Method for forming transistor of semiconductor device

A method for forming a transistor comprises:

• forming a dummy gate having a stacked structure of a gate oxide film and a gate electrode on a semiconductor substrate;
• forming a first insulating film on the entire surface of the resultant structure;
• over-etching the first insulating film to form first insulating film spacers on the lower portion of both sidewalls of the dummy gate;
• forming a second insulating film on entire surface of the resultant structure;
• etching the second insulating film to form second insulating film spacers on the upper portion of the both sidewalls of the dummy gate;
• removing the first insulating film spacers to form openings each of which exposes the sidewall of the gate oxide film;
• performing a thermal oxidation process to form a thermal oxide film filling up the opening;
• ion-implanting low concentration impurities onto the substrate using the dummy gate and the second insulating film spacer as a mask to form low concentration impurity junction regions on both sides of the dummy gate;
• and
• performing a halo implant process using the dummy gate and the second insulating film spacer as a mask to form a halo doped region which is disposed under the both low concentration impurity junction regions and extends into the channel region under the dummy gate.