A method is provided for fabricating a fin field-effect transistor. The method includes providing a semiconductor substrate; and forming a plurality of fins on top of the semiconductor substrate. The method also includes forming isolation structures between adjacent fins; and forming doping sidewall spacers in top portions of the isolation structures near the fins. Further, the method includes forming a punch-through stop layer at the bottom of each of the fins by thermal annealing the doping sidewall spacers; and forming a high-K metal gate on each of the fins.