Heavily carbon-doped InGaP/GaAs HBT's with buried polycrystalline GaAs under the base electrode

This paper describes a new approach to fabricating InGaP/GaAs heterojunction bipolar transistors (HBT's) with a high cutoff frequency (f(T)) high maximum oscillation frequency (f(max)), and low external collector capacitance (C-bc) To attain a high f(T) and f(max), a heavy carbon-doping (1.3 x 10(20) cm(-3)) technique was used with a thin (30-nm-thick) GaAs base layer, while for low C-bc, low-temperature gas-source molecular-beam epitaxial growth on SiO2-patterned substrates was used to bury high-resistance polycrystalline GaAs under the base electrode. An fr of 120 GHz and an f(max) of 230 GHz were achieved for three parallel 0.7 x 8.5 mu m HBT's with an undoped-collector structure, and an fr of 170 GHz and an f(max) of 160 GHz were obtained for a single 0.9 x 10 mu m HBT with a ballistic-collection-transistor structure. Compared to HBT's without buried poly-GaAs, the maximum stable gain was improved by 1.2 dB in the 0.7 x 8.5 mu m HBT and by 23 dB in the 0.9 x 10 mu m HBT due to the reduction in Cbc These results show the high potential of the proposed HBT's for high-speed digital and broadband-amplifier applications.