In one embodiment, a method for fabricating a III-Nitride transistor on a III-Nitride semiconductor body is disclosed. The method comprises etching dielectric trenches in a field dielectric overlying gate, source, and drain regions of the III-Nitride semiconductor body, and thereafter forming a gate dielectric over the gate, source and drain regions. The method further comprises forming a blanket diffusion barrier over the gate dielectric layer, and then removing respective portions of the blanket diffusion barrier from the source and drain regions. Thereafter, gate dielectric is removed from the source and drain regions to substantially expose the source and drain regions. Then, ohmic contacts are formed by depositing contact metal in the source and drain regions. The method results in highly conductive source/drain contacts that are particularly suitable for power transistors, for example, III-Nitride transistors, such as GaN transistors. In another embodiment, a structure for highly conductive source/drain contacts is disclosed.