Third Quadrant Conduction Loss of 1.2-10 kV SiC MOSFETs: Impact of Gate Bias Control

The third quadrant (3rd-quad) conduction of power MOSFETs involves competing current sharing between the metal-oxide-semiconductor (MOS) channel and the body diode controlled by the gate bias (V-G). For 1.2 kV SiC planar MOSFETs, it is well known that a positive V-G higher than the threshold voltage enables parallel conduction through both channels, which reduces the 3rd-quad voltage drop and conduction loss. This work, for the first time, unveils that this fact does not hold for higher voltage (e.g., 3.3 kV and 10 kV) SiC planar MOSFETs. By combining the static characterization, simulation, and modeling, it is revealed that, once the MOS channel turns on, the body diode in high-voltage MOSFETs turns on at a source-to-drain voltage (V-SD) much higher than the built-in potential of the PN junction. In 10 kV SiC MOSFETs, the body diode does not turn on over the entire practical V-SD range if the MOS channel is on. As a result, the positive V-G leads to completely unipolar conduction, which could induce a higher voltage drop than the bipolar body diode at high temperatures. A buck converter based on a 10 kV SiC MOSFET half-bridge module was built and tested, which validated that a negative V-G control provides the smallest 3rd-quad voltage drop and conduction loss at high temperatures. Finally, based on the revealed physics for planar MOSFETs, the optimal V-G control for the 3rd-quad conduction in trench MOSFETs is discussed. These results provide critical device understandings of 1.2-10 kV SiC MOSFETs and important application guidelines for 10 kV SiC MOSFETs.