Semiconductor device and method of manufacturing the same

Provided is a semiconductor device that includes a vertical MOS transistor having a trench structure capable of enhancing a driving performance of the vertical MOS transistor. A thick oxide film is formed next to a gate electrode led out of a trench of the vertical MOS transistor having the trench structure, and is removed to form a stepped portion which has a face lower than a surrounding plane and has slopes as well. This makes it possible to form a heavily doped diffusion layer right under the gate electrode through ion implantation for forming a heavily doped source diffusion layer, thereby solving a problem of no current flow in a part of a driver element and enhancing the driving performance of the vertical MOS transistor.