In 2009, a crystalline oxide semiconductor with a layered structure, which we refer to as c‐axis–aligned crystalline indium‐gallium‐zinc oxide (CAAC‐IGZO), was first discovered. CAAC‐IGZO has a peculiar crystal structure in which clear grain boundaries are not observed despite high c‐axis alignment and absence of a‐b plane alignment. When compared to a Si field‐effect transistor (FET), a metal‐oxide‐semiconductor (MOS) FET, utilizing CAAC‐IGZO, presents lower off‐state current (on the order of yA [10−24 A]). These unique characteristics allow CAAC‐IGZO to realize devices with low power consumption. With the emerging era of artificial intelligence, wherein power saving becomes more significant, CAAC‐IGZO has attracted attention as a potential replacement for Si. This paper describes the characteristics and potentials of CAAC‐IGZO for the development of memory devices with unprecedented functions.

We fabricated a field-effect transistor (FET) with a gate length of 21 nm whose channel material is a c-axis-aligned crystalline In-Ga-Zn oxide (CAAC-IGZO). A CAAC-IGZO FET with a gate length of 21 nm has an extremely low off-state leakage current, a practical driving capability, and tolerance against high temperatures. CAAC-IGZO FETs enable low-power integrated circuits, such as logic and memory in high-temperature environments. Performance estimation of a memory cell using the CAAC-IGZO FET with a gate length of 21 nm revealed that a write time of less than 1 ns and a data retention time of more than 1 h would be possible at 85 • C.INDEX TERMS C-axis-aligned crystalline In-Ga-Zn oxide (CAAC-IGZO), leakage current, nonvolatile memory, random access memory. II. BASIC CHARACTERISTICS OF C-AXIS-ALIGNED CRYSTALLINE IN-GA-ZN OXIDE FET

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