A MOS semiconductor device of a vertical type has: a functional layer, having a first type of conductivity; gate structures, which are formed above the functional layer and have a region of dielectric material and an electrode region; body wells, which have a second type of conductivity, are formed within the functional layer, and are separated by a surface separation region; source regions, which have the first type of conductivity and are formed within the body wells. Each gate structure extends laterally above just one respective body well and does not overlap the surface separation region of the functional layer. The device may further have: at least one shield structure, arranged between adjacent gate structures above the surface separation region; and/or at least one doped control region, having the second type of conductivity, arranged within the surface separation region, which are both set at the source potential.