LOW TURN-ON VOLTAGE OPERATION OF COLLECTOR-UP GE GAAS HETEROJUNCTION BIPOLAR-TRANSISTORS WITH F(MAX)=112 GHZ/

Small-sized collector-up Ge/GaAs Heterojunction Bipolar Transistors (H BTs) with heavily boron-doped Ge-base-layer are successfully fabricate d and they realize a low-power and a high frequency operation. Lowerin g 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 h eterojunction makes it possible to realize lower turn-on voltage opera tion than that of Si or other compound semiconductor bipolar transisto rs. Heavily boron-doped Ge-base-layer and a newly developed self-align ed process reduce the base resistance and the parasitic elements. Intr insic and extrinsic base resistances are 50 Omega/square and 90 Omega/ square, respectively, which are the lowest values among bipolar transi stors. The values of f(T) and f(max) are 23 GHz and 112 GHz, respectiv ely. The large value of f(max) exceeding 100 GHz, might be attributed to extremely low base resistance caused by the heavily boron-doped bas e-layer and the self-aligned process and to low base-collector capacit ance expected from the collector-up structure.