We investigate the role of substrate temperature and gallium flux on the DC and microwave properties of AlGaN/GaN high electron mobility transistors grown by molecular-beam epitaxy on free standing, hydride vapor phase epitaxy grown GaN substrates. The freestanding substrates have threading dislocation densities below 10(7) cm(-2). We find that AlGaN/GaN heterostructures with excellent properties may be grown within a wide range of substrate temperatures and fluxes. Electron Hall mobilities above 1700 cm(2)/V s and sheet resistances below 370 ohm/square are typical. We are able to obtain high saturated drain currents with low gate leakage. Off-state breakdown voltages as high as 200 V with low drain and gate leakage currents have been measured. Further, we have measured microwave output power densities above 5 W/mm at 4 GHz with a power-added efficiency of 46% and an associated gain of 13.4 dB. We attribute improved electrical properties in these devices to the reduced threading dislocation density compared to those grown on non-native substrates. (c) 2006 Elsevier B.V. All rights reserved.