Comparative study of phonon-limited mobility of two-dimensional electrons in strained and unstrained Si metal–oxide–semiconductor field-effect transistors

The phonon-limited mobility of strained Si metal–oxide–semiconductor field-effect transistors (MOSFETs) fabricated on a SiGe substrate is investigated through theoretical calculations including two-dimensional quantization, and compared with the mobility of conventional (unstrained) Si MOSFETs. In order to match both the mobility of unstrained Si MOSFETs and the mobility enhancement in strained Si MOSFETs, it is necessary 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 attributed to the following two factors: the suppression of intervalley phonon scattering due to the strain-induced band splitting, and the decrease in the occupancy of the fourfold valleys which exhibit a lower mobility due to the stronger interaction with intervalley phonons. While the decrease in the averaged conductivity mass, caused by the decrease in the occupancy of the fourfold valleys, contributes to the mobility enhancement in bulk strained Si, it is not necessarily adequate to explain the mobility enhancement for two-dimensional electrons in strained Si. This is suggested by the fact that the mobility limited by intravalley acoustic phonon scattering, which is the dominant scattering mechanism, has almost the same value in the two- and the fourfold valleys, because the difference in the conductivity mass is compensated by differences in the inversion-layer thickness and the valley degeneracy.