AlGaAs/GaAs HBTs for 10-Gb/s ICs using a new base ohmic contact fabrication process

A new basic ohmic contact technology for AlGaAs/GaAs heterojunction bipolar transistors (HBTs) is presented. The effect of the device parameters on the high-frequency performance of HBT ICs for 10-Gb/s systems is analyzed, and it is shown that, at a cutoff frequency (f_T) of 40 GHz or more, reducing base resistance or collector capacitance is more effective than increasing f_T for obtaining high-frequency performance. A process is developed for fabricating base electrodes with a very low ohmic contact resistivity, ~10^-7 Ω-cm², by using a AuZn/Mo/Au alloy, which provides the required high performance. Self-aligned AlGaAs/GaAs HBTs, with a 2.5-μm×5-μm emitter, using a AuZn/Mo/Au alloy base metal and an undoped GaAs collector, are shown to have an f_T and a maximum oscillation frequency of about 45 and 70 GHz, respectively, at 3.5 mA. An AGC amplifier with a 20-dB gain and a bandwidth of 13.7 GHz demonstrates stable performance