In a semiconductor device of the present invention, an N type epitaxial layer is formed on a P type silicon substrate. In the epitaxial layer, P type diffusion layers as a base region, N type diffusion layers as collector regions and an N type diffusion layer as an emitter region are formed. In this event, the P type diffusion layers are formed so as to have a double diffusion structure, and an impurity concentration in a surface of the base region and in a region adjacent thereto is set high. This structure enables improvement in high frequency characteristics and in a current amplification factor while maintaining breakdown voltage characteristics of an NPN transistor.