The average capacitor current method for delay calculation in MOS circuits

The transient response of metal-oxide-semiconductor (MOS) gates is a topic covered in most textbooks on digital integrated circuits and very-large-scale-integration (VLSI) design. One method often used to calculate first-order estimates of gate delays is the average capacitor current method. Using this method, the delay is calculated assuming that the capacitor current is constant and equal to the average of the capacitor current values at the limits of the time interval of interest. In this paper, this method is discussed and compared with other methods of delay calculation using integration and curve-fitting techniques familiar to electrical and computer engineering students. Since the computation of the capacitor current is relatively complicated because it requires the calculation of the MOS transistor currents, for propagation delay calculation there is no benefit in calculating the capacitor current twice. A single current calculation, corresponding to the familiar midpoint integration method, is sufficient to get the same or better accuracy as that of the average capacitor current method. The two-point Gauss quadrature formula is shown to provide excellent results with two capacitor current evaluations.