The wide bandgap semiconductors GaN and AlGaN show promise for high voltage standoff layers in high power devices such as GaN Schottky rectifiers and GaN/AlGaN thyristorlike switches. The material properties which significantly influence the device design and performance are electron and hole diffusion lengths, recombination lifetimes and the critical field for electric breakdown. We have fabricated high standoff voltage (> 450 V) GaN Schot-tky rectifiers, and measured a lower limit for the critical field for electric breakdown to be (2 ± 0.5) · 106 V/cm. Diffusion lengths and recombination lifetimes were measured by electron beam induced current on unintentionally doped, n and p-type GaN samples grown by various epitaxial techniques. To establish the possible effects of linear dislocations and other defects on the transport and breakdown properties, the same sample surfaces were analyzed by AFM. On some of the samples, our measurements indicate that the dislocations appear to be electrically active and that recombination at dislocations occupying grain boundaries limit the minority carrier lifetime to the nanosecond range. Based on the measurements of transport properties, critical fields and the modeling of the devices proposed, our estimates indicate that DARPA/EPRI goals for megawatt electronics set at 5 kV standoff voltage and 200 A on-state current might be achieved with 15 – 20 μm thick layers grown by HVPE, at approximately 1. 1016 cm−3 doping levels, and 1 – 2cm2 device active area.