Effects of arsenic mole fraction x on the gain characteristics of type-II INP/GAASxSB1-x DHBTS

The static gain characteristics of NpN InP/GaAs_xSb_1-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 DeltaE_C 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 GaAs_xSb_1-x base enhances the undesirable emitter-size effects (ESEs) and increases intrinsic recombination currents directly under the emitter contact.