In this paper, we analyse the ionizing irradiation influence on the characteristic current-voltage (V < 0 and V > 0) of the metal/ultra-thin oxide/semiconductor structures where the oxide thickness varies from 40 to 125 Å. The results obtained show that this influence is significant on the current corresponding to the carrier injection by the semiconductor (V > 0). When the thickness of oxide is higher than 100 Å, the radiation creates a positive charge in the oxide and decreases the conduction parameters (K ₁ and K ₂). For oxide thicknesses lower than 100 Å, the radiation does not create any charge in the oxide but improves the conduction parameters. This is attributed to the beneficial effect of the radiation on the defects characterizing an ultra-thin oxide layer.