SEMICONDUCTOR DEVICE AND MANUFACTURE THEREOF

PROBLEM TO BE SOLVED: To minimize the threshold voltage difference between a first and second p-wells by placing first and second p-type doping regions near the surface of the second and first wells, the concn. of the first region being lower than that of the second. SOLUTION: N-type impurity ions are implanted to form a first threshold voltage adjusting layer 12A (first doping region) on the surface of a semiconductor substrate 1, and p-type impurity ions are implanted to form a second threshold voltage adjusting layer 13A (second doping region) on the surface of first and second p-wells 13 and 113, such that both adjusting layers 12A, 13A are formed on the second p-wells 113 to mutually counter-dope them, thereby forming the adjusting layers 12A, 13A having a concn. in the second p-wells lower than that of the adjusting layer 13A in the first p-wells 13. Hence the threshold voltage of the second well 113 becomes lower than that of the first well 13.