBE output resistance τ0 does not apply to devices with significant NBR. In practice, τμ 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

Published in:

IEEE Transactions on Electron Devices  (Volume:46 ,  Issue: 6 )