InP-based double heterojunction bipolar transistors (DHBTs) with different base structures were studied. The base structures are InGaAs with and without graded composition and GaAsSb. Both of the terminal currents of InP/GaAsSb/InP DHBT in forward and reverse modes are limited by the carrier transport across the base layer. This causes the offset voltage to be determined by the difference between the base-collector and base-emitter areas and by the normal common-base current gain. The emitter currents of both graded- and abrupt-base InP/InGaAs/InP DHBTs in the reverse mode are also limited by the carrier transport across the base layer, while their collector currents are limited by the band discontinuity of the base-emitter junction. The different current transport mechanisms of the terminal currents in the forward and reverse modes result in the larger offset voltage.