![]() ![]() As such, it possesses a certain amount of capacitance capable of storing an electric charge potential. The depletion region within that junction acts as an insulating barrier separating gate from source. Why is this? Why doesn’t the JFET’s channel open up again and allow lamp current through like it did before with no voltage applied between gate and source? The answer lies in the operation of the reverse-biased gate-source junction. Opening the control switch again should disconnect the reverse-biasing DC voltage from the gate, thus allowing the transistor to turn back on. This is an advantageous trait of the JFET over the bipolar transistor: there is virtually zero power required of the controlling signal. ![]() As a voltage-controlled device, the JFET requires negligible input current. As a reverse-biased PN junction, it firmly opposes the flow of any electrons through it. Note that there is no current going through the gate. In order to turn the lamp off, we will need to connect another source of DC voltage between the gate and source connections of the JFET like this:Ĭlosing this switch will “pinch off” the JFET’s channel, thus forcing it into cutoff and turning the lamp off: With zero applied voltage between gate and source, the JFET’s channel will be “open,” allowing full current to the lamp. Now, all we need in the circuit is a way to control the JFET’s conduction. ![]() This is no accident, as the source and drain lines of a JFET are often interchangeable in practice! In other words, JFETs are usually able to handle channel current in either direction, from source to drain or from drain to source. Unlike the bipolar junction transistor where the emitter is clearly distinguished from the collector by the arrowhead, a JFET’s source and drain lines both run perpendicular into the bar representing the semiconductor channel. If you haven’t noticed by now, the source and drain connections on a JFET look identical on the schematic symbol. Remembering that the controlled current in a JFET flows between source and drain, we substitute the source and drain connections of a JFET for the two ends of the switch in the above circuit: Let’s begin our investigation of the JFET as a switch with our familiar switch/lamp circuit: Like its bipolar cousin, the field-effect transistor may be used as an on/off switch controlling electrical power to a load.
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