A method forms an HEMT transistor of the normally off type, including: a semiconductor heterostructure, which comprises at least one first layer and one second layer, the second layer being set on top of the first layer; a trench, which extends through the second layer and a portion of the first layer; a gate region of conductive material, which extends in the trench; and a dielectric region, which extends in the trench, coats the gate region, and contacts the semiconductor heterostructure. A part of the trench is delimited laterally by a lateral structure that forms at least one first step. The semiconductor heterostructure forms a first edge and a second edge of the first step, the first edge being formed by the first layer.