It is an object to provide a
semiconductor having a novel structure. In the
semiconductor device, a plurality of memory elements are connected in series and each of the plurality of memory elements includes first to third transistors thus forming a memory circuit. A source or a drain of a first
transistor which includes an
oxide semiconductor layer is in electrical contact with a gate of one of a second and a third
transistor. The extremely low off current of a first
transistor containing the
oxide semiconductor layer allows storing, for long periods of time, electrical charges in the gate
electrode of one of the second and the third transistor, whereby a substantially permanent
memory effect can be obtained. The second and the third transistors which do not contain an
oxide semiconductor layer allow high-speed operations when using the memory circuit.