A method for manufacturing a HEMT transistor comprising the steps of: providing a wafer comprising a semiconductor body including a heterojunction structure formed by semiconductor materials that include elements of Groups III-V of the Periodic Table, and a dielectric layer on the semiconductor body; etching selective portions of the wafer, thus exposing a portion of the heterojunction structure; forming an interface layer by a surface reconstruction process, of a semiconductor compound formed by elements of Groups III-V of the Periodic Table, in the exposed portion of the heterojunction structure; and forming a gate electrode, including a gate dielectric and a gate conductive region, on said interface layer.