Dielectrically isolated substrate and semiconductor device using the same

A semiconductor device, comprising:

• a composite substrate formed by directly bonding a first semiconductor substrate serving as an element region to a second semiconductor substrate serving as a supporting member with a first insulating film interposed therebetween;
• a plurality of low breakdown voltage elements formed in said first semiconductor substrate;
• and
• a high breakdown voltage element formed in said first semiconductor substrate;
• said first semiconductor substrate being divided into first and second island regions which are isolated from each other by an isolating region including an oxide film formed on a side surface of a groove formed in said first semiconductor substrate, and polycrystalline silicon filled in the groove, said second island region including a plurality of element regions, said high breakdown voltage element being formed in said first island region, and said low breakdown voltage elements being formed in said second island region.