Low-turn on voltage operation of collector-up Ge/GaAs heterojunction bipolar transistors with fmax=112 GHz

Small-sized collector-up GeAs Heterojunction Bipolar Transistors (HBTs) with heavily boron-doped Ge-base-layer are successfully fabricated and they realize a low-power and a high frequency operation. Lowering the Ge growth-temperature changes the amount of dipole induced at Ge/GaAs (100) heterojunction to reduce the conduction band discontinuity at Ge/GaAs heterojunction. Tunability of the band lineup at Ge/GaAs heterojunction makes it possible to realize lower turn-on voltage operation than that of Si or other compound semiconductor bipolar transistors. Heavily boron-doped Ge-base-layer and a newly developed self-aligned process reduce the base resistance and the parasitic elements. Intrinsic and extrinsic base resistances are 50 Ω/ ? and 90 Ω/ ?, respectively, which are the lowest values among bipolar transistors. The values of f τ and f max are 23GHz and 112GHz, respectively. The large value of f max , exceeding 100GHz, might be attributed to extremely low base resistance caused by the heavily boron-doped base-layer and the self-aligned process and to low base-collector capacitance expected from the collector-up structure.