In semiconductor devices, methods of forming the same, the semiconductor device include a first gate structure having a first gate oxide layer pattern, a first polysilicon layer pattern containing atoms larger than silicon and a first hard mask layer pattern on substrates under tensile stress. N-type impurity regions are formed under the surface of the substrate on both sides of the first gate structure. A second gate structure having a second gate oxide layer pattern, a second polysilicon layer pattern containing atoms smaller than silicon and a second hard mask layer pattern on substrates under compressive stress. Additionally, P-type impurity regions are formed under the surface of the substrate on both sides of the second gate structure. The semiconductor devices have good device properties.