We report on the structural and electrical properties of AlGaN/GaN heterostructures grown by molecular beam epitaxy on low-dislocation-density, free standing GaN substrates grown by hydride vapor phase epitaxy. Structural characterization by atomic force microscopy, transmission electron microscopy, and X-ray diffractometry reveal a smooth surface morphology, coherent interfaces, an absence of dislocations generated in the epitaxial layers, and narrow X-ray peaks. Hall measurements indicate room temperature electron mobilities of 1750cm(2)/V s at sheet densities of 1.1X 10(13)Cm(-2). High electron mobility transistors exhibit excellent electrical characteristics, including output power densities of 4.8 W/mm at 10 GHz, off-state breakdown voltages of up to 200 V, and extrinsic cutoff frequencies of 36 GHz on devices with 0.45-mu m gate lengths. (c) 2007 Elsevier B.V. All rights reserved.