Fabrication method of semiconductor device and fabrication method of dynamic threshold transistor

A method of fabricating a semiconductor device, comprising:

• forming a first trench and a second trench, that is located respectively at a first side and a second side of a silicon substrate portion in a silicon substrate;
• forming a semiconductor layer that has etching selectivity against silicon in said first trench and said second trench by epitaxially growing;
• forming a silicon layer on said semiconductor layer by epitaxially growing in said first trench and said second trench;
• forming a void underneath said silicon layer by removing said semiconductor layer;
• forming a buried insulation film in said voids;
• forming a gate insulation film directly on said silicon substrate portion and a gate electrode on said gate insulation film;
• and
• forming a source region in said silicon layer at said first side of said silicon substrate portion and a drain region in said silicon layer at said second side of said silicon substrate portion.
• wherein the method further comprises, after said forming said buried insulation film but before said forming the gate insulation film, exposing a top surface of said silicon substrate portion by removing said buried insulation film covering said top surface, said gate insulation film being formed directly on said top surface of said silicon substrate portion.