A semiconductor device, including: a first layer including monocrystalline material and first transistors, the first transistors overlaid by a first isolation layer; a second layer including second transistors and overlaying the first isolation layer, the second transistors including a monocrystalline material; at least one contact to the second transistors, where the at least one contact has a diameter of less than 200 nm; a first set of external connections underlying the first layer to connect the device to external devices; a second set of external connections overlying the second layer to connect the device to external devices; and an interconnection layer in-between the first layer and the second layer, where the interconnection layer includes copper or aluminum.