This investigation proposes an InP/InGaAs composite-collector double heterojunction bipolar transistor (CC-DHBT) grown by metalorganic chemical vapor deposition. The improved structure exhibits the advantages of no knee-shaped characteristics, no switching effect, low output conductance, a high two-terminal base–collector breakdown voltage (BV) that exceeds 20 V, and high three-terminal breakdown voltages (BVCEO>15 V, BVCBO>20 V). The current gain is over unity at ultralow collector current density of 10−4 A/cm2. These characteristics are attributed to the optimized collector design. Furthermore, this study elucidates complex breakdown mechanisms in the CC-DHBTs.