1. Field of the Invention
The present invention relates to an object, a method, or a manufacturing method. In addition, the present invention relates to a process, a machine, manufacture, or a composition of matter. In particular, one embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a memory device, a driving method thereof, or a manufacturing method thereof. In particular, one embodiment of the present invention relates to a semiconductor memory device.
In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics. A display device, an electro-optical device, a semiconductor circuit, and an electronic device include a semiconductor device in some cases.
2. Description of the Related Art
Dynamic random access memory (DRAM) is widely used as a main memory device of a semiconductor device. DRAM has high degree of integration and large storage capacity per unit area. However, since DRAM is a volatile memory, regular refresh operation is required to hold stored data for a long period.
In recent years, use of an oxide semiconductor for a transistor in a memory cell included in DRAM is suggested. Since the off-state current of a transistor including an oxide semiconductor in a channel formation region can be low, the interval between refresh operations can be significantly increased. Thus, power consumption of DRAM can be reduced (see Patent Document 1).
USPTO Backgrounds
USPTO Backgrounds