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Degradation of SiO2 surface passivation for 4H-SiC power bipolar junction transistors (BJTs) as a result of ion irradiation has been studied to assess the radiation hardness of these devices. Fully functional BJTs with 2700 V breakdown voltage are implanted with 600 keV helium ions at fluences ranging from 1 ×1012 to 1 ×1016 cm-2 at room temperature. These ions are estimated to reach the SiO2/ SiC interface. The current-voltage characteristics before and after irradiation show that the current gain of the devices starts degrading after a helium fluence of 1×1014 cm-2 and decreases up to 20% for the highest fluence of ions. Simulations show that the helium ions induce ionization inside the SiO2, which increases the interface charge and leads to a degradation of the BJT performance. Thermal annealing of the irradiated devices at 300 °C, 420 °C, and 500 °C further increases the amount of charge at the interface, resulting in increased base current in the low-voltage range.