An (Au/Ti)/Al2O3/n-GaAs structure with thin (30 angstrom) interfacial oxide layer (Al2O3), formed by atomic layer deposition technique is fabricated to investigate both frequency and applied bias voltage dependences of real and imaginary parts of dielectric constant (epsilon' and epsilon '') and electric modulus (M' and M ''), loss tangent tan delta and ac electrical conductivity sigma(AC) in a wide frequency range from 1000 Hz to 1 MHz at room temperature. The dielectric properties of the (Au/Ti)/Al2O3/n-GaAs metal-insulator-semiconductor structure are obtained using the forward and reverse bias capacitance-voltage (C-V) and conductance-voltage (G/omega-V) measurements in the applied bias voltage range from -4 V to +4 V, at room temperature. Experimental results show that the dielectric parameters were strongly frequency and voltage dependent. For each frequency the (C - V) plots show a peak and the change in frequency has effect on both the intensity and position of the peak. epsilon', epsilon '' and tan delta decrease with increasing frequency, whereas sigma(AC) increases with increasing frequency at applied bias voltage. M' increases with the increasing frequency and reaches a maximum. M '' shows a peak and peak position shifts to higher frequency with increasing applied voltage. It can be concluded that the epsilon', epsilon '', tan delta, M', M '' and sigma(AC) values of the (Au/Ti)/Al2O3/n-GaAs structure are strongly dependent on both the frequency and applied bias voltage especially in the depletion and accumulation region. Also, the results can be deduced to imply that the interfacial polarization is easier at low frequencies, therefore contributing to the deviation of dielectric properties and AC electrical conductivity of (Au/Ti)/Al2O3/n-GaAs structure.