Alloying elemental high-k metal oxides (such as HfO2) with other metals is seen as an effective method of controlling the properties of the dielectric based on the concentration of cations in the mixture; in particular, mixing HfO2 with Al2O3, and forming hafnium aluminate layers which will still have a relatively high dielectric constant (typically k similar to 15) and remain amorphous up to high processing temperatures. This paper summarizes the results of physical and electrical characterisation of hafnium aluminate (HfAlxOy) films prepared by Metal-Organic Chemical Vapour Deposition. We show how, using ultraviolet-visible, single angle ellipsometry, the thickness and composition of the deposited and of the transition/interfacial layers can be extracted, and further used for the estimation of the relative dielectric constant. Moreover, a methodology for extracting the band gap of these materials and its dependence on the aluminium concentration is presented. This has been achieved by using a simple parameterization model (Wemple-Di Domenico) to account for the optical dispersion of the films. Preparing thin films with a relatively high dielectric constant and with an amorphous structure even at high processing temperatures, are not the only requirements to be achieved when such layers are to be used as gate dielectrics. The electrical characteristics - such as leakage current, density of interface states, fixed charge in the oxide - are extremely important. The results obtained through capacitance-voltage and current-voltage measurements show the possibility of adjusting the relative dielectric constant of the layers in a wide range (9-16), when the aluminium, concentration varies between 4% and 38%. The minimum leakage current occurs for Al concentrations up to 9%. The thinner films show Fowler-Nordheim conduction even at higher concentrations of Al into the film, while thicker films show a higher hysteresis due to an increased number of slow trapping centres in the film. (c) 2006 Elsevier B.V. All rights reserved.