The Au/Ti/n-GaAs structures with and without Al2O3 interfacial layer have been fabricated. The Al2O3 interfacial layer has been formed on the GaAs substrate by atomic layer deposition. The effects of the interfacial layer on the current-voltage (I-V) and capacitance-voltage (C-V) characteristics of the devices have been investigated in the temperature range of 60-300 K. It has been seen that the carrier concentration from C-V characteristics for the MIS (metal/insulating layer/semiconductor) diode with Al2O3 interfacial layer has a higher value than that for the reference diode without the Al2O3 interfacial layer (MS). Such a difference in the doping concentration has been attributed not to doping variation in the semiconductor bulk but to the presence of the Al2O3 interfacial layer because both diodes have been made on the pieces cut from the same n-type GaAs wafer. The temperature-dependent I-V characteristics of the MIS diode do not obey the thermionic emission current theory because of the presence of the Al2O3 layer. An electron tunneling factor, a delta(chi)(1/2), value of 20.64 has been found from the I-V-T data of the MIS diode. An average value of 0.627 eV for the mean tunneling barrier height, chi, presented by the Al2O3 layer has been obtained.