A semiconductor device includes first and second channels, first and second gate structures, first and second source/drain layers, first and second fin spacers, and first and second etch stop patterns. The first channels are disposed vertically on a first region of a substrate. The second channels are disposed vertically on a second region of the substrate. The first gate structure is formed on the first region and covers the first channels. The second gate structure is formed on the second region and covers the second channels. The first and second source/drain layers contact the first and second channels, respectively. The first and second fin spacers contact sidewalls and upper surfaces of the first and second source/drain layers, respectively. The first and second etch stop patterns are formed on the first and second fin spacers, respectively, and do not contact the first and second source/drain layers, respectively.