A semiconductor device includes a first transistor structure on a substrate, the first transistor structure including first channel layers spaced apart from each other, a first gate electrode surrounding the first channel layers, a first source/drain region connected to the first channel layers on a first side of the first gate electrode, and a second source/drain region connected to the first channel layers on a second side of the first gate electrode that is opposite to the first side of the first gate electrode, and a second transistor structure on the first transistor structure, the second transistor structure including second channel layers spaced apart from each other, a second gate electrode surrounding the second channel layers, and a third source/drain region connected to the second channel layers on a first side of the second gate electrode.