In this paper, we show that through electrical characterization and detailed quantum simulations of the capacitance-voltage and current-voltage (I-V) characteristics, it is possible to extract a series of material parameters of alternative gate dielectrics. We have focused on HfO2 and HfSiXOYNZ gate stacks and have extracted information on the nature of localized states in the dielectric responsible for a trap-assisted tunneling (TAT) current component and for the temperature behaviour of the I-V characteristics. Simulations are based on a one-dimensional Poisson-Schrodinger solver capable to provide the pure tunneling current and TAT component. Energy and capture cross section of traps responsible for TAT current have been extracted.