Q. What is Pinch - off voltage?

 The reverse bias is relatively large near the source. As a result, the depletion region intrudes into the channel near the drain, and the effective channel area is constructed. Since the resistance of the constituent channel is higher, the I-V plot for the channel begins to depart from the straight line that was valid at low current levels. As the voltage V_D and current I_D are increased still further, the channel region near the drain becomes more constituted by the depletion region and the channel resistance continues to increase. As V_D is increased, there must be some bias voltage at which the depletion region meet near the drain and eventually pinch off the channel. When this happens, the current Id cannot increase significantly with further increase in V_D.