Compact MOSFET Models for VLSI Design

6

Quasistatic, Non-quasistatic, and Noise Models

6.1 Introduction

In Chapter 4 we discussed the operation of a MOS transistor under static bias conditions, where it was assumed that the terminal voltages remain constant with respect to time. We also discussed various modeling approaches to model the DC current–voltage characteristics. The effect of short channel and high field on DC characteristics was discussed in Chapter 5. For the DC bias conditions it was assumed that under normal operating conditions there was no flow of current through the gate and the substrate terminals.

The MOSFET, however, behaves differently under dynamic conditions where terminal voltages vary with time. To illustrate the dynamic operation we consider an n-channel MOSFET with a gate driven by a voltage pulse having a finite rise and fall time as shown in Figure 6.1(a). Initially, for t < t₁ we assume that the MOSFET is operating in the strong inversion with a drain current I_DS1. As the gate voltage increases with time for t < t₁ the inversion charge density also increases. The increase in the inversion charge in the channel can occur through an increase in the source current −I_S(t) and a decrease in the drain current I_D(t). It may be recalled that under static conditions the channel current is independent of the position y making −I_S = I_D = I_DS, which means the electrons supplied from the source are collected at the drain. But under transient conditions the requirement of build up of inversion charge ...