A method for fabricating a semiconductor device includes forming a buried gate electrode in a semiconductor substrate. An insulating layer is formed over the buried gate electrode and is etched to form a contact hole exposing the semiconductor substrate. A sacrificial spacer is formed on sidewalls of the insulating layer defining the contact hole. A polysilicon layer pattern is formed in the contact hole. The sacrificial spacer is removed to form an air gap around the polysilicon layer pattern. A thermal process is performed to remove a seam existing in the polysilicon layer pattern.