Fabrication of a Group III-nitride transistor device can include implanting dopant ions into a stacked Group III-nitride channel layer and Group III-nitride barrier layer to form source/drain regions therein with a channel region therebetween. The channel layer has a lower bandgap energy than the barrier layer along a heterojunction interface between the channel layer and the barrier layer. The source/drain regions have a lower defect centers energy than the channel region. The source/drain regions and the channel region are exposed to a laser beam with a wavelength having a photon energy that is less than the bandgap energy of the channel region and higher than the defect centers energy of the source/drain regions to locally heat the source/drain regions to a temperature that anneals the source/drain regions.