Nonvolatile Memory With Extremely Low-Leakage Indium-Gallium-Zinc-Oxide Thin-Film Transistor

Emerging nonvolatile memory with an oxide-semiconductor-based thin-film transistor (TFT) using indium-gallium-zinc-oxide (IGZO) was developed. The memory is called nonvolatile oxide-semiconductor random access memory (NOSRAM). The memory cell of the NOSRAM (NOSRAM cell) consists of an IGZO TFT for data writing, a normal Si-based p-channel metal-oxide-semiconductor (PMOS) for data reading, and a cell capacitor for storing charge and controlling the PMOS gate voltage. The IGZO TFT and the cell capacitor are formed over the PMOS. Owing to extremely low-leakage-current characteristics of the IGZO TFT, the charge stored in the 2-fF cell capacitor is maintained for a long time. This long data retention realized innovative nonvolatile memory. The NOSRAM cell fabricated with the 0.8-μ m process technology demonstrated an on/off ratio of 10⁷ and an endurance over 10¹² write cycles. In addition, NOSRAM with a memory capacity of 1 Mb was fabricated; the cell size was 12.32 μm² and the cell array size was 13.5 mm². The 1-Mb NOSRAM achieved basic operation at 4.5 V or less, write operation at 150 ns/page, read distribution of data “1” with 3σ = 0.10 V, and a data retention over 60 days at 85^°C.