Depletion And Pinchoff:

The JFET works as the voltage at the gate causes an electrical field which interferes, more or less, with the flow of charge carriers all along the channel. Since the drain voltage E_D rises, and hence, does the drain current I_D, up to a definite level-off value. This is correct as long as the gate voltage E_G is constant and is not so large. Since E_G rises (i.e., negatively in an n channel or positively in a p channel), a depletion area develops in the channel. The charge carriers cannot flow in the depletion area; therefore whenever there is such an area, they should pass via a narrowed channel. The bigger E_G becomes, the wider the depletion area gets, and the more constricted the channel becomes. When E_G is high sufficient, the depletion area totally obstructs the flow of charge carriers, and the channel can't conduct current at all. This condition is termed as pinchoff. It is similar to pressing down on a garden hose until the water cannot flow.