Oxide-based thin-film transistor (TFT) semiconductor memory device having source/drain electrode of one transistor connected to gate electrode of the other

摘要

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.