SEMICONDUCTOR DEVICE AND FABRICATION THEREOF

PROBLEM TO BE SOLVED: To prevent a leakage current due to impurity depletion in an active region by defining the active region with a field insulator, forming an impurity region of same conductivity type as a substrate and then forming a gate pattern on a gate insulator and the field insulator. SOLUTION: A field oxide 32 is deposited on an initial P type semiconductor substrate 31 and the active region AA of an NMOS transistor having low threshold voltage is defined. A screen insulator 90 is then deposited on the active regions AA, AA' and a P well 33 is formed on the substrate to include an extended active region, i.e., the projecting part AA', while surrounding the field oxide 32. Subsequently, the screen insulator 90 is removed and a gate insulator 34 is deposited on the active regions AA, AA'. Finally, a gate 35 is formed on the substrate. According to the method, the active region AA beneath the field oxide 32 underlying the gate 35 of an NMOS transistor having a low threshold voltage is extended to the P well 33.