A semiconductor device includes a lower insulating pattern on a semiconductor substrate, a lower gate pattern on the lower insulating pattern and formed of a doped polysilicon layer, a residual insulating pattern with an opening exposing a portion of a top surface of the lower gate pattern, an upper gate pattern on the residual insulating pattern, the upper gate pattern filling the opening, and a diffusion barrier pattern in contact with the portion of the top surface of the lower gate pattern and extending between the residual insulating pattern and the upper gate pattern.