Drive strength of a cell is the measure of the ability to drive the load connected to it. More the drive strength more is the current carrying capability of the cell and more easy it is for the cell to sustain a larger load at output than one with a lower drive strength.
Standard cells used in design may be according to different drive strengths of the cells available in the input library. For example AND1X, AND2X, AND3X, INV1X, INV2X, INV3X and so on are a few standard cells found in the library file. Likewise different filler cells or ECO cells may be classified based on different drive strengths.
Now what does drive strength of a cell depend upon and how cells can be differentiated based on this. Well, drive strength depends on the ratio (w/l) of the transistor. Deciding on w/l ratio comes into sizing concepts for transistors. This can be manipulated during the standard cell design by increasing the number of fingers or the multiplication factor in the transistors. Smaller cells have a lower drive strength which is a result of more delay in cells. This is because the oxide thickness (tox i.e, w in the ratio expression above) is inversely related to the threshold voltage of the cell and smaller the w the higher is the threshold voltage VT which would cause a higher transition time in the signal passing through it. The reverse happens for the cells with higher w whereby the VT is less and the transition time or slew introduced to the signal passing through it is less.
The images below show the schematic and layout of buffers having different drive strengths. Fig1 shows the cell BUFF2X which has lesser area and lesser drive that can be seen prominently while fig2 shows the same for other cell BUFF8X which has a much higher size and drive than BUFF2X
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| Fig 1: Schematic and layout of BUFFX2 |
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| Fig 2: Schematic and layout of BUFFX8 |
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