A method of forming a semiconductor structure includes growing a second III-V compound layer over a first III-V compound layer, wherein the second III-V compound layer has a different band gap from the first III-V compound layer. The method further includes forming a source feature and a drain feature over the second III-V compound layer. The method further includes forming a gate dielectric layer over the second III-V compound layer, the source feature and the drain feature. The method further includes implanting at least one fluorine-containing compound into a portion of the gate dielectric layer. The method further includes forming a gate electrode over the portion of the gate dielectric layer.