A low contact resistivity of 4.410-7 Ωcm2 for AlGaAs/GaAs HBTs was realized using Pt/Ti/Pt/Au base metal and a 81019 cm-3 highly-doped base. A high fmax of 170 GHz was achieved by reducing a base resistance. The formation of oxide-free interface between an AlGaAs graded base and Pt-based metal was demonstrated with Auger electron spectroscopy. The optimization of the growth condition conquered the rapid current-induced degradation in the highly Be-doped HBTs. An extremely wide bandwidth of 40 GHz was attained by a Darlington feeback amplifier fabricated using these high-fmax HBTs. These properties indicate that the application of AlGaAs/GaAs HBTs can be expected to extend to future ultrahigh-speed optical transmission systems.