A semiconductor device includes gate structures including a tunnel insulating layer pattern, a floating gate, a dielectric layer pattern and a control gate sequentially disposed on a substrate. The control gate includes an impurity doped polysilicon layer pattern and a metal layer pattern. The gate structures are spaced apart from each other on the substrate. A capping layer pattern is disposed on a sidewall portion of the metal layer pattern and includes a metal oxide. An insulating layer covers the gate structures and the capping layer pattern. The insulating layer is formed on the substrate and includes an air-gap therein.