Q.What is pinch off ? What is the value of drain current at pinch off ?

Pinch off is the limiting condition of gate control in a JFET, and the drain current has a definite value there. …

[!TLDR]

Pinch off is the condition where the depletion regions meet and close the channel at the pinch off voltage $V_P$, and the drain current at pinch off is zero.

As the reverse gate-to-source voltage of an n-channel JFET is increased, the depletion regions on the two gate-channel junctions grow and extend further into the channel, steadily narrowing the conducting path. When the reverse bias is increased sufficiently, a stage is reached at which the two depletion regions extend right across the channel and touch each other, completely blocking it. This condition is called pinch off, and the gate-to-source voltage at which it occurs is called the pinch off voltage …