Chapter 5 The Field-Effect Transistor - MHHE.com
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C H A P T E R 5 The Field-Effect Transistor 5.0 PREVIEW In the last two chapters, we looked at the bipolar junction transistor and BJT circuits. In this chapter, we introduce the second major type of transistor, the ®eld-effect transistor (FET). There are two general classes of FETs: the metal- oxide-semiconductor FET (MOSFET) and the junction FET (JFET). The MOSFET has led to the second electronics revolution in the 1970s and 1980s, in which the microprocessor has made possible powerful desktop computers and sophisticated hand-held calculators. The MOSFET can be made very small, so high-density VLSI circuits and high-density memories are possible. We begin the chapter with a look at the physical structure and operation of the MOSFET. The current±voltage characteristics of the device are developed, and then the dc analysis and design of MOSFET circuits are considered. We will see how the MOSFET can be used in place of resistors in a circuit, so that circuits containing only MOSFETs can be designed. In the junction FET, the junction may be a pn junction, which forms a pn JFET, or a Schottky barrier junction, which forms a metal-semiconductor FET, or MESFET. MESFETs are used in very high speed or high-frequency applications, such as microwave ampli®ers. Since JFETs are specialized devices, the discussion of JFET circuits is brief. Although the emphasis of this chapter is on dc circuits, we discuss how the FET can be used in switch, digital, and linear ampli®er applications. A major goal of this chapter is to enable the reader to become very familiar and com- fortable with the MOSFET properties and to be able to quickly analyze and design the dc response of FET circuits. 5.1 MOS FIELD-EFFECT TRANSISTOR The metal-oxide-semiconductor ®eld-effect transistor (MOSFET) became a prac- tical reality in the 1970s. The MOSFET, compared to BJTs, can be made very small (that is, it occupies a very small area on an IC chip). Since digital circuits can be designed using only MOSFETs, with essentially no resistors or diodes required, high-density VLSI circuits, including microprocessors and memories, can be fabricated. The MOSFET has made possible the hand-held calculator and 243 C H A P T E R 5 The Field-Effect Transistor 5.0 PREVIEW 5.1 MOS FIELD-EFFECT TRANSISTOR 243
