In a semiconductor device, an n.sup.+ -type polysilicon layer is formed on a substrate through a gate oxide layer. A p.sup.+ -type source or drain diffusion layer is formed on both sides of an impurity layer in the substrate. The n.sup.+ -type polysilicon layer is positioned over an intermediate portion of a channel formation layer, and has an oxide layer on an upper surface thereof. The n.sup.+ -type polysilicon layer has at its side portions a p.sup.+ -type polysilicon layer to make a gate electrode together with the n.sup.+ -type polysilicon layer. The gate electrode semiconductor layer is formed on the channel formation layer through the gate insulation layer in such a manner that in a portion contacting with the gate insulation layer, the nearer the portion approaches the impurity layers of the source and drain regions, the larger the work function of the portion becomes.