Evidence of optical gain improvement in AlGaAs/GaAs heterojunction phototransistors using an emitter shoulder structure

N-n-p Al/sub 0.3/Ga/sub 0.7/As/GaAs heterojunction phototransistors have been fabricated with a novel thinned emitter-edge shoulder structure. Varying sized MOVPE-grown circular devices have been assessed both electrically and optically to determine the influence of the shoulder structure, In this paper, we demonstrate an internal quantum efficiency estimated to be up to around 0.6 compared to around 0.38 for no shoulder devices. An average threefold increase in the optical gain of the shoulder devices has also been observed particularly with decreasing device sizes in contrast to no-shoulder devices. Furthermore, electrical characterization of the transistors indicates that the base current of the with-shoulder transistors is more nearly proportional to the emitter-base junction area than its perimeter. The improvement in optical gain can therefore be attributed to the suppression of perimeter-related edge leakage current by introducing the novel shoulder structure in heterojunction phototransistors.