112-GHz collector-up Ge/GaAs heterojunction bipolar transistorswith low turn-on voltage

This paper reports small-sized collector-up Ge/Ga/As heterojunction bipolar transistors (HBT's) operating at low power and high frequency. A heavily B-doped Ge base-layer and a newly-developed self-aligned process reduce the base resistance and the parasitic elements. Intrinsic base resistance is 50 Ω/□; this is the lowest value reported for bipolar transistors. With limiting the active emitter area through B ion implantation, these collector-up HBT's with a collector size of 2×5 μm² exhibit a current gain of 60. They exhibit a maximum oscillation frequency f_max of 112 GHz with an associated current gain cutoff frequency f_T of 25 GHz. The large value of f_max, exceeding 100 GHz, is attributed to the extremely low base resistance caused by the heavily B-doped base-layer and the self-aligned process and to the low base-collector capacitance expected from the collector-up structure. The turn-on voltage of these HBT's is approximately 0.7 V smaller than that of AlGaAs/GaAs HBT's. These results show that these HBT's have excellent potential for low-power dissipation circuits