A semiconductor device includes a substrate having a conductive area, a first pattern formed on the substrate and having a contact hole through which the conductive area is exposed, and a contact plug in the contact hole. The contact plug includes first and second silicon layers. The first silicon layer, formed from a first compound including at least two silicon atoms, is formed in the contact hole to contact a top surface of the conductive area and a side wall of the first pattern. The second silicon layer, formed from a second compound including a number of silicon atoms less than the number of the silicon atoms of the first compound, is formed on the first silicon layer and fills a remaining space of the contact hole, the second silicon layer being spaced apart from the first pattern at an entrance of the contact hole.