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The static gain characteristics of NpN InP/GaAsxSb1-x/InP double heterojunction bipolar transistors (DHBTs) were studied as a function of the base arsenic (As) mole fraction x. Compared to the devices with a lattice-matched base (x=0.51), a current gain improvement arising principally from a base current reduction is observed in DHBTs having higher As- base mole fractions (and consequently, with a reduced type-II conduction band discontinuity DeltaEC at the emitter-base junction). Both the surface periphery and the intrinsic recombination currents decrease markedly as the base arsenic concentration increases. The present work therefore unambiguously demonstrates that the emitter type-II conduction band discontinuity between the InP emitter and the GaAsxSb1-x base enhances the undesirable emitter-size effects (ESEs) and increases intrinsic recombination currents directly under the emitter contact.