Fig2: A six-transistor CMOS SRAM cell
Other types of SRAM Cells:
The SRAM cells are categorized based on the type of load used in the elementary inverter of the flip-flop cell. There are commonly three types of SRAM memory cells:
1. 4T cell (four NMOS transistors plus two poly load resistors)
2. 6T cell (six transistors-four NMOS transistors plus two PMOS transistors)
3. TFT cell (four NMOS transistors plus two loads called TFTs)
4T (Four Transistor) Cell:
This design consists of four NMOS transistors plus two poly-load resistors. 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 inverters consist of a very high poly-silicon resistor. 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 DRAM cell.