Abstract: A field-effect semiconductor device has a gate pad at the outside of an area of a semiconductor element and island regions of a conductivity type opposite that of a substrate of the device at surfaces of the device under the gate pad. When voltage is applied to the semiconductor device's drain, depletion layers form and extend to the opposite side of the substrate from each of the island regions and become continuous with one another. Thus, the voltage applied to the device's insulation layers is limited and a high rated voltage of the device can be obtained. Further, this arrangement provides a wide contact area between the gate pad and the gate wiring because meshed gate wiring is formed in the area between the island regions. In this way, insulation film having no contact holes in the island regions and contact holes in the body region may be formed without replacing masks by alternating the opening with for introducing impurities in the island region and the opening width in the body region.