A semiconductor structure includes a substrate and a fin protruding from the substrate along a first direction, wherein the fin includes a first semiconductive layer over the substrate, a second semiconductive layer over the first semiconductive layer along the first direction, and a dielectric layer disposed between the first semiconductive layer and the second semiconductive layer and electrically isolated from the first semiconductive layer and the second semiconductive layer. The semiconductor structure also includes a gate electrode including: a first conductive portion extending in a second direction different from the first direction and including an upper surface level with an upper surface of the first semiconductive layer; and a second conductive portion electrically isolated from the first conductive portion and including a bottom surface level with a bottom surface of the second semiconductive layer.