A semiconductor device includes a substrate, a first transistor and a second transistor disposed on the substrate, and a first contact structure. The first transistor includes first semiconductor channel layers stacked and separated from one another, and a first source/drain structure and a second source/drain structure disposed at two opposite sides of and connected with each first semiconductor channel layer. The second transistor includes second semiconductor channel layers disposed above the first semiconductor channel layers, stacked, and separated from one another, and a third source/drain structure and a fourth source/drain structure disposed at two opposite sides of and connected with each second semiconductor channel layer. The first contact structure penetrates through the third source/drain structure. The first source/drain structure is electrically connected with the third source/drain structure via the first contact structure, and a part of the first source/drain structure is disposed between the substrate and the first contact structure.