Abstract

A novel approach to small-signal MOSFET modeling is presented in this book. As a result, time- and frequency-domain physics-based quasi-2D NQS four-terminal small-signal MOSFET models are proposed. The time-domain model provides the background to a novel DIBL-included quasi‑2D NQS four-terminal frequency-domain small-signal MOSFET model. Parameters and electrical quantities of the frequency-domain model are described by explicit functions. The models take into account: the velocity saturation effect of carriers in the channel, the dependence of the mobility on the electric field, the electrical coupling between the perturbed charge in the channel and the gate and the body, local variations in the channel thickness, and the DIBL effect. Derivation of the models is based on an analysis of a current density vector field and the following newly introduced phenomena: gradual channel detachment effect (GCDE), channel thickness modulation effect CTME), and channel-lengthening effect (CLE). A set of partial differential equations for the new physics-based small-signal MOSFET models is derived. The set consists of a quasi-2D small-signal continuity equation, a quasi-2D small-signal Poisson’s equation, and a quasi-2D small-signal transport equation. All the equations give a mathematical description of the behavior of the carriers in the channel and charges in the gate and the body. A set of supplementary equations for coupling and non-capacitive displacement currents in the MOSFET under dynamic operation is also derived. Based on the quasi‑2D dc MOSFET representation, a useful formula for the gate-to-body capacitance Cgb is derived, and some rules dealing with channel-to-gate and channel-to-body coupling currents are established. Only reciprocal capacitances are present in these models. The quasi-2D approach to the MOSFET modeling shows that two types of waves can propagate from the source to the drain, i.e., a longitudinal wave of disturbance in the carrier density and a transverse wave of disturbance in the channel thickness. It is shown that the magnitudes of both gate and body transadmittances are decreasing functions of frequency. The new frequency-domain small-signal MOSFET model has been successfully verified experimentally up to 30 GHz. The model parameters have realistic values. The new model is valid from zero Hz to well above the cut-off frequency fT. Each model parameter describes some physical phenomenon.

Authors (3)