A method includes epitaxially growing a gallium nitride (GaN) layer over a silicon substrate. The method further includes epitaxially growing a donor-supply layer over the GaN layer. The method further includes forming a source and a drain on the donor-supply layer. The method further includes forming a gate structure between the source and the drain on the donor-supply layer. The method further includes plasma etching a portion of a drift region of the donor-supply layer to a depth of less than 60% of a donor-supply layer thickness. The method further includes depositing a dielectric layer over the donor-supply layer.