Abnormal extracted value of effective channel length for off-set gate MOSFET

Effective channel length of off-set gate MOSFET was extracted as its substrate voltage was changed. It is found that the extracted value is abnormally large when the absolute substrate voltage is large. This phenomenon is explained by considering the off-set region as a fringe MOSFET and a JFET which are parallel connected. When the absolute substrate voltage is large, the JFET is pinched off and the drain current in the off-set region is modulated by the field effect from the gate electrode (fringe MOSFET). This makes the extracted effective channel abnormally long.