Substrate current I SUB of an n-channel metal–oxide–semiconductor field-effect transistor (nMOSFET) depending on source/drain voltage V DS can be applied as a stress index in the hot carrier test. Usually, the substrate bias directly influences device performance, such as the threshold voltage and the source/drain current. On the other hand, the substrate biasing circuit benefits turn-on current and restrains the turn-off current. However, few studies assessed the change to substrate current when forcing different drain voltages and substrate biases. Furthermore, a unique phenomenon was observed: separation of I SUB curves and consentient trends existed while gate voltage V GS increased from 0 to 1.8V and a turning point located around at the peak value of I SUB . Here, this study identifies the increase in surface inversion charge Q i from substrate effect in weak inversion layer more than in strong one is evidently correlated with this interesting symptom. In this study, the gate length L G and the gate width W of a measured nMOSFET device is 0.18μm and 10μm with a 90nm process.