Recent studies regarding MOSFET's on SiC reveal that 4H-SiC devices suffer from a low inversion layer mobility, while in 6H-SiC despite of a higher channel mobility the bulk mobility parallel to the c-axis is too low, making this polytype unattractive for power devices. This work presents experimental mobility data of MOSFET's fabricated on different polytypes as well as capacitance-voltage (C-V ) measurements of corresponding n-type MOS structures which give evidence that the low inversion channel mobility in 4H-SiC is caused by a high density of SiC-SiO 2 interface states close to the conduction band. These defects are believed to be inherent to all SiC polytypes and energetically pinned at around 2.9 eV above the valence band edge. Thus, for polytypes with band gaps smaller than 4H-SiC like 6H-SiC and 15R-SiC, the majority of these states will become resonant with the conduction band at room temperature or above, thus remarkably suppressing their negative effect on the channel mobility. In order to realize high performance power MOSFET's, the results reveal that 15R-SiC is the best candidate among all currently accessible SiC polytypes.

Hall effect measurements in a Hall-bar configuration are performed on nitrogen-doped n-type bulk 4H, 6H, and 15R SiC single crystals cut into small parallelepipeds with their longest edges either parallel or perpendicular to the ĉ axis. In the temperature range investigated (40–700 K), an anisotropy of the electron Hall mobility is observed in all three polytypes. While the mobility perpendicular to the ĉ axis—with magnetic field perpendicular or parallel to the ĉ axis—is greater than the mobility parallel to the ĉ axis for 6H and 15R SiC, 4H SiC shows the opposite behavior.

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