PHYSICAL UNDERSTANDING OF LOW-FIELD CARRIER MOBILITY IN SILICON MOSFET INVERSION LAYER

From both experimental and theoretical studies of the gate field dependences of the low-field mobilities of electrons and holes by changing surface orientations and oxidation conditions we found that two-dimensional electron gas formulation can successfully explain eta = 1/3 (eta is the weighting factor of mobile charge density in calculating the effective field for universal mobility curve) for (111) electrons and holes and eta = 1/2 for (100) electrons; the mobility limited by the phonon scattering has E(eff)-0.3 dependences both for electrons and holes; the mobility data on different crystal orientations for both carriers can indeed be explained by the Matthiessen's rule quantitatively, only when we consider valley repopulation; the mobility limited by the surface roughness has beta-E(eff)-2 dependences both for electrons and holes. The coefficients-beta are found to be independent of oxidation conditions and measurement temperature, but are very dependent on the crystal orientations only through the effective mass and the product of the correlation length L and the mean asperity height-DELTA. Surprisingly enough, the values of DELTA-L calculated from the mobility data for various orientations agree well with those calculated from the atomic distance by microscopic crystallography. Our observation has both theoretically and technologically important implications, because the only technology dependence of the mobility is from the ionized impurity scattering which dominates only at weak inversion.