The physics of excess electron velocity in submicron-channel FET’s

A transport-equation model is used to examine the processes by which the velocity of an electron injected from a source contact into an n channel can overshoot the saturated drift velocity by a factor of 3 or more (depending upon the material). The results agree semiquantitatively with earlier Monte Carlo calculations and show that the excess electron velocity can persist for distances on the order of 1 μm, with obvious benefit to the transit time of the carrier (and hence the fT) in submicron-channel FET’s. Data for Ge, Si, and GaAs are given.