The present invention relates in general to power ICs, etc. having the SOI structure, and more specifically to the structure in which an
SOI substrate comprises a base substrate, an SOI
oxide film formed on the base substrate, and active
layers formed on the SOI
oxide film, and also integrates on itself power devices and the corresponding control elements monolithically. Between this base substrate and this SOI
oxide film is formed heavily-doped
semiconductor regions having a
conductivity type opposite to that of this base substrate. Hence, the junction
capacitance between the base substrate and the heavily-doped
semiconductor regions decreases an actual
capacitance between the base substrate and the
active layer so that to inhibit or prevent inversion
layers from being formed at the bottom of the active
layers.