A method of manufacturing a transistor according to some embodiments includes sequentially forming a dummy gate oxide layer and a dummy gate electrode on an active region of a semiconductor substrate, ion-implanting a first conductive impurity into source/drain regions to form first impurity regions, and ion-implanting the first conductive impurity to form second impurity regions that are overlapped by the first impurity regions. The method includes forming a pad polysilicon layer on the source/drain regions, sequentially removing the pad polysilicon layer and the dummy gate electrode from a gate region of the semiconductor substrate, annealing the semiconductor substrate, and ion-implanting a second conductive impurity to form a third impurity region in the gate region. The method includes removing the dummy gate oxide layer, forming a gate insulation layer, and forming a gate electrode on the gate region.