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The electrical performances of sub-50-nm n-metal-oxide-semiconductor field effect transistors (n-MOSFETs) are improved when a plasma nitridation process is used after the gate electrodes are formed. The maximum drive current is increased by 2% and the minimum gate length is shrunk by 5% while the off-leakage current is maintained. Inverse modeling suggested that these improvements were due to nitridation-induced changes in the two-dimensional carrier profile, and scanning tunneling microscopy confirmed that they were. The plasma nitridation decreased the overlapping length from 12 nm to 10 nm and increased the steepness of the lateral abruptness of the extension region from 3.6 nm/decade to 1.8 nm/decade. Such an optimized profile is thought to be mainly due to nitrogen suppressing the lateral anomalous diffusion of the arsenic piled-up along the interface between the silicon substrate and the insulating layer.