Simple and accurate size calculation of MOS transistor

The letter describes an analytical solution to the problem of finding a correct size of a MOS transistor required to charge a load capacitance to a wanted voltage level within a given time. The resultant formula is extremely simple, yet very accurate. It could be utilised by an LSI designer, as well as in automatic design algorithms. The expression for the channel width over channel length ratio was derived from the second-order drain current equations for both regions, saturated as well as linear (not from simplified, first-order approximations).