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VLSI Design
Volume 8 (1998), Issue 1-4, Pages 437-442

Study on Possible Double Peaks in Cutoff Frequency Characteristics of AlGaAs/GaAs HBTs by Energy Transport Simulation

Department of Electronic Information Systems, Faculty of Systems Engineering, Shibaura Institute of Technology, 307 Fukasaku, Omiya 330, Japan

Copyright © 1998 Hindawi Publishing Corporation. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


By using an energy transport model, we simulate cutoff frequency fT versus collector current density IC characteristics of npnn AlGaAs/GaAs heterojunction bipolar transistors (HBTs) with various n-collector thickness and n-doping densities. It is found that the calculated fT characteristics show double peak behavior when the n- layer is thick enough and the n-doping is high enough to allow existence of neutral n- region. The mechanism of the double peak behavior is discussed by studying energy band diagrams, electron-energy profiles and electron-velocity profiles. Particularly, we discuss the origin of the second peak (at higher IC) which is not usually reported experimentally.