ZnO is a wide band gap semi-conductor that has attracted tremendous interest for its potential applications in optoelectronic, solar cell, gas detection... In this work, aluminium doped zinc oxide (ZnO:Al) films were deposited by RF magnetron sputtering on glass substrates with different RF power densities of 1.2, 2.5, 3.7 and 4.9 W/cm(2). We notice that the films grown at 1.2 W/cm(2) were very thin and their physical properties were not precisely determined. The electrical properties of ZnO films were investigated using the impedance spectroscopy technique in the frequency range from 5 Hz to 13 MHz. The impedance data, represented by Nyquist diagrams showed that the resistivity of the films changed during the first three months after deposition. The deposited films show good optical transmittance (over 80%) in the visible and near infrared spectra. The band gap is around 3 eV and decreases with the increasing of the RF power density (from 3.35 to 3.05 eV). The results of this study suggest that the variation of the RF power density used for deposition allow the control of the electrical and optical properties of the films.