The field-effect transistor

Chapter The field-effect transistor

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • 23CHAPTERThe field-effect transistorBIPOLAR TRANSISTORS BEHAVE AS THEY DO BECAUSE CURRENT VARIATIONS AT one P-N junction produce larger current variations at another. You’ve seen a simpli-fied picture of how this happens, and how the effect can be exploited to get currentamplification.The bipolar transistor isn’t the only way that semiconductors can be combined toget amplification effects. The other major category of transistor, besides the bipolar de-vice, is the field-effect transistor or FET. There are two main types of FET: the junc-tion FET (JFET) and the metal-oxide FET (MOSFET).Principle of the JFETA JFET can have any of several different forms. They all work the same way: the cur-rent varies because of the effects of an electric field within the device.The workings inside a JFET can be likened to the control of water flow through agarden hose. Electrons or holes pass from the source (S) electrode to the drain (D).This results in a drain current, ID, that is generally the same as the source current,IS. This is analogous to the fact that the water comes out of a garden hose at the samerate it goes in (assuming that there aren’t any leaks in the hose).The rate of flow of charge carriers—that is, the current—depends on the voltage ata regulating electrode called the gate (G). Fluctuations in gate voltage, EG, causechanges in the current through the channel, IS or ID. Small fluctuations in the controlvoltage EG can cause large variations in the flow of charge carriers through the JFET.This translates into voltage amplification in electronic circuits.416Copyright © 2002, 1997, 1993 by The McGraw-Hill Companies. Click here for terms of use.