The transistor as a switch

Chapter 5.2 The transistor as a switch

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

  • 5.2. THE TRANSISTOR AS A SWITCH285With no voltage applied between gate and source, the channel is a wide-open path for elec-trons to flow. However, if a voltage is applied between gate and source of such polarity that itreverse-biases the PN junction, the flow between source and drain connections becomes lim-ited, or regulated, just as it was for bipolar transistors with a set amount of base current.Maximum gate-source voltage ”pinches off” all current through source and drain, thus forcingthe JFET into cutoff mode. This behavior is due to the depletion region of the PN junctionexpanding under the influence of a reverse-bias voltage, eventually occupying the entire widthof the channel if the voltage is great enough. This action may be likened to reducing the flow ofa liquid through a flexible hose by squeezing it: with enough force, the hose will be constrictedenough to completely block the flow.waterhosenozzleHose constricted by squeezing,water flow reduced or stoppedwaterNote how this operational behavior is exactly opposite of the bipolar junction transistor.Bipolar transistors are normally-offdevices: no current through the base, no current throughthe collector or the emitter. JFETs, on the other hand, are normally-ondevices: no voltageapplied to the gate allows maximum current through the source and drain. Also take notethat the amount of current allowed through a JFET is determined by a voltagesignal ratherthan a currentsignal as with bipolar transistors. In fact, with the gate-source PN junctionreverse-biased, there should be nearly zero current through the gate connection. For thisreason, we classify the JFET as a voltage-controlled device, and the bipolar transistor as acurrent-controlled device.If the gate-source PN junction is forward-biased with a small voltage, the JFET channelwill ”open” a little more to allow greater currents through. However, the PN junction of aJFET is not built to handle any substantial current itself, and thus it is not recommended toforward-bias the junction under any circumstances.This is a very condensed overview of JFET operation. In the next section, we’ll explore theuse of the JFET as a switching device.5.2The transistor as a switchLike its bipolar cousin, the field-effect transistor may be used as an on/off switch controllingelectrical power to a load. Let’s begin our investigation of the JFET as a switch with ourfamiliar switch/lamp circuit: