Ch 5a 2006 - 5.0 PREVIEW In the last two chapters we looked...

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5.0 PREVIEW In the last two chapters, we looked at BJT circuits. In this chapter, we introduce the second major type of transistor, the field-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. In JFETs, 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 amplifiers. Although the emphasis of this chapter is on dc circuits, we discuss how the FET can be used in switch, digital, and linear amplifier applications. A major goal of this chapter is to enable the reader to become very familiar and comfortable 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 MOSFET compared to a BJT, can be made very small. Since digital circuits can be designed using only MOSFETs with essentially no resistors or diodes, high-density VLSI circuits, including microprocessors and memories, can be fabricated. In a MOSFET the current is controlled by an electric field applied applied perpendicular to both the semiconductor surface and to the direction of current. The phenomenon used to modulate the conductance of a semiconductor, or control the current in a semiconductor, by applying an electric field perpendicular to the surface is called the field effect. Again, the basic transistor principle is that the voltage between two terminals controls the current through the third terminal. In the following two sections, we will discuss, the various types of MOSFETs, develop their I - V characteristics, and then consider the dc biasing of various MOSFET circuit configurations. 5.1.1 Two-Terminal MOS Structure The heart of the MOSFET is the metal-oxide-semiconductor capacitor shown in Figure 5.1. EE 329 Introduction to Electronics 226
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The metal may be aluminum or some other type of metal. In many cases, the metal is replaced by a high- conductivity polycrystalline silicon layer deposited on the oxide. However, the term metal is usually still used in referring to MOSFETs. In the above figure, the parameter t OX is the thickness of the oxide and ε OX is the oxide permittivity.
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