A semiconductor device includes a semiconductor substrate, a first semiconductor stack, a second semiconductor stack, a first gate structure, and a second gate structure. The semiconductor substrate comprising a first device region and a second device region. The first semiconductor stack is located on the semiconductor substrate over the first device region, and has first channels. The second semiconductor stack is located on the semiconductor substrate over the second device region, and has second channels. A total number of the first channels is greater than a total number of the second channels. The first gate structure encloses the first semiconductor stack. The second gate structure encloses the second semiconductor stack.