Chapter 4 A Brief Review of Technology

Chapter Chapter 4 A Brief Review of Technology

Radio Frequency Integrated Circuit Design Second Edition Book
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Radio Frequency Integrated Circuit Design Second Edition Book

  • 7575c h a p t e r 4A Brief Review of Technology4.1  IntroductionAt the heart of RF integrated circuits are the transistors used to build them. The basic function of a transistor is to provide gain. Unfortunately, transistors are never ideal because along with gain comes nonlinearity and noise. The nonlinearity is used to good effect in mixers and in the limiting function in oscillators. Transistors also have a maximum operating frequency beyond which they cannot produce gain. Metal oxide semiconductor (MOS) and bipolar transistors will be discussed in this chapter. CMOS is the technology of choice in any digital application because of its very low quiescent power dissipation and ease of device isolation. However, traditionally MOS field-effect transistors (MOSFETs) have had inferior speed and noise compared to bipolar transistors. Also, CMOS devices have proved challeng-ing to model for RF circuit simulation, and without good models, RFIC design can be a very frustrating experience. In order to design RFICs, it is necessary to have a good understanding of the high-speed operation of the transistors in the technology that is being used. Thus, in this chapter a basic introduction to some of the more important properties will be provided. For more detail on transistors, the interested reader should consult [1–10].4.2  Bipolar Transistor DescriptionFigure 4.1 shows a cross section of a basic npn bipolar transistor. The collector is formed by epitaxial growth in a p- substrate (the n- region). A p region inside the collector region forms the base region; then an n+ emitter region is formed inside the base region. The basic transistor action all takes place directly under the emit-ter in the region shown with an oval. This can be called the intrinsic transistor. The intrinsic transistor is connected through the diffusion regions to the external contacts labeled e, b, and c. More details on advanced bipolar structures, such as using SiGe heterojunction bipolar transistors (HBTs), and double-poly self-aligned processes can be found in the literature [1, 2]. Note that although Si is the most common substrate for bipolar transistors, it is not the only one; for example, GaAs HBTs are often used in the design of cellular radio power amplifiers and other high power amplifiers. Figure 4.2 shows the transistor symbol and biasing sources. When the tran-sistor is being used as an amplifying device, the base-emitter junction is forward biased while the collector-base junction is reverse biased, meaning the collector is