COMPARATIVE-STUDY OF PHONON-LIMITED MOBILITY OF 2-DIMENSIONAL ELECTRONS IN STRAINED AND UNSTRAINED SI METAL-OXIDE-SEMICONDUCTOR FIELD-EFFECT TRANSISTORS

The phonon-limited mobility of strained Si metal-oxide-semiconductor f ield-effect transistors (MOSFETs) fabricated on a SiGe substrate is in vestigated through theoretical calculations including two-dimensional quantization, and compared with the mobility of conventional (unstrain ed) Si MOSFETs. In order to match both the mobility of unstrained Si M OSFETs and the mobility enhancement in strained Si MOSFETs, it is nece ssary to increase the coupling of electrons in the two-dimensional gas with intervalley phonons, compared to the values used in conventional models. The mobility enhancement associated with strain in Si is attr ibuted to the following two factors: the suppression of intervalley ph onon scattering due to the strain-induced band splitting, and the decr ease in the occupancy of the fourfold valleys which exhibit a lower mo bility due to the stronger interaction with intervalley phonons. While the decrease in the averaged conductivity mass, caused by the decreas e in the occupancy of the fourfold valleys, contributes to the mobilit y enhancement in bulk strained Si, it is not necessarily adequate to e xplain the mobility enhancement for two-dimensional electrons in strai ned Si. This is suggested by the fact that the mobility limited by int ravalley acoustic phonon scattering, which is the dominant scattering mechanism, has almost the same value in the two- and the fourfold vall eys, because the difference in the conductivity mass is compensated by differences in the inversion-layer thickness and the valley degenerac y. (C) 1996 American Institute of Physics.