Junction gate field effect transistor (JFET or JUGFET) is the simplest type of field effect transistor. These are three-terminal semiconductor devices that can be used as electronically controlled switches, amplifiers or electrically controlled resistors.
The JFET operation can be compared to that of a garden pipe. The flow of water through a pipe can be controlled by crushing it to reduce the cross section and the electric charge flow through a JFET is controlled by forcing the current carrying channel. The current also depends on the electrical field between the source and the discharge (similar to the pressure difference on both ends of the tube).
Conduction of the conducting channel is performed using the field effect: a voltage between the gate and the source is applied to reverse the gate n-source pn junction, thereby widening the jumper layer invading channel conduction and limiting its cross-sectional area. The depletion layer is so called because it is depleted of mobile carriers and is therefore electrically non-conductive for practical purposes.
When the drainage layer extends along the width of the conduit channel, reduction is achieved and drain-to-source conduction is interrupted. Pinching occurs at a particular reverse polarization (VGS) of the gate-source junction. The switch-off voltage (Vp) varies considerably, even between devices of the same type. For example, VGS (off) for the Temic J202 device varies from -0.8 V to -4 V. The typical values range from -0.3 V to -10 V.
To turn off a channel device n requires a negative gate-source voltage (VGS). Conversely, to turn off a p-channel device, a positive VGS is required.
In normal operation, the electric field developed by the gate blocks to some extent the conduction of the source-drain. Some JFET devices are symmetrical with respect to origin and discharge.