We propose that silicon-on-insulator (SOI) metal-oxide-semiconductor f
ield effect transistors (MOSEETs) with SOI films thinner than the inve
rsion layer of the bulk MOSFETs can provide higher inversion-layer mob
ility than the bulk Si MOSFETs, because of the significant modulation
of the subband structure due to the size effect of the ultrathin SOI f
ilms. In order to examine the effectiveness of this device structure o
n the mobility enhancement, a theoretical calculation of the inversion
-layer mobility at room temperature is performed. It is found that the
mobility of the SOI MOSFETs with a SOI thickness of around 3 nm can b
e higher than that of the bulk MOSFETs. This mobility enhancement is a
ttributed to an increase in the occupancy of the 2-fold valleys and th
e suppression of inter-valley phonon scattering, associated with the e
nergy splitting between the 2-fold and the 4-fold valleys due to the s
ize effect of the SOI films.