High-speed InP-InGaAs heterojunction phototransistors employing a nonalloyed electrode metal as a reflector

High-speed InP-InGaAs heterojunction phototransistors (HPT´s) with a base terminal (three-terminal HPT´s) have been fabricated. These HPT´s have nonalloyed electrodes functioning as reflectors and a configuration in which light is incident through the substrate. These features lead to an increase in quantum efficiency in spite of the thin base and collector light-absorbing layers. Optical gain dependence on collector current is weak because of the low recombination current at the emitter-base interface. Maximum optical-gain cutoff frequencies of 22 and 14 GHz are obtained for a 3×3-μm² emitter HPT illuminated by 1.3- and 1.55-μm light, respectively. This HPT has the capability of operating as a high-speed heterojunction bipolar transistor (HBT) as well. A current-gain cutoff frequency (f_T) of 128 GHz is obtained for a 3×9-μm² emitter HBT fabricated on the same wafer. Equivalent circuit analysis, in which all the components are determined by measuring both the electrical and optical characteristics of a three-terminal HPT, shows good agreement with experimental results