Chapter 6: Active RF Components
Our focus in the first five chapters has been primarily geared toward passive RF devices and their electric circuit behavior. In this chapter, we extend and broaden our scope to include an investigation of various active circuit elements. Of specific interest for the design of amplifiers, mixers, and oscillators 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 emphasize a number of key concepts driving the technological RF/MW evolution. These concepts are utilized later for the design of amplifiers, mixers, oscillators, and other circuits developed in subsequent chapters. Our goal is to enable the reader to formulate and develop his or 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 device level. This is needed because
- at high-frequencies additional capacitive and inductive effects enter the solid-state devices and affect their performance
- the high-frequency behavior of many active devices markedly departs from that of low-frequency components and therefore requires special treatment
- to utilize simulation tools such as SPICE or more specialized RF CAD programs, a working knowledge of the physical parameters that directly or indirectly influence the circuit behavior must be obtained.
Chapter 6 provides a concise summary of the most important semiconductor fundamentals that are encountered at high frequencies. By analyzing the pn-junction and the Schottky contact, we gain a more complete picture of electronic circuit functions that 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 operation. The RF industry 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 and Schottky contact. We learn about the construction, functionality, and temperature as well as noise performance of the bipolar and the metal-semiconductor field effect transistors.