A method for forming a high electron mobility transistor is disclosed. A substrate is provided. A channel layer is formed on the substrate. An electron supply layer is formed on the channel layer. A dielectric passivation layer is formed on the electron supply layer. A gate recess is formed into the dielectric passivation layer and the electron supply layer. A surface modification layer is conformally deposited on an interior surface of the gate recess. The surface modification layer is then subjected to an oxidation treatment or a nitridation treatment. A P-type GaN layer is formed in the gate recess and on the surface modification layer.