A method includes fabricating a first-type active-region semiconductor and a second-type active-region semiconductor structure stacked with each other and both extending in a first direction at a front side of a substrate. The method includes forming a front-side power rail and a front-side signal line extending in the first direction in a front-side metal layer. The front-side power rail is conductively connected to a source terminal of a second-type transistor in the second-type active-region semiconductor structure. The method includes forming a back-side metal layer on a backside of the substrate, and forming a back-side power rail and a back-side signal line extending in the first direction in the back-side metal layer. The back-side power rail is conductively connected to a source terminal of a first-type transistor in the first-type active-region semiconductor structure.