The importance of 2-dimensional (2D) features of carriers in Si MOSFETs on the device performance is re-examined experimentally and theoretically from the viewpoint of low-field mobility, velocity in high tangential fields and the inversion-layer capacitance. It is confirmed that low-field mobility and inversion-layer capacitance can be understood well in terms of the 2D subbands and the 2D carrier transport. In order to obtain fully-quantitative understanding of low-field mobility, however, it is still necessary to more accurately determine the amount of the scattering parameters in the inversion layer. On the other hand, saturation velocity is considered to be less influenced by the 2D quantization, while it is found experimentally that saturation velocity is slightly dependent on surface carrier concentration.According to the knowledge of 2-dimensional carrier transport in Si inversion layer, an effective way to have higher current drive is to increase the occupancy of the 2-fold valleys, which have lower conductivity mass, on a (100) surface. From this viewpoint, two device structures, strained Si MOSFETs and SOI MOSFETs with ultra-thin SOI films, are introduced and the behavior of low-field mobility is analyzed through the calculations of the subband structures and phonon-limited mobility.