Novel AlGaAslGaAs double heterojunction bipolar transistors (DHBTs) with composite cellector have been proposed and simulated to replace the conventional AlGaAs/GaAs DHBT. The composite collector combines both wide-bandgap (AlGaAs or InGaP) and narrow-bandgap (GaAs) materials. In the composite collector, AlGaAs (or InGaP) provides high breakdown field and thus is able to be employed to reduce thickness of collector while keeping the same breakdown voltage; GaAs provides high electron mobility and thus is able to be employed to reduce on-resistance and transit time. The simulation results demonstrate that novel AlGaAs/GaAs DHBTs have higher f t than conventional DHBT, higher BV CBO than HBTs, and. lower Knee voltage (on-resistance) than DHBT. The results also show the current gains of all -(D)HBTs with difference in the collector are almost identical. Three InGaP/GaAs (D)HBTs with different structures in collector have also been fabricated to confirm the idea of the proposed structures.