Semiconductor device manufacturing method and semiconductor device

A semiconductor device having an insulated-gate transistor of a first conductivity type, said semiconductor device comprising:

• a semiconductor substrate having a predetermined semiconductor region of a second conductivity type in its surface;
• a gate insulating film selectively formed on said predetermined semiconductor region, the surface of said predetermined semiconductor region under said gate insulating film being defined as a channel region;
• a gate electrode formed on said gate insulating film;
• first and second extension-forming side walls formed on sides of said gate electrode and containing an extension-forming impurity of the first conductivity type;
• first and second auxiliary side walls adjacent respectively to said first and second extension-forming side walls such that the first and second extension-forming side walls are disposed between the sides of the gate elect rode and said first and second auxiliary side walls;
• and
• first and second source/drain regions separated by said channel region in the surface of said predetermined semiconductor region, wherein
• said gate insulating film, said gate electrode and said first and second source/drain regions define said transistor, and
• said source/drain regions include first and second extension regions which are shallower than other regions and are formed under said first and second extension-forming side walls.