Phonon-limited inversion layer electron mobility in extremely thin (100) Si layers of silicon-on-insulator field-effect transistors has been studied at 300 K using a relaxation time approximation and a one-dimensional self-consistent calculation. For the Si layer thicknesstSiof more than approximately 5 nm, the mobility behavior as a function of an effective vertical electric field is found to be almost identical with that of bulk Si inversion layers. For a thickness of less than that, however, the mobility behavior is considerably affected by the change in the electronic structures due to a confinement effect. As the Si layer thickness decreases, the phonon-limited electron mobility&mgr;phincreases to a maximum attSiof∼3 nmand decreases monotonically. The increase in mobility results from the increase of the fraction of electrons in the lowest energy subband that has a higher mobility than other subbands. The mobility decrease in the extremely thintSiregion is attributed to the enhancement of phonon scattering rates caused by a reduction of the spatial widths of the subbands. ©1997 American Institute of Physics.