Advanced SRAM technology-the race between 4T and 6T cells

This work discusses the trade-offs between 4T SRAM cells which use four bulk transistors (and have poly resistor or TFT loads) and 6T SRAM cells which use six bulk transistors (and use bulk PMOS loads). 4T SRAM cells have dominated the stand-alone SRAM market since first introduced in the 1970´s, but 6T SRAM cells have been dominant for on-chip storage in advanced microprocessors and other logic circuits. However, recently there has been a resurgence of interest in 6T cells for stand alone SRAM applications. While 4T cells are typically smaller, they generally require a more complex process, and have poorer stability, especially at low voltage. This paper quantitatively examines several different trade-offs in SRAM cell design.