Semiconductor device having shallow trenches and method for manufacturing the same

A semiconductor device, comprising:

• a substrate;
• an epitaxial layer of a first conductivity type formed on top of said substrate;
• a body layer of a second conductivity type opposite to the first conductivity type formed on top of said epitaxial layer;
• a trench formed in the body layer, wherein said trench reaches the epitaxial layer;
• a source region of the first conductivity type formed on said body layer, with at least part of the source region being exposed to inner surfaces of the trench;
• a gate insulating film that is formed on the inner surfaces of the trench;
• a gate electrode that is formed within said trench and electrically isolated from said epitaxial layer, said body layer and said source region by the gate insulating film;
• and
• a source electrode that is insulated from the gate electrode and connected to the source region;
• wherein
• a lowermost end face of said gate electrode is spaced from a first interface between the body layer and the epitaxial layer and a second interface between the epitaxial layer and the substrate by first and second distances, respectively, said distances being measured in a thickness direction of said trench, and the first distance is smaller than the second distance;
• and
• in a region above the first interface, said gate insulating film has a thickness that increases towards the first interface.