A semiconductor body has source and drain regions (4 and 5; 4' and 5') spaced apart by a body region (6; 6') and a drain drift region (50; 50') and both meeting the same surface (3a) of the semiconductor body. An insulated gate structure (7; 70'; 700) is provided within a trench (80; 80'; 80'') extending in the semiconductor body. The gate structure has a gate conductive region (70b; 70'b; 70''b) separated from the trench by a dielectric layer (70a; 70'a) such that a conduction channel accommodation portion (60; 60') of the body region extends along at least side walls (80a; 80'a; 80''a) of the trench and between the source (4; 4') and drain drift (50; 50') regions. The trench extends from the body region into the drain drift region (50; 50') and the dielectric layer has, at least on side walls (80a; 80'a; 80''a) of the trench, a greater thickness in the portion of the trench extending into the drain drift region (50; 50') than in the remaining portion of the trench so that an extension (71; 71'; 71''; 710) of the gate conductive region extending within the trench through the drain drift region (50; 50') towards the drain region (5; 5') forms a field plate.
