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Anomalous increase in saturation-region drain current Id(sat) but serious on-resistance degradation (decrease in linear-region drain current) is observed in n-type high-voltage lateral diffused MOS transistors stressed under medium gate voltage Vg. However, Id(sat) is degraded for the devices stressed under low and high Vg. Experimental data reveal that two competing mechanisms are responsible for the shift of Id(sat). One is the interface state Nit formation in the N- drift region. The other is the Nit formation in the channel region. The former mechanism leads to the anomalous increase in Id(sat), whereas the latter mechanism causes the to decrease. Experimental data and technology computer-aided-design simulations confirm that the impact ionization rate of the device is enhanced if significant Nit formation in the N- drift region is present. According to the results presented in this paper, significant formation in the drift region is identified to be the main mechanism responsible for the anomalous increase in Id(sat).