A semiconductor device includes a substrate having a first region and a second region, first and second nanowires disposed sequentially on the substrate in the first region, and extending respectively in a first direction, third and fourth nanowires disposed sequentially on the substrate in the second region, and extending respectively in the first direction, a first inner spacer between the first nanowire and the second nanowire, and including hydrogen of a first hydrogen mole fraction, and a second inner spacer between the third nanowire and the fourth nanowire, and including hydrogen of a second hydrogen mole fraction that is greater than the first hydrogen mole fraction.