Large-signal transient behavior of metal-oxide-semiconductor transistors (MOST)

The plane of the drain characteristics of the MOST is divided into three regions: cut-off. (I), conduction beyond (II) and below (III) pinch-off. The large-signal operation of the device is represented by a non-linear equivalent circuit, derived from C. T. Sah´s small-signal equivalent circuit, where the current source is modified in order to describe the drain characteristics of the device in the three regions. A basic inverter circuit is used for switching analysis. The amplitude of the voltage step applied to the input is sufficient to drive the MOST from region I to II or III and vice versa. Both the turn-on and turn-off times are expressed as sums of two terms: a delay and a switching time. The delay is due to the gate to drain capacitance which causes a fraction of the input step to appear at the output: the delay is defined as the time required for the output voltage to recover its initial value. The switching times are defined at the 90% at the output voltage excursion. While the turn-off transient lends itself to a very simple analysis, the turn-on transient may consist of two parts (in case region III is reached): a constant current transient (region II) followed by a transient (region III) where a square-law current -- voltage relationship applies. The analysis leads to rather complex formulae: a normalized diagram is supplied for quick calculations. Experimental results are shown. Finally the intrinsic parameters and the external variables affecting the switching behavior of the device are summarized.