A method of forming a semiconductor structure, the method includes epitaxially growing a second III-V compound layer on a first III-V compound layer. A carrier channel is located between the first III-V compound layer and the second III-V compound layer. The method further includes forming a source feature and a drain feature on the second III-V compound layer, forming a third III-V compound layer on the second III-V compound layer, depositing a gate dielectric layer on a portion of the second III-V compound layer and a top surface of the third III-V compound layer, treating the gate dielectric layer on the portion of the second III-V compound layer with fluorine and forming a gate electrode on the treated gate dielectric layer between the source feature and the drain feature.