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 fT  versus collectorcurrent density IC characteristics of npn−n AlGaAs/GaAs heterojunction bipolartransistors (HBTs) with various n−-collector thickness and n−-doping densities. It isfound 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 energyband diagrams, electron-energy profiles and electron-velocity profiles. Particularly, wediscuss the origin of the second peak (at higher IC) which is not usually reportedexperimentally