Vertical Schottky rectifiers have been fabricated on a free-standing n-GaN substrate. Circular Pt Schottky contacts with different diameters (50 mu m, 150 mu m and 300 mu m) were prepared on the Ga-face and full backside ohmic contact was prepared on the N-face by using Ti/Al. The electron concentration of the substrate was as low as similar to 7 x 10(15) cm(-3). Without epitaxial layer and edge termination scheme, the reverse breakdown voltages (V-B) as high as 630 V and 600 V were achieved for 50 pm and 150 pm diameter rectifiers, respectively. For larger diameter (300 mu m) rectifiers, V-B dropped to 260 V. The forward turn-on voltage (V-F) for the 50 mu m diameter rectifiers was 1.2 V at the current density of 100 A/cm(2), and the on-state resistance (R-on) was 2.2 m Omega cm(2), producing a figure-of-merit (V-B)(2)/R-on Of 180 MW cm(-2). At 10 V bias, forward currents of 0.5 A and 0.8 A were obtained for 150 mu m and 300 mu m diameter rectifiers, respectively. The devices exhibited an ultrafast reverse recovery characteristics, with the reverse recovery time shorter than 20 ns. (c) 2006 Elsevier Ltd. All rights reserved.