Figure shows the operation of a JFET.
The JUGFET has a physical structure that can be represented by the diagram shown in Figure. In Figure, if the N-Type material is connected to a power source, current will flow through it. It can be seen that the power source could be connected either way, the source and drain being interchangeable. The current flow consists of electrons moving through the N-Type semiconductor.
In Figure, a negative potential is applied to the gate regions. The junction between the P-Type and N-Type regions forms a reversed-biased diode, so no current flows. However, an electric filed extends into the N-Type bar from the P-Type regions. This charge forces current carriers (electrons) away from the region, reducing the amount of the N-Type material for conducting between the source and the drain.
If the potential applied to the gate is made sufficiently negative, the electric field will extend across the whole thickness of the N-Type material. Hardly any charge carriers will be available for current flow, and the current available from the drain will drop to a very low value (never to zero, for it is physically impossible for the channel to close completely).
Changes in the voltage applied to the gate will cause corresponding changes in the current flowing between the source and the drain, which makes the operation of the FET very similar to a bipolar transistor.