112-GHZ COLLECTOR-UP GE GAAS HETEROJUNCTION BIPOLAR-TRANSISTORS WITH LOW TURN-ON VOLTAGE/

This paper reports small-sized collector-up Ge/GaAs heterojunction bip olar transistors (HBT's) operating at low power and high frequency. A heavily B-doped Ge base-layer and a newly-developed self aligned proce ss reduce the base resistance and the parasitic elements, Intrinsic ba se resistance is 50 Omega/square; this is the lowest value reported fo r 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 oscil lation frequency f(max) of 112 GHz with an associated current gain cut off frequency f(tau) of 25 GHz, The large value of f(max), exceeding 1 00 GHz, is attributed to the extremely low base resistance caused by t he heavily B-doped base-layer and the self-aligned process and to the low base-collector capacitance expected from the collector-up structur e. The turn-on voltage of these HBT's is approximately 0.7 V smaller t han that of AlGaAs/GaAs HBT's, These results show that these HBT's hav e excellent potential for low-power dissipation circuits.