A dielectrically separated wafer and a fabrication method of the same makes it possible to expand the device fabrication surface area of the dielectrically separated silicon islands by laminating a low concentration impurity layer including a dopant of the same conductivity on a high concentration impurity layer formed on the bottom of the island. A dielectrically separated wafer and a fabrication method for the same which can grow a polysilicon layer without producing voids in the dielectrically separating oxide layer includes forming a seed polysilicon layer at low temperature and under low pressure and by forming, on the seed polysilicon layer, a high temperature polysilicon layer. A dielectrically separated wafer and a fabrication method for the same in which the surface between dielectrically separated islands is flattened includes polishing the surface of the dielectrically separated wafer only the amount needed for the surface of a dielectrically separated wafer to become a flat surface between dielectrically separated silicon islands, without projections or indentations.