However, under the circumstance V_DS > V_GS - V_th , when the gate voltage relative to drain voltage is insufficient to form the channel (that is, V_GD< V_th ), the channel is terminated before the drain end. The channel is then said to be pinched off. This region of operation, known as saturated or pinch-off condition, is portrayed in Figure 2.5. The effective channel length is thus reduced as the inversion layer near the drain end vanishes. As the majority carriers (electrons) reach the end of the channel, they are swept to the drain by the drift action of the field due to the drain voltage. In the saturated state, the channel current is controlled by the gate voltage and is almost independent of the drain voltage.

Chapter 2 : Operating Principles of MOS Transistors
However, under the circumstance V_DS > V_GS - V_th , when the gate voltage relative to drain voltage is insufficient to form the channel (that is, V_GD< V_th ), the channel is terminated before the drain end. The channel is then said to be pinched off. This region of operation, known as saturated or pinch-off condition, is portrayed in Figure 2.5. The effective channel length is thus reduced as the inversion layer near the drain end vanishes. As the majority carriers (electrons) reach the end of the channel, they are swept to the drain by the drift action of the field due to the drain voltage. In the saturated state, the channel current is controlled by the gate voltage and is almost independent of the drain voltage.
In short, the nMOS transistor possesses the three following regions of operation : Cutoff, sub-threshold or non-conducting zone Non-saturation or linear zone Saturation region
Figure 2.5: An nMOS enhancement mode transistor in saturated (pinch-off) mode. Note that V_GS > V_th , and V_DS > V_GS - V_th .