Meter check of a transistor

Chapter 5.3 Meter check of a transistor

Lessons In Electric Circuits Volume III – Semiconductors Book
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Lessons In Electric Circuits Volume III – Semiconductors Book

  • 288CHAPTER 5. JUNCTION FIELD-EFFECT TRANSISTORSuniversal figure like 0.7 volts is for a silicon BJT’s base-emitter junction voltage.• REVIEW:• Field-effect transistors control the current between source and drain connections by avoltage applied between the gate and source. In a junctionfield-effect transistor (JFET),there is a PN junction between the gate and source which is normally reverse-biased forcontrol of source-drain current.• JFETs are normally-on (normally-saturated) devices. The application of a reverse-biasingvoltage between gate and source causes the depletion region of that junction to expand,thereby ”pinching off” the channel between source and drain through which the controlledcurrent travels.• It may be necessary to attach a ”bleed-off” resistor between gate and source to dischargethe stored charge built up across the junction’s natural capacitance when the controllingvoltage is removed. Otherwise, a charge may remain to keep the JFET in cutoff modeeven after the voltage source has been disconnected.5.3Meter check of a transistorTesting a JFET with a multimeter might seem to be a relatively easy task, seeing as how ithas only one PN junction to test: either measured between gate and source, or between gateand drain.COMAVVAAOFFCOMAVVAAOFF+-+-N-channel transistorgatedrainsourcephysical diagramP NgatedrainsourceBoth meters show non-continuity(high resistance) through gate-channel junction.