The most common SRAM cell consists of four NMOS transistors plus two poly-load resistors
(Figure 8-6). This design is called the 4T cell SRAM. Two NMOS transistors are pass-transistors.
These transistors have their gates tied to the word line and connect the cell to the columns. The
two other NMOS transistors are the pull-downs of the flip-flop inverters. The loads of the invert-
ers consist of a very high polysilicon resistor.
This design is the most popular because of its size compared to a 6T cell. The cell needs room only
for the four NMOS transistors. The poly loads are stacked above these transistors. Although the
4T SRAM cell may be smaller than the 6T cell, it is still about four times as large as the cell of a
comparable generation DRAM cell.