MOBILITY ENHANCEMENT OF SOI MOSFETS DUE TO SUBBAND MODULATION IN ULTRATHIN SOI FILMS

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.