We have grown AlGaN/GaN high electron mobility transistor (HEMT) structures by plasma-assisted molecular beam epitaxy on free-standing n-GaN substrates grown by hydride vapor phase epitaxy. Reflection high energy electron diffraction patterns of the as-loaded wafers exhibit narrow streaks which persist throughout the growth. Atomic force microscopy shows smooth AlGaN surfaces with root-mean-square roughness of 10 angstrom over a 20 X 20 mu m(2) area. High resolution x-ray diffractometry indicates that the AlGaN peak is similar to 20% narrower than for similar structures grown on SiC. Hall mobilities, electron sheet densities, and sheet resistances were measured on ten 60 X 60 mu m(2) Hall test patterns defined photolithographically across the surface of the 10 X 10 mm2 sample. Buffer leakage measurements demonstrate that a Be:GaN layer effectively isolates the channel from the conductive substrate. Average sheet resistances and sheet densities were 380 Omega/rectangle and 0.94 X 10(13) cm(-2), respectively. These HEMT structures exhibit room-temperature Hall mobilities in excess of 1900 cm(2)/V S. In addition, devices on these structures exhibit excellent pinch-off, low gate leakage, and saturated drain current densities of almost 700 mA/mm. Further details regarding the structural and electrical properties will be described along with device testing. (c) 2005 American Vacuum Society.