Transistor device and fabrication method

A method for forming a transistor, comprising:

• providing a substrate;
• forming a dummy gate film on the substrate;
• doping an upper portion of the dummy gate film to convert into a modified film on a remaining dummy gate film;
• etching the modified film and the remaining dummy gate film to expose a surface of the substrate to form a modified layer and a dummy gate layer on the substrate;
• forming a first sidewall spacer on a sidewall surface of each of the dummy gate layer and the modified layer;
• forming source/drain regions in the substrate and on both sides of the first sidewall spacer and the dummy gate layer;
• forming a dielectric film on each of the substrate, the source/drain regions, and the dummy gate layer;
• and
• planarizing the dielectric film and the modified layer to provide a dielectric layer, and to remove the modified layer to expose the dummy gate layer, wherein the dielectric film has a planarization rate lower than the modified layer, and the formed dielectric layer has a surface higher than the exposed dummy gate layer.