Semiconductor device having trench gate structure and manufacturing method thereof

A method of manufacturing a semiconductor device comprising:

• forming a first semiconductor layer of a first conductivity type;
• forming a first part of a base layer of a second conductivity type in the first semiconductor layer;
• forming a trench in the first and the second semiconductor layers;
• forming an insulating film on side walls and a bottom portion of the trench;
• forming a second semiconductor layer of the first conductivity type in the first semiconductor layer near the bottom of the trench, the second semiconductor layer having an impurity concentration higher than that of the first semiconductor layer;
• forming a second part of the base layer of the second conductivity type in the first semiconductor layer using the second semiconductor layer as a stopper in a region deeper than the first part of the base layer;
• and
• forming a gate electrode at least partly on the insulating film in the trench,
• wherein the base layer is formed by performing ion-implantation a plurality of times before and after forming the second semiconductor layer.