A vertical semiconductor device, comprising: - a semiconductor body having a top surface (S b); - a trench formed in the semiconductor body starting from the top surface and having side walls (22a, 22b) and a bottom; - a gate region of conductive material, extending within the trench; - an insulating region extending along bottom portions of the side walls of the trench and on the bottom of the trench; - a gate insulating layer extending along top portions of the side walls of the trench on top of the insulating region and laterally with respect to the gate region; - a conductive region extending within the trench, a top portion of the conductive region being surrounded at the top and laterally by the gate region, a bottom portion of the conductive region being surrounded at the bottom and laterally by the insulating region; and - a field insulating layer arranged between the gate region and the conductive region so as to electrically separate the gate region and the conductive region from one another; characterized in that the field insulating layer is connected to the insulating region by means of field-connecting thickened portions (72a, 72b), each of which has a thickness that increases as the depth increases; and wherein the gate insulating layer is connected to the insulating region by means of gate-connecting thickened portions (70a, 70b), each of which has a thickness that increases as the depth increases; and wherein the side walls (22a, 22b) of the trench are undulated in such a way that the width of the trench varies in a non-monotonic way with the depth, thereby assuming a first value at the top surface (S b), then decreasing as the depth increases until a first relative minimum is assumed, then increasing again, until assuming a relative maximum at a depth equal at the most to the maximum depth of the gate region, then decreasing monotonically as far as the bottom of the trench; and wherein the conductive region has corresponding side walls (24a, 24b), which are undulated, in such a way that the width of the conductive region varies in a non-monotonic way with the depth, thereby increasing, starting from a top wall of the conductive region, until exhibiting a relative maximum, at a depth comprised between the depth of the top wall of the conductive region and the maximum depth of the gate region, then decreasing until assuming a minimum at a depth not greater than the maximum depth of the gate region, and then increasing monotonically as far as the bottom of the conductive region. - a semiconductor body having a top surface (S b); - a trench formed in the semiconductor body starting from the top surface and having side walls (22a, 22b) and a bottom; - a gate region of conductive material, extending within the trench; - an insulating region extending along bottom portions of the side walls of the trench and on the bottom of the trench; - a gate insulating layer extending along top portions of the side walls of the trench on top of the insulating region and laterally with respect to the gate region; - a conductive region extending within the trench, a top portion of the conductive region being surrounded at the top and laterally by the gate region, a bottom portion of the conductive region being surrounded at the bottom and laterally by the insulating region; and - a field insulating layer arranged between the gate region and the conductive region so as to electrically separate the gate region and the conductive region from one another;