The subband structure of (100)Si inversion layers can be altered by either uniaxial stress or substrate bias due to deformation potential and quantizing surface field, respectively. The reduced splitting between E0 and E0 caused by compressive stress is shown to be exactly compensated by the additional surface field via substrate bias at high compressive stress. At low compressive stress (≲ 109 dyn cm-2) and low carrier concentration (< 1012 cm 2) we observe a marked decrease of electron mobility at 4.2 K. The decrease in mobility can be recovered by suitable negative substrate bias. We attribute these results to the influence of the density of state tails of E0 subband on the free electrons in the E0 subband. At high stress, E0 and E0 cross. The measured Ef crossing point from E0 to mixed subband at various stresses and depletion charges agrees well with Takada and Ando's calculation. The results for  and  stresses indicate further splitting of the E0 subband for the former. For (111) surfaces, the mobility increases with either eompressive or tensile stress in  direction. From the point of view of the influence of tail states of the higher subbands, this even function dependence of μ(P) seems to support 6-valley degeneracy in the absence of sttess. © 1982.