InAs-InGaAs graded emitter contact layer was incorporated into the InGaAsSb-GaAsSb/InP graded base double heterojunction bipolar transistors (DHBTs). Compared to a uniform In0.53Ga0.47As emitter contact layer, the graded InAs-InGaAs emitter contact layer effectively reduced specific contact resistance from 0.378 to 0.091 Omega-mu m(2). Measured sheet resistance also decreased from 58.3 to 47.4 Omega/square. A graded base DHBT with a 0.5 x 8 mu m(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 x 3 mu m(2)) emitter is utilized. (c) 2006 Elsevier B.V. All rights reserved.