In the present work, we report on the evidence for a high negative charge density in aluminum oxinitride (AlON) coating on silicon. A comparative study was carried out on the composition and electrical properties of AlON and aluminum nitride (AlN). AlON films were deposited on p-type Si (1 0 0) substrate by RF magnetron sputtering using a mixture of argon and oxygen gases at substrate temperature of 300 degrees C. The electrical properties of the AlON, AlN films were studied through capacitance-voltage (C-P) characteristics of metal-insulator semiconductor (MIS) using the films as insulating layers. The flatband voltage shift V-FB observed for AlON is around 4.5 V, which is high as compared to the AlN thin film. Heat treatment caused the V-FB reduction to 3 V, but still the negative charge density was observed to be very high. In the AlN film, no fixed negative charge was observed at all. The XRD spectrum of AlON shows the major peaks of AlON (2 2 0) and AlN (0 0 2), located at 20 value of 32.96 degrees and 37.8 degrees, respectively. The atomic percentage of Al, N in AlN film was found to be 42.5% and 57.5%, respectively. Atomic percentages of Al, N and O in EDS of AlON film are 20.21%, 27.31% and 52.48%, respectively. (c) 2007 Elsevier Ltd. All rights reserved.