InAs–InGaAs graded emitter contact layer was incorporated into the InGaAsSb–GaAsSb/InP graded base double heterojunction bipolar transistors (DHBTs). Compared to a uniform In 0.53Ga 0.47As emitter contact layer, the graded InAs–InGaAs emitter contact layer effectively reduced specific contact resistance from 0.378 to 0.091 Ω-μm 2. Measured sheet resistance also decreased from 58.3 to 47.4 Ω/□. A graded base DHBT with a 0.5×8 μm 2 emitter showed a unity current gain cutoff frequency ( f T) and maximum oscillation frequency ( f MAX) of 500 GHz and 260 GHz, respectively. The f MAX improves to 350 GHz when a smaller (0.4×3 μm 2) emitter is utilized.