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Bias-temperature instabilities (BTIs) are investigated for n- and p-substrate 4H-SiC metal–oxide–semiconductor (MOS) capacitors. The midgap voltage (V \rm mg) shifts positively under positive bias stress at high temperatures for n-substrate capacitors with 67.5-nm nitrided oxides and shifts negatively under negative bias for p-substrate capacitors with 55-nm nitrided oxides. The magnitudes of the Vmg shifts are less than 0.5 V for electric fields of magnitudes of approximately \pm3.1 MV/cm for up to one day of stress at 150 ^\circ\C or 20 min of stress at 300 ^\circ\C. Switched-bias stressing at 150 ^\circ\C causes partially reversible shifts for the n-substrate capacitors, while the p-substrate capacitors show monotonically increasing negative shifts. Based on the measured temperature dependence of the Vmg shifts, the effective activation energy for BTI that is measured between room temperature and 250 ^\circ\C is 0.12 \pm 0.02 eV for the n-substrate capacitors (positive shifts) and 0.23 \pm 0.02 eV for the p-substrate capacitors (negative shifts). The midgap voltage shifts in these wide-bandgap devices are caused by charge capture at deep interface traps and N-related defects at or near the \SiC-\SiO2 interface, which can be enhanced at elevated temperatures by the generation of additional carriers due to the ionization of deep dopants in the SiC during bias-temperature stress. [ABSTRACT FROM PUBLISHER]