A semiconductor device includes a first substrate structure having a first substrate, circuit elements disposed on the first substrate, and first bonding pads disposed on the circuit elements. A second substrate structure is connected to the first substrate structure. The second substrate structure includes a second substrate having first and second surfaces, first and second conductive layers spaced apart from each other, a pad insulating layer having an opening exposing a portion of the second conductive layer and gate electrodes stacked to be spaced apart from each other in a first direction and electrically connected to the circuit elements. First contact plugs extend on the second surface in the first direction and connect to the gate electrodes. A second contact plug extends on the second surface in the first direction and electrically connects to the second conductive layer. Second bonding pads electrically connect to the first and second contact plugs.