A method for manufacturing a HEMT transistor includes: realizing an undoped epitaxial layer on a substrate; realizing a barrier epitaxial layer on the undoped epitaxial layer so as to form a heterojunction; realizing source and drain structures, separated from one other, on the barrier epitaxial layer; depositing an insulating layer on the barrier epitaxial layer and on the source and drain structures; and photolithographic defining the insulating layer, defining first and second insulating portions in correspondence of the source and drain structures, respectively, and exposing a portion of the barrier epitaxial layer. The method further comprises: forming first and second spacers lying at the corners of the first and second insulating portions; and depositing a gate metal structure at least partially covering said first and second insulating portions, and said first and second spacers, said gate metal structure being a field plate of the HEMT transistor.