Method and structure for forming high-K/metal gate extremely thin semiconductor on insulator device

A method of forming a semiconductor device comprising:

• forming a dummy gate on a substrate, the dummy gate having sidewalls with an undercut region present therein;
• forming a spacer adjacent to the sidewalls of the dummy gate, wherein the spacer includes a first spacer portion including an extension portion with a first width that fills the undercut region in the sidewall of the dummy gate, and a second spacer portion that is present atop the first spacer portion, the second spacer portion having a second width that is less than the first width;
• forming a raised source region and a raised drain region, wherein the raised source region and the raised drain region are separated from the dummy gate by the extension portion of the spacer;
• removing the dummy gate to provide an opening to the substrate;
• and
• filling the opening with a gate structure including a gate conductor having an undercut region filled with the extension portion of the spacer.