Effective on-current of MOSFETs for large-signal speed consideration

To gauge the speed capability of a MOSFET, the maximum saturation on-current is usually quoted as an important parameter. Although the effect of the linear characteristics has been known to be important, it is the purpose of this paper to provide an analytical and a quantitative answer, using a simple circuit analysis. The results show that the charging/discharging time in the linear region is not small, and consequently that the drain saturation voltage to reach the saturation current is a critical parameter. A new parameter "effective on-current" is proposed which captures the linear characteristics in addition to the saturation current, and should be more indicative of the large-signal speed capability in digital circuits. Finally, the impacts of series resistance are also examined.