Bias Stress-Induced Threshold-Voltage Instability of SiC MOSFETs

We have observed instability in the threshold voltage, VT, of SiC metal-oxide semiconductor field-effect transistors (MOSFETs) due to gate-bias stressing. This effect has routinely been observed by us in all 4H and 6H SiC MOSFETs from three different manufacturers—even at room temperature. A positive-bias stress, applying an electric field of about 1 to 2 MV/cm across the gate oxide, for 3 minutes followed by a negative-bias stress for another 3 minutes typically results in a shift of the ID-VGS current-voltage characteristic in the range of 0.25 to 0.5 V and is repeatable. We speculate that this effect is due to the presence of a large number of near-interfacial oxide traps that presumably lie in the oxide transition region that extends several nm into the oxide from the SiC interface, caused by the presence of C and strained SiO2. This instability is consistent with charge tunneling in and out of these near-interfacial oxide traps, which in irradiated Si MOSFETs has been attributed to border traps. Also consistent with charge tunneling is the observed linear increase in the magnitude of the SiC VT instability with log (time).