DC and RF characteristics of InP/InAlAs/GaAsSb/InP double heterojunction bipolar transistors (DHBTs) are reported. The device heterostructures include InAlAs spacer layer between InP emitter and GaAsSb base layer. The impact of thin InAlAs spacer layer was investigated by comparing the DC characteristics of large-area devices fabricated on InP/lnAlAs/GaAsSb/InP and conventional Inp/GaAsSb/InP heterostructures. By suppressing the base tunneling current and surface recombination current, the DHBTs with thin InAlAs spacer layer exhibited 5 decade lower crossover current of 7 x 10(-11) A (collector current at unity current gain) than conventional InP/GaAsSb/InP DHBT that exhibited crossover current of 4 x 10(-6) A. The current gain also improved twice by the impact of thin InAlAs layer. To investigate the high-frequency characteristics of InP/InAlAs/GaAsSb/lnP DHBTs, small-area devices employing laterally etched undercut micro-airbridges were fabricated., The unity current gain cut-off frequency, f(T), of 100 GHz was obtained from a 1 x 30 mu m(2) emitter DHBT. (c) 2006 Elsevier Ltd. All rights reserved.