Velocity saturation

The velocity saturation (english velocity saturation ) designates one of the short-channel effects in field-effect transistors ( FETs).

At high drain voltages or smaller channel length () is very large, the longitudinal electric field in the channel. After the drift velocity of the charge carriers should increase steadily. However, the charge carrier mobility is decreased at high speeds. This hot carrier induced mobility degradation due to a scattering of charge carriers at the crystal lattice to produce optical phonons. Finally, the speed does not increase with the field strength, it saturates.

As a consequence, the drain current in the transistor increases less strongly than one would expect, namely only due to increasing carrier concentration. This manifests itself, for example in the transfer characteristic of the transistor, which instead of a quadratic curve exhibits only a linear increase in current. The transconductance is thus independent of the current, which is usually very unfavorable.

This effect is particularly relevant in DSM technologies (DSM, Eng. Deep submicron ) due to the very short transistors.