Understanding fingering of transistors

What is the need for fingering?? 
                                                      The problem is really with putting those cells or let us call instances on the rows which have a fixed height. So in that case if you would like to increase the the height of the transistor in order to have more drive strength (or to deliver more current to the load capacitance ) it is not possible. The solution would be to use the fingering technique.

Understanding fingering.
                                    Let us understand fingering by comparing it with a real life scenario. Let us say you have a tap which draws out water. How can you make the tap pull out more water?? It is by increasing the diameter of the tap. More is the diameter more is the current that goes out.

water coming out of small pipe

increasing the diameter of pipe causes more water to flow out 

                       Increasing the diameter of the tap is analogous to increasing the height or length of the transistor region so as to deliver more current.
                     Now i give a constraint that you cannot increase the diameter of the pipe to draw out more water. So what do you do to get more water???. Simple. Put more pipes in parallel so as to increase the water flow.

putting more pipes in parallel without increasing the diameter causes more water flow 

                    This is what exactly we are doing in fingering. We keep the transistor of fixed size so that the height remains constant but then we put more number of transistors in parallel so as to deliver more current to the load.
                       In this case we join the sources and drains in chained fashion so that they act like taps in parallel. In general when we say 2x and 4x drive strength buffers what exactly it means is that the it could deliver 8 times more current than the normal 1x buffer by employing 8 fingers or even more.

Drastic reduction in resistance
                       Another advantage of fingering is that resistance reduces drastically by fingering. Let us say you have a resistance of R from the given figure. Now when fingering is done, all these resistances come in parallel hence, the resistance reduces by a factor of N. This is another remarkable advantage of doing fingering,