Metal-insulator-semiconductor (MOS) structures with insulator layer thickness of 290 Angstrom were irradiated using a Co-60 (gamma-ray) source and relationships of electrical properties of irradiated MOS structures to process-induced surface defects have been investigated both before and after gamma-irradiation. The density of surface state distribution profiles of the sample Au/SnO2/n-Si (MOS) structures obtained from high-low frequency capacitance technique in depletion and weak inversion both before and after irradiation. The measurement capacitance and conductance are corrected for series resistance. Series resistance (Rs) of MOS structures were found both as function of voltage, frequency and radiation dose. The C(f)-V and G(f)-V curves have been found to be strongly influenced by the presence of a dominant radiation-induced defects. Results indicate interface-trap formation at high dose rates (irradiations) is reduced due to positive charge build-up in the semiconductor/insulator interfacial region (due to the trapping of holes) that reduces the flow rate of subsequent holes and protons from the bulk of the insulator to the Si/SnO2, interface. The series resistance decreases with increasing dose rate and frequency the radiation-induced flat-band voltage shift in 1V. Results indicate the radiation-induced threshold voltage shift (DeltaV(T)) strongly dependence on radiation dose rate and frequency. (C) 2003 Elsevier Ltd. All rights reserved.