Depletion mode versus enhancement mode

Chapter Depletion mode versus enhancement mode

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

  • Depletion mode versus enhancement modeThe JFET works by varying the width of the channel. Normally the channel is wideopen; as the depletion region gets wider and wider, choking off the channel, the chargecarriers are forced to pass through a narrower and narrower path. This is known as thedepletion mode of operation for a field-effect transistor.A MOSFET can also be made to work in the depletion mode. The drawings andschematic symbols of Figs. 23-7 and 23-8 show depletion-mode MOSFETs.However, MOS technology also allows an entirely different means of operation. Anenhancement-mode MOSFET normally has a pinched-off channel. It is necessary toapply a bias voltage, EG, to the gate so that a channel will form. If EG = 0 in such a MOS-FET, that is, if the device is at zero bias, the drain current ID is zero when there is no sig-nal input.The schematic symbols for N-channel and P-channel enhancement-mode devicesare shown in Fig. 23-10. The vertical line is broken. This is how you can recognize an en-hancement-mode device in circuit diagrams.Common source circuit42523-10Schematic symbols for enhancement-mode MOSFETs. At A,N-channel; at B, P-channel.Common-source circuitThere are three different circuit hookups for FETs, just as there are for bipolar tran-sistors. These three arrangements have the source, the gate, or the drain at signalground.The common-source circuit places the source at signal ground. The input is at thebase. The general configuration is shown in Fig. 23-11. An N-channel JFET is used here,