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/sub T/), high maximum oscillation frequency (f/sub max/), and low external collector capacitance (C/sub bc/). To attain a high f/sub T/ and f/sub max/, a heavy carbon-doping (1.3/spl times/10/sup 20/ cm/sup -3/) technique was used with a thin (30-nm-thick) GaAs base layer, while for low C/sub bc/, low-temperature gas-source molecular-beam epitaxial growth on SiO/sub 2/-patterned substrates was used to bury high-resistance polycrystalline GaAs under the base electrode. An f/sub T/ of 120 GHz and an f/sub max/ of 230 GHz were achieved for three parallel 0.7/spl times/8.5 /spl mu/m HBT's with an undoped-collector structure, and an f/sub T/ of 170 GHz and an f/sub max/ of 160 GHz were obtained for a single 0.9/spl times/10 /spl 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/spl times/8.5 /spl mu/m HBT and by 2.3 dB in the 0.9/spl times/10 /spl mu/m HBT due to the reduction in C/sub bc/. These results show the high potential of the proposed HBT's for high-speed digital and broadband-amplifier applications.