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Phosphorous from P2O5 is more effective than nitrogen for passivating the 4H-SiC/SiO2 interface. The peak value of the field-effect mobility for 4H-SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) after phosphorus passivation is approximately 80 cm2/V·s. However, P2O5 converts the SiO2 layer to phosphosilicate glass (PSG)-a polar material that introduces voltage instabilities which negate the benefits of lower interface trap density and higher mobility. We report a significant improvement in voltage stability with mobilities as high as 72 cm2/V·s for MOSFETs fabricated with a thin PSG gate layer ( ~ 10 nm) capped with a deposited oxide.