High quality undoped AlGaN/GaN high electron mobility transistors (HEMTs) structures have been grown by hydride vapor phase epitaxy (HVPE), for the first time. The morphology of the films grown on Al2O3 substrates is excellent, with a root-mean-square roughness of similar to0.2 nm over 10 X 10 mum(2) measurement area. Capacitance voltage measurements show the formation of a dense sheet of charge at the AlGaN/GaN interface. This is the first ever report of the formation of a two-dimensional electron gas in a nitride structure grown by HVPE. HEMTs with 1 mum gate length fabricated on these structures show transconductances in excess of 110 mS/mm and drain-source current above 0.6 A/mm. (C) 2003 Elsevier Science Ltd. All rights reserved.