HIGH-PERFORMANCE SMALL-SCALE COLLECTOR-UP ALGAAS GAAS HBTS WITH A CARBON-DOPED BASE FABRICATED USING OXYGEN-ION IMPLANTATION

We report the development of high-performance small-scale AlGaAs/GaAs collector-up heterojunction bipolar transistors (C-up HBT) with a carbon (C)-doped base layer. Oxygen-ion (O+) implantation is used to define their intrinsic emitter/base junctions and zinc (Zn)-diffusion is used to lower the resistivity of thier O+-implanted extrinsic base layers. The highly resistive O+-implanted AlGaAs layer in the extrinsic emitter region sufficiently suppresses electron injection even under high-forward-bias conditions, allowing high collector current densities. The use of a C-doped base is especially effective for small-scale C-up HBT's because it suppresses the undesirable turn-on voltage shift caused by base dopant diffusion in the intrinsic area around the collector-mesa perimeter that occurs during the high-temperature Zn-diffusion process after implantation. Even in a small-scale transistor with a 2 mum x 2 mum collector, a current gain of 15 is obtained. A microwave transistor with a 2 mum x 10 mum collector has a cutoff frequency f(T) of 68 GHz and a maximum oscillation frequency f(max) of 102 GHz. A small-scale C-up HBT with a 2 mum x 2 mum collector shows a higher f(max) of 110 GHz due to reduced base/collector capacitance C(BC) and its f(max) remains above 100 GHz, even at a low collector current of 1 mA. The C(BC) of this device is estimated to be as low as 2.2 fF. Current gain dependence on collector size is also investigated for C-up HBT's and it is found that the base recombination current around the collector-mesa perimeter reduces the current gain.