Semiconductor device and method of fabricating the same

A method of forming a semiconductor substrate, the method comprising:

• performing an ion implanting process over a semiconductor substrate in a cell/core region to form an ion implanted region in the cell/core region;
• selectively etching a portion of the semiconductor substrate located at a portion of a boundary of the ion implanted region to form a first recess exposing the ion implanted region;
• removing the ion implanted region using the first recess to form an under-cut space in the substrate;
• forming a first insulating film to fill the under-cut space and the first recess;
• selectively etching a portion of the semiconductor substrate at a portion of a boundary of the first insulating film to form a second recess exposing the first insulating film, wherein the first recess and the second recess are formed in different locations of the semiconductor substrate;
• and
• forming a second insulating film to form a SOI structure in the substrate of the cell/core region, the second insulating film filling the second recess,
• wherein the first insulating film and the second insulating film form a barrier layer that electrically separates a floating body of the SOI structure in the cell/core region from a bulk body in a peripheral region of the semiconductor substrate.