Depletion and pinchoff

Chapter Depletion and pinchoff

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

  • In electronic circuits, N-channel and P-channel devices can do the same kinds ofthings. The main difference is the polarity. An N-channel device can almost always bereplaced with a P-channel JFET, and the power-supply polarity reversed, and the cir-cuit will still work if the new device has the right specifications. Just as there are differ-ent kinds of bipolar transistors, there are various types of JFETs, each suited to aparticular application. Some JFETs work well as weak-signal amplifiers and oscillators;others are made for power amplification.Field-effect transistors have some advantages over bipolar devices. Perhaps themost important is that FETs are available that generate less internal noise than bipolartransistors. This makes them excellent for use in sensitive radio receivers at very highor ultra-high frequencies.Field-effect transistors have high input impedances. The gate controls the flow ofcharge carriers by means of an electric field, rather than via an electric current.Depletion and pinchoffEither the N-channel or the P-channel JFET works because the voltage at the gatecauses an electric field that interferes, more or less, with the flow of charge carriersalong the channel. A simplified drawing of the situation for an N-channel device isshown in Fig. 23-3. For a P-channel device, just interchange polarity (minus/plus) andsemiconductor types (N/P) in this discussion.As the drain voltage ED increases, so does the drain current ID, up to a certainlevel-off value. This is true as long as the gate voltage EG is constant, and is not too largenegatively.But as EG becomes increasingly negative (Fig. 23-3A), a depletion region (solidblack) begins to form in the channel. Charge carriers cannot flow in this region; theymust pass through a narrowed channel. The more negative EG becomes, the wider thedepletion region gets, as shown at B. Ultimately, if the gate becomes negative enough,418 The field-effect transistor23-2Simplified cross-sectional drawing of a P-channel JFET (at A) and itsschematic symbol (at B).