Q. In The Drain Characteristics Of FET What Does The Region At Pinch Off?

Field Effect Transistors (FETs) utilize a conductive channel whose resistance is controlled by an applied potential.

Junction Field Effect Transistor (JFET) : In JFETs a conducting channel is formed of n or p-type semiconductor (GaAs, Ge or Si).Connections are made to each end of the channel, the Drain and Source. In the implementation shown below a pair of gate electrodes of opposite doping with respect to the channel are placed at opposite sides of the channel. Applying a reverse bias forms a depletion region that reduces the cross section of the conducting channel. (from Sze) Changing the magnitude of the reverse bias on the gate modulates the cross section of the channel.

First assume that the drain voltage is 0. Increasing the reverse gate potential will increase the depletion width, i.e. reduce the cross section of the conducting channel, until the channel is completely depleted. The gate voltage where this obtains is the "pinch-off voltage" VP.

Now set both the gate and drain voltages to 0. The channel will be partially depleted due to the "built-in" junction voltage. Now apply a drain voltage. Since the drain is at a higher potential than the source, the effective depletion voltage increases in proximity to the drain, so the width of the depletion region will increase as it approaches the drain. If the sum of the gate and drain voltage is sufficient to fully deplete the channel, the device is said to be "pinched off".

Increasing the drain voltage beyond this point moves the pinch-off point towards to the source.