Q. What do you mean by Pinch off voltage?
As the voltage Vds is increased from 0 to a few volts, the drain current will increase as determined by Ohm's law and the plot of Id versus Vds will appear as shown below. The relative straightness of the plot reveals that for a region of lower values of Vds, the resistance is essentially a constant. As Vds increases and reaches a level called Vp the depletion region widens causing reduction in the channel width, this reduction in channel width causes the resistance to increase and forces a curve in the graph. This level of voltage Vp is called pinch off voltage. Actually pinch off is the condition at which both the p parts touch thus blocking the current. But this doesn't really happen. The current Id maintains a saturation level referred to as Idss because of the existence of a very small channel with high density current. As Vds is increased after attaining Vp, the region of close encounter between the 2 deletion regions will increase, but the kevel of Id remains essentially the same. In essence therefore once Vds > Vp, the JFET has the characteristics of a current source.