Schottky diodes were fabricated by evaporation of Al on a strongly etched n-type Si surface for 3 min after mechanical cleaning. The measurements of one of the better working of the Al-nSi diodes has been carried out at room temperature. Two expressions were found for the ideality factor n by supposing that all the interface states at first are in equilibrium with the metal and then with the semiconductor. The diode showed non-ideal I-V behaviour with an ideality factor of 1.46. The density distribution of interface states was obtained from the forward bias I-V characteristics. Non-linearity or curvature in the reverse bias C-2-V plots was explained by a quantity called the "excess capacitance" C0 caused by the presence of the interface states. The excess capacitance was observed to decrease with increasing frequency: this behaviour was ascribed lo the fact that the apparent density of the interface states decreases with increasing frequency. In addition. the parameters obtained from C-V characteristics were corrected by means of a simple graphical method for excess capacitance suggested by Vasudev et al. and of a theoretical model of an MIS structure introduced by Fonash.