7.1 Introduction

Active device modeling is one of the most important areas of microwave CAD. Most frequently-used approaches in today's circuit design are based on lumped equivalent circuits, an example of which is shown in Figure 7.1. A large variety of such equivalent circuit models have been developed in the past, because no single equivalent model can represent all kinds of transistor behaviors. The specific equivalent circuit structure in a model optimized for one type of device becomes a limitation of the model for other devices. With the rapid technology change, new types of semi-conductor devices are constantly evolving, and development of models to represent the new transistor behaviors is a continuous activity. Developing new equivalent circuit models requires human experience and judgment. Often a time-consuming trial-and-error process is used for formulating new equivalent circuit topology and for creating formulas fornonlinear elements. Recently, researchers have started investigating neural network approaches for representing transistor DC, small-signal, and large-signal behaviors. Neural networks have the potential of modeling new transistor behaviors through computerized training, thus possibly making...