The first commercially viable high voltage (>600V) gallium nitride (GaN) Schottky barrier devices are reported. Though GaN does not have any “micropipe” defects, which commonly exists in SiC material, defects like dislocations due to lattice mismatch hamper the material development of GaN high power devices. Improvements in the nitride epitaxial film growth have led to significant reduction of conductive dislocations. Conductive Atomic Force Microscope (CAFM) analysis of conductive dislocations shows only on the order of 103 cm-2 density of conductive dislocations, which are believed to be responsible for the undesired leakage current. GaN diodes compare to SiC or Si devices demonstrate a significant advantage in the thermal resistance. The insulating properties of Sapphire substrates allow fabrication of the devices in TO220 packages with insulating frame and thermal resistance better than 1.8°C/W compare to 3°C/W of SiC or Si devices with insulating frame. Performance of GaN, SiC and Si devices in the switch mode power supplies is compared.