A semiconductor device includes a first transistor and a second transistor. The first transistor includes a first nanowire extending through a first gate electrode and between first source and drain regions. The second transistor includes a second nanowire extending through a second gate electrode and between a second source and drain regions. The first nanowire has a first size in a first direction and a second size in a second direction, and the second nanowire has a second size in the first direction and substantially the second size in the second direction. The first nanowire has a first on current and the second nanowire has a second on current. The on current of the first nanowire may be substantially equal to the on current of the second nanowire based on a difference between the sizes of the first and second nanowires. In another arrangement, the on currents may be different.