Study on possible double peaks in cutoff frequency characteristics of AlGaAs/GaAs HBTs by energy transport simulation

By using an energy transport model, we simulate cutoff frequency f_T- versus collector current density I_c characteristics of npn^-n AlGaAs/GaAs heterojunction bipolar transistors (HBTs) with various n^--collector thickness and n^--doping densities. It is found that the calculated fr 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 I_c) which is not usually reported experimentally.