Methods of manufacturing semiconductor devices include providing a substrate including a NMOS region and a PMOS region, implanting fluorine ions into an upper surface of the substrate, forming a first gate electrode of the NMOS region and a second gate electrode of the PMOS region on the substrate, forming a source region and a drain region in portions of the substrate, which are adjacent to two lateral surfaces of the first gate electrode and the second gate electrode, respectively, and performing a high-pressure heat-treatment process on an upper surface of the substrate by using non-oxidizing gas.