To provide a semiconductor device which makes it possible to avoid deterioration in the step coverage property at a gate electrode provided on an operating region and decrease a leakage current between the operating region and the gate electrode. The semiconductor device arranged as a HEMT is made to include an operating region composed of multilayer films, such as a channel layer, an electron supplying layer and other semiconductor layer, and having an island structure independently mesa-isolated from one another. The semiconductor device also includes a gate electrode and an impurity diffusion layer provided on the surface of the operating region, the impurity diffusion layer being doped with an impurity having a conductivity type inverse to the impurity doped into the electron supplying layer.