Provided are semiconductor devices including a semiconductor substrate, an insulating layer including a contact hole through which the semiconductor substrate is exposed, and a polysilicon layer filling the contact hole. The polysilicon layer is doped with impurities and includes an impurity-diffusion prevention layer. In the semiconductor devices, the impurities included in the polysilicon layer do not diffuse into the insulating layer and the semiconductor substrate due to the impurity-diffusion prevention layers.