A 4-Mb CMOS SRAM with a PMOS thin-film-transistor load cell

A 4-Mb (512 K words by 8-b) CMOS static RAM (SRAM) with a PMOS thin-film transistor (TFT) has been developed. The RAM can obtain a much larger data-retention margin than a conventional high-resistive load-type well by using the PMOS TFT as a memory cell load. An internal voltage down-converter architecture with an external supply voltage-level sensor not only realizes a highly reliable 0.5- mu m MOS transistor operation but also a sufficiently low standby-power dissipation characteristic for data battery-backup application. A self-aligned equalized level sensing scheme can minimize the sensing delay for a local sense amplifier to drive a large load capacitance of a global sensing bus line. The RAM is fabricated using a 0.5 mu m, triple-poly, and double-aluminum with dual gate-oxide-thickness CMOS process technology. The RAM operates under a single 5-V supply voltage with 23-ns typical address access time and 20- and 70-mA operation current at 10 and 40 MHz, respectively.>