Q.What is pinch off ?

Pinch-off describes what happens when the reverse-bias gate voltage is made large enough to close the channel of a JFET completely.

As the reverse gate-to-source voltage is increased, the depletion regions on the two sides of the channel widen and the conducting channel narrows. At a particular gate voltage the two depletion regions just touch each other in the middle of the channel, blocking it entirely so that carriers can no longer flow through. The drain current $I_D$ then falls to zero.

The gate-to-source voltage at which this happens is called the pinch-off voltage $V_P$. Its value depends on the doping level of the channel and on the channel width. For an n-channel JFET $V_P$ is a negative voltage (for example $-3$ V or $-6$ V in typical problems). …