MOSFET devices with HfO2 gate oxides were irradiated with high-energy (120 MeV) Au ions. It was observed that the irradiation ca n cause both little or severe damage to the structures under test. We associate these effects with the soft and hard breakdown failure modes occurring in ultra-thin oxides. In both cases there is a large increment of the transistor off current. If the damage is not catastrophic a decrease of the transconductance and drain current characteristics can also be detected. This indicates that the transistor is still operational but with worst performance parameters.