A method for forming a MOSFET includes the steps of forming cobalt silicide layers on a polysilicon gate electrode and source/drain regions, implanting impurity ions to form source/drain extensions and diffusing the impurity ions in the source/drain extensions The temperature of the heat treatment for diffusing step is lower than the maximum of the temperatures of the heat treatment for forming the silicide layer, whereby a MOSFET having excellent short-channel characteristics and a higher reliability can be obtained.