Using quantum mechanical methods that include the full-band structure of Si and SiO2, we study two non-classical phenomena that occur in MOS transistors at the nanometer-scale: tunneling through ultrathin oxides and quantum confinement in Si layers. SiO2 models based on beta-cristobalite, beta-quartz and tridymite polymorphs have been implemented for the calculation of tunneling current. Limitations of the effective-mass approximation are investigated. In particular, we obtain good agreement between calculated and measured tunneling current densities for a n-poly-Si/SiO2/p-Si MOS capacitor and predict the full-band structure in the channels of bulk and double-gate MOSFETs. (C) 2003 Elsevier Ltd. All rights reserved.