The interface states and reliability of 4H-SiC metal-oxide-semiconductor field effect transistors (MOSFETs) with thermal gate oxides have been researched widely. Several reports have researched the gate oxide process itself, but the effects of subsequent processes should not be ignored. In this paper, the reliability of thermal gate oxide films followed by polysilicon gate (poly-gate) process, which are widely used in MOSFET manufacture, and Al gates were compared. The poly-gate samples markedly affected the performance measured by time-zero dielectric breakdown and time-dependent dielectric breakdown methods because the phosphorus content diffused during poly-gate formatting; this was especially advantageous in reducing leakage current and improving the charge-to-breakdown (Q (BD)). After electronic characteristics measurements, scanning electron microscopy cross-sections were also used to analyze the breakdown mechanism. We observed an intermediate layer between the Al gate and the oxide that may cause the barrier height to be smaller than that of the poly-gate. The Al work function and polysilicon Fermi level determine the gate leakage currents and the resultant gate oxide reliability, whereas the Al2O3 gate sample has a smaller work function offset (0.7 eV) than ideal Al gate and poly-gate samples. The results imply that the reliability of the Al gate samples may be an intrinsic problem.
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