A semiconductor device includes a first transistor in a first region and a second transistor in a second region. The first transistor includes: a first nanowire, a first gate electrode, a first gate dielectric layer, a first source/drain region, and an inner-insulating spacer. The first nanowire has a first channel region. The first gate electrode surrounds the first nanowire. The first gate dielectric layer is between the first nanowire and the first gate electrode. The first source/drain region is connected to an edge of the first nanowire. The inner-insulating spacer is between the first gate dielectric layer and the first source/drain region. The second transistor includes a second nanowire, a second gate electrode, a second gate dielectric layer, and a second source/drain region. The second nanowire has a second channel region. The second gate electrode surrounds the second nanowire. The second gate dielectric layer is between the second nanowire and the second gate electrode. The second source/drain region is connected to an edge of the second nanowire.