A semiconductor device includes a first substrate including an impurity region including impurities of a first conductivity type, circuit devices on the first substrate, a lower interconnection structure electrically connected to the circuit devices, a second substrate on the lower interconnection structure and including semiconductor of the first conductivity type, gate electrodes on the second substrate and stacked and spaced apart from each other in a first direction perpendicular to an upper surface of the second substrate, channel structures penetrating the gate electrodes, and a connection structure. The channel structures may extend perpendicular to the second substrate. The channel structures may include a channel layer. The connection structure may connect the impurity region of the first substrate to the second substrate, and the connection structure may include a via including a semiconductor of a second conductivity type.