DRAM Using Crystalline Oxide Semiconductor for Access Transistors and Not Requiring Refresh for More Than Ten Days

We fabricated a dynamic random access memory (DRAM) using crystalline oxide semiconductor (OS) transistors and not requiring refresh for more than ten days. We call this memory a dynamic oxide semiconductor random access memory (DOSRAM). A crystalline oxide semiconductor is an In-Ga-Zn-oxide (IGZO) semiconductor and has a c-axis aligned crystal (CAAC) structure. A crystalline OS transistor has extremely low off-state current. The DOSRAM uses this device for access transistors, and can have a very long refresh cycle. A memory cell array made of a crystalline OS layer can be stacked on peripheral circuits made of a silicon (Si) semiconductor layer; thus, the area of the DOSRAM can be decreased.