OVERVIEW

Before one can design a circuit to be integrated in CMOS technology, one must first have a model describing the behavior of all the components available for use in the design. A model can take the form of mathematical equations, circuit representations, or tables. Most of the modeling used in this text will focus on the active and passive devices discussed in the previous chapter as opposed to higher-level modeling such as macromodeling or behavioral modeling.

It should be stressed at the outset that a model is just that and no more it is not the real thing! In an ideal world, we would have a model that accurately describes the behavior of a device under all possible conditions. Realistically, we are happy to have a model that predicts simulated performance to within a few percent of measured performance. There is no clear agreement as to which model comes closest to meeting this "ideal" model [ ^[1]]. This lack of agreement is illustrated by the fact that, at this writing, HSPICE [ ^[2]] offers the user 43 different MOS transistor models from which to choose!

This text will concentrate on only three of these models. The simplest model, which is appropriate for hand calculations, was described in Section 2.3 and will be further developed here to include capacitance, noise, and ohmic resistance. In SPICE terminology, this simple model is called the LEVEL 1 model. Next, a small-signal model is derived from the LEVEL 1 large-signal model and is...