An InP/InGaAs/InP double heterojunction bipolar transistor was fabricated and its Ka-band power performance characterized. The device employed a 30 nm highly doped InGaAs base, and a 150 nm collector with an InAlGaAs linearly graded at the base-collector junction to prevent current blocking and maintain breakdown voltage. The dc Current gain is 28.4 at a current density of J(C) = 666 kA/cm(2) and the breakdown voltage (BVCEO) is larger than 5V. A submicrometer InP/InGaAs DHBT with an emitter size of 0.6 x 12 mu m(2) demonstrated a maximum cutoff frequency (f(T)) of 230 GHz, and a maximum output power density of 3.7 mW/mu m(2) at 29 GHz. (C) 2007 Elsevier Ltd. All rights reserved.