A semiconductor device includes a semiconductor substrate, a cell region
in a surface portion of the substrate for operating as a transistor, a
gate lead wiring region having a gate lead pattern on the substrate, a
trench in the surface portion of the substrate extending from the cell
region to the gate lead wiring region, an oxide film on an inner surface
of the trench, and a gate electrode in the trench insulated with at least
the oxide film from the substrate. A speed of formation of a main portion
of the sidewalls of the trench at the gate lead wiring region is greater
than that of a main portion of the sidewalls of the trench at the cell
region, so that a thickness of the oxide film at the gate lead wiring
region is greater than that at the cell region.