Microwave heterojunction bipolar transistors

GaAs-GaAlAs HBTs are promising devices for microwave operation, combining many of the advantages of the GaAs MESFET and the Si BJT. The high transconductance and threshold voltage uniformity of the bipolar transistor are retained in the HBT. The GaAlAs-GaAs materials system makes use of the high electron mobility of GaAs giving a low base transit time. Base transit time is an important factor in determining the current-gain bandwidth (f_T) of the transistor. A further advantage of GaAs is the availability of a semi-insulating substrate which greatly reduces the interconnection capacitance. The importance of the heterojunction is that a wide bandgap emitter allows the base to be heavily doped and hence low resistance. The heavy doping also permits operation at high current densities. These factors make HBTs particularly attractive for microwave applications. The authors discuss this and as examples they present results on small signal transistors, power transistors and linear amplifiers