Method of forming transistor with dual spacer

A method of forming a transistor with dual spacers, comprising:

• forming a gate dielectric layer and a gate electrode on a substrate, wherein the gate dielectric layer protrudes from the gate electrode;
• forming a first dual spacer comprising a first inner spacer and a first outer spacer on the gate dielectric layer beside the gate electrode;
• forming a second dual spacer comprising a second inner spacer having an L-shaped profile and a second outer spacer on the gate dielectric layer beside the first dual spacer;
• removing the second outer spacer, thereby the gate dielectric layer comprising a first part and a second part, wherein the first part is right below the second inner spacer, and the second part is uncovered by the second inner spacer, wherein a thickness of the first part is larger than a thickness of the second part;
• and
• forming a source/drain in the substrate beside the second dual spacer after the second dual spacer is formed, and then the second outer spacer is removed, wherein the second part vertically overlaps the source/drain completely.