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6
MOSFET Capacitance Models
6.1 Introduction
This chapter presents the dynamic compact MOSFET (metal-oxidesemiconductor field-effect transistor) models for analyzing the device perfor -
mance under time-varying terminal voltages in circuit operation. The MOSFET
device models developed in Chapters 4 and 5 are applicable to devices under
DC or steady-state biasing condition, that is, when the terminal voltages do
not vary with time. However, the real circuit operates under time-varying
terminal voltages. Under such biasing condition, the device behavior is
described by dynamic models. If the rate of change of terminal voltages is
sufficiently small, the device operation can be described by a small signal
dynamic model. On the other hand, if the rate of change of terminal voltages
is large, the device is represented by a large signal dynamic model. In dynamic
models, the device is represented by capacitors, resistors, current sources,
and so on. The dynamic MOSFET models are essential part of circuit CAD
(computer-aided design).
The dynamic operations of MOSFET devices are due to the capacitive
effects of the device, resulting from the stored charges in the device. Thus,
a capacitance model describing the intrinsic and extrinsic components
of the device capacitance is an essential part of a compact model for circuit simulation besides DC model. In most circuit simulators, the same
capacitance model is used for both the small signal AC analysis and the
large signal transient analysis. A capacitance model is always based on
quasistatic assumptions, that is, charge in a device can follow the varying
terminal voltage instantaneously without any delay. In this chapter, first
of all, a large signal dynamic model is described by developing models
for intrinsic charges and capacitances of a large geometry device (large
L and wide W). Then the models for short channel devices are discussed.
Finally, the small signal linear model parameters required for small signal
analysis are discussed.
6
MOSFET Capacitance Models
6.1 Introduction
This chapter presents the dynamic compact MOSFET (metal-oxidesemiconductor field-effect transistor) models for analyzing the device perfor -
mance under time-varying terminal voltages in circuit operation. The MOSFET
device models developed in Chapters 4 and 5 are applicable to devices under
DC or steady-state biasing condition, that is, when the terminal voltages do
not vary with time. However, the real circuit operates under time-varying
terminal voltages. Under such biasing condition, the device behavior is
described by dynamic models. If the rate of change of terminal voltages is
sufficiently small, the device operation can be described by a small signal
dynamic model. On the other hand, if the rate of change of terminal voltages
is large, the device is represented by a large signal dynamic model. In dynamic
models, the device is represented by capacitors, resistors, current sources,
and so on. The dynamic MOSFET models are essential part of circuit CAD
(computer-aided design).
The dynamic operations of MOSFET devices are due to the capacitive
effects of the device, resulting from the stored charges in the device. Thus,
a capacitance model describing the intrinsic and extrinsic components
of the device capacitance is an essential part of a compact model for circuit simulation besides DC model. In most circuit simulators, the same
capacitance model is used for both the small signal AC analysis and the
large signal transient analysis. A capacitance model is always based on
quasistatic assumptions, that is, charge in a device can follow the varying
terminal voltage instantaneously without any delay. In this chapter, first
of all, a large signal dynamic model is described by developing models
for intrinsic charges and capacitances of a large geometry device (large
L and wide W). Then the models for short channel devices are discussed.
Finally, the small signal linear model parameters required for small signal
analysis are discussed.