The lowest conduction band of Si is often approximated by three pairs of equivalent parabolic valleys located near the X-points of the Brillouin zone. There are recent experimental indications that the effective mass depends on shear strain and the silicon film thickness. The parabolic band structure ignores these effects completely. By comparison with numerical pseudopotential calculations, we show that the recently evaluated two-band k . p model accurately describes the dependences of the valley shifts and the effective masses on the shear strain component. Furthermore, we demonstrate that the two-band model is valid in a larger portion of the Brillouin zone as compared to the parabolic approximation with strain dependent effective masses and can be successfully used to describe analytically the subband dispersions in ultra-thin Si films, with or without strain. In the latter case, the model provides an analytical expression for the thickness-dependent non-parabolicity parameter in the unprimed subbands. Finally, the low-field mobility with the dependence of the non-parabolicity parameter on the film thickness taken into account is compared with the mobility computed with the bulk value of the non-parabolicity parameter. (C) 2008 Elsevier Ltd. All rights reserved.