Substrate current verifying lateral electrical field under forward substrate biases for nMOSFETs

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 1.8 V 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 0.18 μm and 10 μm with a 90 nm process.