A semiconductor device and a forming method thereof are provided. The semiconductor device includes a device layer including a front side and a back side opposite to each other, a bonding layer disposed on the back side of the device layer, and a carrier substrate underlying the bonding layer. The device layer includes source/drain (S/D) structures, semiconductor channel layers connecting the S/D structures, and a gate structure disposed between the S/D structures and around each of the semiconductor channel layers, where the back side is planar and includes the S/D structures and the gate structure.