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

N-n-p Al0.3Ga0.7As/GaAs heterojunction phototransistors have been fabricate d 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 dem onstrate an internal quantum efficiency estimated to be up to around 0.6 co mpared to around 0.38 for no-shoulder devices. An average threefold increas e in the optical gain of the shoulder devices has also been observed partic ularly with decreasing device sizes in contrast to no-shoulder devices. Fur thermore, 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 op tical gain can therefore be attributed to the suppression of perimeter-rela ted edge leakage current by introducing the novel shoulder structure in het erojunction phototransistors,