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

This paper reports small-sized collector-up Ge/GaAs 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 Omega/square; 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 x 5 mu m(2) 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(tau) 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.