Hole injection into silicon dioxide (SiO₂) films (8–40 nm thick) is investigated for the first time during substrate electron injection via Fowler–Nordheim (FN) tunneling in n-type 4H- and 6H–SiC (silicon carbide) based metal–oxide–semiconductor (MOS) structures at a wide range of temperatures (T) between 298 and 598 K and oxide electric fields _Eox${}_{\mathrm{}}$ from 6 to 10 MV/cm. Holes are generated in heavily doped n  -type polycrystalline silicon (n⁺${}^{}$-polySi) gate serving as the anode as well as in the bulk silicon dioxide (SiO₂) film via hot-electron initiated band-to-band ionization (BTBI). In absence of oxide trapped charges, it is shown that at a given temperature, the hole injection rates from either of the above two mechanisms are higher in n-4H–SiC MOS devices than those in n-6H–SiC MOS structures when compared at a given _Eox${}_{\mathrm{}}$ and SiO₂ thickness (_tox${}_{\mathrm{}}$). On the other hand, relative to n-4H–SiC devices, n-6H–SiC structures exhibit higher hole injection rates for a given _tox${}_{\mathrm{}}$ during substrate electron injection at a given FN current density _je,_FN${}_{\mathrm{}\mathrm{}}$ throughout the temperature range studied here. These two observations clearly reveal that the substrate material (n-6H–SiC and n  -4H–SiC) dependencies on time-to-breakdown (_tBD${}_{\mathrm{}}$) or injected charge (electron) to breakdown (_QBD${}_{\mathrm{}}$) of the SiO₂ film depend on the mode of FN injections (constant field/voltage and current) from the substrate which is further verified from the rigorous device simulation as well.