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.