Study on physical models for understanding the properties of strained-Si MOS Transistors

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The effects of the channel strain on the electrical characteristics of MOSFETs have not been fully understood yet in spite of the importance. Thus, the establishment of the physical models to describe the electrical properties is of paramount importance. In this study, following issues have been clarified by the analyses using bi-axial tensile strain Si MOSFETs.* It has been found that inversion-layer electron and hole mobility do hold the universality against the effective field with the value of η of 1/2 and 1/3, respectively, as similar with unstrained-Si n-and p-MOSFETs.* It has been found that gate current of n-MOSFETs under the positive bias, attributed to tunneling of electrons from the inversion layers, decreases with an increase in bi-axial tensile strain. We have demonstrated that this tunnel current reduction is quantitatively explained by the increase in the barrier height between Si and SiO2, caused by the lowering of the conduction band edge due to the tensile strain.* It has been found that the density and the energy distribution of interface states, generated by FN stress injection, do not change with applying tensile strain to channels, while the threshold voltage shift of n-MOSFETs after the stressing does change with strain because of the reduction in the band bending under the inversion condition in strained-Si nMOSFETs.* While electron mobility limited by Coulomb scattering due to substrate impurities in Si n-MOSFETs increases with an increase in tensile strain, the electron mobility limited by Coulomb scattering due to interface charges does decrease. These results are explained by the reduction in the averaged conductivity mass and the increase in the scattering probability with interface Coulomb scattering centers, both of which are provided by the increase in the occupancy of the 2-fold valley electrons due to applying tensile strain.