Semiconductor device and manufacturing method thereof

A manufacturing method of a semiconductor device comprising the steps of:

• forming a trench in the main surface of a semiconductor substrate;
• forming a conductive layer on said main surface so as to fill said trench;
• forming a potential fixing electrode filling said trench and having an expanding part expanding on said main surface so that a width thereof is larger than that of said trench, and forming a gate electrode on said main surface by patterning said conductive layer;
• forming insulation layers so as to cover said gate electrode and expose said expanding part of said potential fixing electrode;
• and
• forming main electrodes so as to be electrically insulated from said gate electrode and connected to a whole upper surface of said expanding part of said potential fixing electrode.