Disclosed are a semiconductor device and a method for manufacturing the same. The semiconductor device includes a gate structure which includes a silicon oxynitride (SiON) layer formed on a semiconductor substrate, a hafnium silicon oxynitride (HfSiON) layer formed on the silicon oxynitride (SiON) layer, a polysilicon layer formed on the hafnium silicon oxynitride (HfSiON) layer, and a silicide layer formed on the polysilicon layer, spacers at sidewalls of the gate structure, and source and drain regions at opposite sides of the gate structure.