In this article we present results from heterojunction bipolar transistor (HBT) devices that weredesigned for low power consumption by taking advantage of the small bandgap of high-indium composition In0.86Ga0.14As in the base layer. These were grown by solid-source molecular-beam epitaxy. Both single- and double-HBT devices were grown and fabricated. The 6.00 Angstrom lattice parameter of these device structures was accommodated by first growing metamorphic, relaxed, linearly graded InxAl1-xAs buffers on semi-insulating InP substrates, grading from In0.52Al0.48As lattice-matched with InP at 5.87 Angstrom to In0.86Al0.14As at 6.00 Angstrom. The total thickness of each buffer was 1.2 mum or greater. These graded buffer layers developed the crosshatched morphology that is associated with the strain-relaxation process. The density of threading dislocations reaching the buffer surface was measured by EPD to be as low as 2 x 10(6) cm(-2). Although this density is still more than an order of magnitude greater than that measured for lattice-matched material on InP, these 6.00 Angstrom HBT devices exhibited good electrical characteristics. For these electrical tests, both large (70 x 70 mum(2)) and small (1.5 x 10 or 2 x 10 mum(2)) devices were fabricated. All devices exhibited significantly lower turn-on and knee voltages compared to the GaAs and lattice-matched In0.52Ga0.48As-on-InP HBT technologies. The 6.00 A single HBT demonstrated a V(BE)similar to0.4 V for operation at 1 mA compared to 0.7 V for lattice-matched In0.52Ga0.48As on InP and 1.3 V for GaAs. The 6.00 A HBT approach additionally benefits from a favorable valence band alignment between the p-type In0.86Ga0.14As base and the n-type In0.86Al0.14As emitter, which reduces hole injection into the emitter and increases device current gain. The rf measurements of this structure exhibited a transistor cut-off frequency f(T) exceeding 100 GHz. Small double-HBT devices presented a breakdown voltage BVCEO>2 V, which was twice that of small single-HBT devices. This enables further vertical scaling of the device to reduce the collector transit time. (C) 2004 American Vacuum Society.