GaN bulk rectifiers show excellent on-state resistances (in the mOmega cm-2 range), forward turn-on voltages of similar to 1. 8 V and reverse-recovery times of <50 ns. A key requirement is to develop effective edge termination techniques in order to prevent premature surface-induced breakdown. We have performed a simulation study of the effects of varying the dielectric passivation material (SiO2, SiN, AIN, SC2O3, or MgO), the thickness of this material, the extent of metal overlap onto the dielectric and the ramp oxide angle on the resulting reverse breakdown voltage (V-B) of bulk rectifiers. We find that SiO2 produces the highest. V-B of the materials investigated, that there is an optimum metal overlap distance for a given oxide thickness and small oxide ramp angles produce the highest V-B.