Ultrathin oxide layers, 2-5 nm thick, have been grown on (100) n-Si by Rapid Thermal Oxidation (RTO) at 900 degrees C. RTO is an effective method to control the oxide thickness in this range to within 10%. The direct tunnelling through these ultrathin layers is examined with current-voltage and impedance measurements on Al/SiO2/n-Si structures with an oxide layer thickness between 2 and 4 nm. After the determination of the surface potential vs bias relation and the oxide layer capacitance from the capacitance-voltage measurements, a quantitative analysis of the current-voltage characteristic based on electron tunnelling from a degenerate accumulation layer through the SiO2 barrier into the metal is made. A very good agreement with the theory is obtained assuming a simple trapezoidal tunnel barrier for the SiO2, from which the tunnel barrier height and the electron effective mass in the SiO2 bandgap are derived. The density of interface traps at the Si/SiO2 interface is determined using the conductance method. Only a very small increase of interface trap density with decreasing oxide layer thickness is found. The very high density of interface traps (more than 3 x 10(12) cm(-2) eV(-1)) can be reduced to the 10(10) cm(-2) eV(-1) level by application of a conventional Post Metallization Anneal (PMA).