A semiconductor device includes a substrate; a 1st transistor formed above the substrate, and having a 1st transistor stack including a plurality of 1st channel structures, a 1st gate structure surrounding the 1st channel structures, and 1st and 2nd source/drain regions at both ends of the 1st transistor stack in a 1st channel length direction; and a 2nd transistor formed above the 1st transistor in a vertical direction, and having a 2nd transistor stack including a plurality of 2nd channel structures, a 2nd gate structure surrounding the 2nd channel structures, and 3rd and 4th source/drain regions at both ends of the 2nd transistor stack in a 2nd channel length direction, wherein the 3rd source/drain region does not vertically overlap the 1st source/drain region or the 2nd source/drain region, and the 4th source/drain region does not vertically overlap the 1st source/drain region or the 2nd source/drain region.