Packaged semiconductor devices including backside power rails and methods of forming the same

Methods for forming packaged semiconductor devices including backside power rails and packaged semiconductor devices formed by the same are disclosed. In an embodiment, a device includes a first integrated circuit device including a first transistor structure in a first device layer; a front-side interconnect structure on a front-side of the first device layer; and a backside interconnect structure on a backside of the first device layer, the backside interconnect structure including a first dielectric layer on the backside of the first device layer; and a first contact extending through the first dielectric layer to a source/drain region of the first transistor structure; and a second integrated circuit device including a second transistor structure in a second device layer; and a first interconnect structure on the second device layer, the first interconnect structure being bonded to the front-side interconnect structure by dielectric-to-dielectric and metal-to-metal bonds.