The static gain characteristics of N-p-N 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 largely arising from a base current reduction is observed in DHBTs with higher As base mole fractions (and consequently a reduced type-II conduction band discontinuity Delta E-C at the emitter-base heterojunction). 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. (C) 2008 Elsevier Ltd. All rights reserved.