We discuss the growth and characterization of homoepitaxial GaN layers and AlGaN/GaN high electron mobility transistor (HEMT) structures grown by plasma-assisted molecular beam epitaxy (MBE) on freestanding n-GaN substrates. The GaN substrates were fabricated by hydride vapor phase epitaxy and exhibit low dislocation densities of similar to 10(7) cm(-2). The best MBE-grown homoepitaxial epilayers on these substrates were grown in the gallium droplet regime. Root-mean-square roughnesses of these layers were 3.5-4.0 angstrom over 5 x 5 mu m(2) regions. AlGaN/GaN HEMT structures were grown on these substrates and exhibit room-temperature Hall mobilities of 1920 cm(2)/V s at an electron sheet density of 0.9 x 10(13) cm(-2). Electrical isolation of the two-dimensional electron gas from the conductive substrate was accomplished using a Be:GaN buffer. HEMT devices were photolithographically defined and DC and RF device characteristics were measured. Off-state breakdown voltages of 90 V, saturated drain currents of nearly 700 mA/mm, and gate leakage currents of 0.07 mA/mm were observed on unpassivated devices. Preliminary results on RF performance and device reliability are presented and discussed. (c) 2005 Elsevier B.V. All rights reserved.