Our focus in the first five chapters has been primarily geared towards passive RF devices and their electric circuit behavior. In this chapter we are going to extend and broaden our scope to include an investigation of various active circuit elements. Of specific interest for the design of amplifiers, mixers, modulators and demodulators are solid state devices such as diodes and transistors. What complicates a unified treatment is the wealth of special purpose components developed and marketed by a range of companies for a wide host of industrial applications. We cannot adequately address the multitude of technological advances currently shaping the RF/MW commercial markets. This is not the intent of this text; rather we would like to emphasize a number of key concepts driving the technological RF/MW evolution. These concepts are utilized later for the design of amplifiers, matching networks and other circuits developed in subsequent chapters. Our approach is geared towards enabling the reader to formulate and develop his/her own network descriptions as part of an integrated strategy to construct suitable models of analog RF circuits.

Before developing appropriate network models for active devices, a short discussion of solid state physics involving pn and metal-semiconductor junctions is presented. The aim is to provide a solid state perspective of the electric circuit representations derived from the physical devices. This is needed because

• at high frequency modes of operation, additional capacitive and inductive effects enter the solid state devices and affect their performance,
• the high frequency behavior of many active devices markedly depart from those of low frequency components and therefore require special treatment,
• to utilize simulation tools such as SPICE, or more dedicated RF CAD programs, a working knowledge of the physical parameters must be obtained that directly or indirectly influence the circuit behavior.

By analyzing the pn-junction and the Schottky contact, we gain a more complete picture of electronic circuit functions which form the foundation of rectifier, amplifier, tuning, and switching systems. In particular, the metal-semiconductor interface is shown to be especially useful for high-frequency operations. It is the RF domain that has seen many specialized diode developments. Chief among them are the Schottky, PIN, and Tunnel diode to name but a few.

Next, our attention is turned toward the bipolar and field effect transistors which are more complex implementations of the previously investigated pn-junction or Schottky contact. In these sections we learn construction, functionality, temperature, and noise performance of the bipolar and the Gallium Arsenide metal semiconductor field effect transistors.