Simulation and fabrication of InGaP/Al_0.98Ga_0.02As/ GaAs oxide-confined collector-up heterojunction bipolar transistors

Two-dimensional numerical simulations were performed to investigate the proposed oxide-confined collector-up heterojunction bipolar transistors (C-up HBTs). To achieve a high current gain, the effective base-emitter junction area should be decreased to be smaller than that of the base-collector junction. However, at the same time the offset voltage was increased by the decrease of the base-emitter junction area. A trade-off between the current gain and the offset voltage was observed for C-up HBTs. In the device fabrication, the current confinement of the partially oxidized Al_0.98Ga _0.02As layer was demonstrated by a simple current-voltage measurement applied to the base-emitter junction. With applying a 400°C oxidation process for 70 min, a current gain of 79 can be achieved with a low base sheet resistance of 203 Ω/sq. and offset voltage of 160 mV. The relationship between the current gain and the offset voltage obtained from the experimental results shows the same trend as that observed in the simulations. The optimized condition of the oxidation depth is that in which the effective base-emitter junction area is slightly smaller than that of the base-collector junction.