The FAST model

The low complexity and the accuracy that can be obtained by taking care of the phenomenon of carriers velocity saturation, which is dominant in sub-micron technologies, suggested the use of the classical charge-control analysis and the gradual-channel approximation (Hodges model), described in §3.1.1.

Estimation accuracy and low computational effort can be achieved by operating both on the waveforms of internal signals and on the topology considerations: in particular all the waveforms in the circuit are approximated with linear ramps.

By approximating the input waveform with a ramp, a strong simplification of the $I(V)$ equations is obtained. Figure 3.1 shows the output voltage of an inverter driven by a ramp input. It can be noticed that a ramp can properly approximate the output voltage variation, especially in the central phases of the commutation. The increasing error on the tail of the switching does not affect significatively the delay and power estimation.

Figure 3.1: Inverter voltages waveform

The voltage ramp approximation are described in §3.1.2.