Method of manufacturing field effect transistor

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

• forming a separating and insulating film on a main surface of a semiconductor substrate of a first conductivity type for separation of an element forming region;
• forming a region which has a peak concentration of the first conductivity type, is located in said semiconductor substrate and extends from the vicinity of a lower surface of said separating and insulating film to a position at a depth in said element forming region;
• forming a gate electrode, which is located in said element forming region, on said main surface of said semiconductor substrate with a gate insulating film therebetween;
• forming first impurity regions of a second conductivity type having a first concentration in regions below said main surface of said semiconductor substrate, which are located at opposite sides of a region under said gate electrode, with a space from each other;
• forming sidewall oxide films covering side surfaces of said gate electrode;
• forming second impurity regions, which have a second impurity concentration higher than said first impurity concentration, in said main surface of said semiconductor substrate, each of said second impurity regions being spaced apart from each other by said region under said gate electrode and a region under sidewall oxide films, and being located in said element forming region adjacent to said separating and insulating film;
• and
• forming third impurity regions of the second conductivity type, in a step separate from and subsequent to forming said first impurity regions of the second conductivity type and forming said second impurity regions, each of which is located between one of said second impurity regions and said peak concentration of the first conductivity type, is adjacent to one of said second impurity regions and has a third concentration lower than said second concentration.