There is disclosed a single-gate type conductivity-modulation field effect transistor having a semiconductive substrate, a base layer, and a source layer formed in the base layer. A source electrode is provided on a surface of the substrate, for electrically shorting the base layer with the source layer. A drain layer is provided in the substrate surface. A drain electrode is formed on the substrate surface to be in contact with the drain layer. A gate electrode is insulatively provided above the substrate surface, for covering a certain surface portion of the base layer which is positioned between the substrate and the source layer to define a channel region below the gate electrode. A lightly doped semiconductor diffusion layer is formed in the substrate surface so as to overlap said base layer and said drain layer. The diffusion layer having an impurity density which is varied continuously through the thickness of the diffusion layer.