This paper presents novel structures of AlGaAs GaAs heterojunction bipolar transistors with buried SiO 2 in the extrinsic collector. Transistor layer structures are grown by molecular beam epitaxy on GaAs substrates patterned with SiO 2. Single-crystal and polycrystal structures are grown in the SiO 2 openings and on the SiO 2, respectively. Due to the low dielectric constant of SiO 2, about one-third that of GaAs, and the complete carrier depletion of polycrystalline GaAs, a 40–65% reduction in the base-collector capacitance is realized without sacrificing the current gain or cutoff frequency.