Third Quadrant Operation of 1.2-10 kV SiC Power MOSFETs

The third quadrant (3rd-quad) conduction (or reverse conduction) of power transistors is critical for synchronous power converters. For power metal-oxide-semiconductor field-effect-transistors (MOSFETs), there are two current paths in the 3rd-quad conduction, namely the MOS channel path and the body diode path. It is well known that, for 1.2 kV silicon carbide (SiC) planar MOSFETs, the conduction loss in the 3rd-quad is reduced by turning on the MOS channel with a positive gate bias (VGS) and keeping the dead time as small as possible. Under this scenario, the current is conducted through both paths, allowing the device to take advantage of the zero 3rd-quad forward voltage drop (VF3rd) of the MOS channel path and the small differential resistance of the body diode path.

However, in this thesis work, this popular belief is found to be invalid for power MOSFETs with higher voltage ratings (e.g., 3.3 kV and 10 kV), particularly at high temperatures and current levels. The aforementioned MOS channel and body diode paths compete in the device’s 3rd-quad conduction, and their competition is affected by VGS and device structure.

This thesis work presents a comparative study on the 3rd-quad behavior of 1.2 kV to 10 kV SiC planar MOSFET through a combination of device characterization, TCAD simulation and analytical modeling. It is revealed that, once the MOS channel turns on, it changes the potential distribution within the device, which further makes the body diode turn on at a source-to-drain voltage (VSD) much higher than the built-in potential of the pn junction. In 10 kV SiC MOSFETs, with the MOS channel on, the body diode does not turn on over the entire practical VSD range. As a result, the positive VGS leads to a completely unipolar conduction via the MOS channel, which could induce a higher VF3rd than the bipolar body diode at high temperatures. Circuit test is performed, which validates that a negative VGS control provides the smallest 3rd-quad voltage drop and conduction loss at high temperatures in 10 kV SiC planar MOSFET. The study is also extended to the trench MOSFET, another major structure of commercial SiC MOSFETs. Based on the revealed physics for planar MOSFETs, the optimal VGS control for the 3rd-quad conduction in different types of commercial trench MOSFETs is discussed, which provides insights for the design of high-voltage trench MOSFETs. These results provide key guidelines for the circuit applications of medium-voltage SiC power MOSFETs.