Question

difference between velocity saturation and channel pinchoff

Answer 1

Pinch-off saturation

When the Drain-Source voltage reaches a particular value, the induced charge near the drain becomes zero and the channel(shown in black) gets pinched-off. The voltage difference between the pinch-off point and the source remains constant even if VDS

V

D

S

is increased. Thus, the current flow IDS

I

D

S

saturates and the transistor is said to be in saturation.

AVelocity Saturation

This phenomenon also results in saturation of the current IDS

I

D

S

, but occurs mostly in short-channel MOSFETs (the channel length being smaller than in the previous device).

The velocity of carriers are supposed to vary linearly with the applied electric field as the mobility is considered to be a constant parameter. However, in a short channel, due to excessive collisions suffered by the carriers, their velocity saturates after a critical electric field.

Since current is the rate of flow of electrons, it also attains a saturated value once the velocity saturates.