The junction field effect transistor is one of the earliest silicon based transistors. The JFET is the closest to what was originally envisioned (a solid-state analog of the vacuum tube) when development of the transistor began. The first created bi-polar junction transistor of Brattain, Bardeen, and Schockley at Bell Labs, was actually closer to an accidental discovery than a planned event, for which a new theory of transport of charge carriers had to be developed.

A doubly-diffused JFET (simplified) is shown at the left in cross-section. The channel -- colored here in violet -- determines the type of JFET. If the device is doped so that the channel is p-type material the device is called a p-channel JFET. The channel is a pure resistance that depends upon the doping characteristics.

All junctions of the JFET operate in reverse-bias mode (i.e. if the device is an n-channel JFET then the drain is the most positive junction, followed by the source -- the gate then operates from 0 to a negative voltage with respect to the source). Having all junctions reverse biased means that only leakage currents flow, and are in the range of one to tens of microamps maximum).

The diagram at the right shows the depletion (i.e. absence of carrier) regions -- shown in yellow -- growing under an increasing reverse voltage from gate to source. The width of the channel is decreased and therefore the resistance that is inversely proportional to the channel width increases. The active region between the channel fully open and pinch-off is where signals can be amplified by modulating the gate to source voltage. This is a simplified diagram. In reality the depletion region tapers along the voltage gradient from drain to source.

The diagram on the left, then, is biased to what is called pinch-off. Where sufficient reverse gate to source voltage is applied to drive the depletion region to maximum (pinching off the channel) and causing the current flow to be reduced to a minimum.

The JFET, being a resistive device more like the pentode vacuum tube, makes a very good radio-frequency amplifier without the large output capacitances and tendency to oscillate of their bi-polar compatriots. The JFET also makes a very good switch and will switch signals in either direction (drain-to-source or source-to-drain). An unique application of the JFET is to use it as a variable voltage-controlled resistor. As long as discrete transistors are used the JFET will find applications for which only it is best suited.