Impact of Ionizing Radiation on the $\hbox {SiO}_{2}/ \hbox {SiC}$ Interface in 4H-SiC BJTs

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.