A semiconductor device includes; an active pattern on a substrate, a source/drain pattern on the active pattern, a channel pattern connected to the source/drain pattern and including semiconductor patterns spaced apart in a vertical stack, and a gate electrode extending across the channel pattern. The semiconductor patterns includes a first semiconductor pattern and a second semiconductor pattern. The gate electrode includes a first part between the substrate and the first semiconductor pattern and a second part between the first semiconductor pattern and the second semiconductor pattern. A width of the first part varies with a depth of the first part, such that a width of a middle portion of the first part is less than a width of a lower portion of the first part and a width of an upper portion of the first part.