A stack-type semiconductor device includes a lower device and an upper device disposed on the lower device. The lower device includes a lower substrate, a lower interconnection on the lower substrate, a lower pad on the lower interconnection, and a lower interlayer insulating layer covering side surfaces of the lower interconnection and the lower pad. The upper device includes an upper substrate, an upper interconnection under the upper substrate, an upper pad under the upper interconnection, and an upper interlayer insulating layer covering side surfaces of the upper interconnection and the upper pad. Each of the pads has a thick portion and a thin portion. The thin portions of the pads are bonded to each other, the thick portion of the lower pad contacts the bottom of the upper interlayer insulating layer, and the thick portion of the upper pad contacts the top of the lower interlayer insulating layer.