High and low field transport properties of electrons confined to quasi
-two-dimensional sheets in semiconductor heterostructures are discusse
d within the context of their importance in the operation of field eff
ect transistors. Modulation-doped heterostructures, in which the two-d
imensional electron gas is in undoped material, have extremely large m
obilities; these mobilities are larger than what can actually be fully
used by the transistor. Doped-channel quantum well structures have lo
wer mobilities and velocities, but field effect transistors fabricated
from such structures operate very well in part because of the larger
electron concentration which is possible.