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Wet thermal oxidation of aluminum containing III-V semiconductors is a potent alternative technique for fabricating optoelectronic and microelectronic devices. Oxides have previously been used to define emitter openings of AlGaAs heterojunction bipolar transistors (HBTs) in low capacitance collector-up configurations. In the present work, the unique metal formation of AlAsSb oxidation is used to reduce the extrinsic base resistance under the collector in HBTs on InP. In particular, oxidation of AlAsSb can produce an insulating alumina film with a self-aligned, adjacent elemental antimony layer. We report on the electrical properties of this metal layer and the demonstration of HBTs with embedded metal underlying the extrinsic base. The combined low resistance and capacitance of such structures can potentially yield higher speed operation.