SiC technology is attractive for high power and high temperature applications due to its wide band gap (~3.26 eV for 4H-SiC). Bias-temperature instability (BTI) is a critical reliability issue for SiC devices and circuits, but only limited information is available. We have investigated bias-temperature-instabilities (BTIs) for 4HSiC based nMOSFETs before and after total ionizing dose irradiation (TID), including switching the bias polarity. We find that hole traps are more stable than electron traps in these wide band gap materials, and the BTI response for switched biases changes after total dose radiation. nMOSFETs were fabricated on an aluminum doped 4H-SiC epitaxial layer with a 55 nm, NO-nitrided gate oxide. Switched BTI experiments were performed under electrical fields of ± 3 MV/cm at 150 °C for stress times of 20 min. to 60 min. Total ionizing dose (TID) irradiation was performed with 10-keV X-rays at room temperature with a gate bias of 1.5 MV/cm. Drain current vs. gate voltage (ID-VG) measurements were performed at room temperature with a HP4156A parameter analyzer. The threshold voltage (VTH) shifts monotonically to more negative values with time under negative stress bias of −3MV/cm at 150 C, as shown in Fig.1. The magnitude of the midgap voltage increases monotonically at early stress times, with saturation at longer times. This saturation represents the filling of process and stressinduced interface and near-interfacial oxide (border) traps with holes during the negative-bias stress.