DEPLETION AND PINCHOFF

A JFET works because the voltage at the gate causes an electrical field that interferes, more or less, with the flow of charge carriers along the channel. As the drain voltage E_D increases, so does the drain current I_D, up to a certain level-off value. This is true as long as the gate voltage E_G is constant and is not too large. As E_G increases (negatively in an n channel or positively in a p channel), a depletion region develops within the channel.
Charge carriers cannot flow in the depletion region, so when there is such a region, they must pass through a narrowed channel. The larger E_G becomes, the wider the depletion region gets, and the more constricted the channel becomes. If E_G is high enough, the depletion region completely obstructs the flow of charge carriers, and the channel cannot conduct current at all. This condition is known as pinchoff. It is like pressing down on a garden hose until the water can’t flow.