A semiconductor device having a polysilicon gate containing a doping impurity and nitrogen, and method of manufacture

A semiconductor device comprises a plurality of MOS transistors (T41, T42, T42), to have different characteristics, which are formed on a common substrate. The control electrode (4A, 4B, 4C) of each of the transistors comprises a polysilicon layer containing internally an impurity doping and nitrogen. Nitrogen is introduced into a lower portion (N1, N2, N3) of the polycrystalline silicon layer, so that the concentration of the dopant impurity is relatively high in an upper portion of the polycrystalline silicon layer, but relatively low in a lower part. The nitrogen concentration modulates the effective thickness of the gate oxide film of each transistor and allows to individually adjust the characteristics thereof.