High voltage semiconductor device including buried oxide layer

A semiconductor device, comprising:

• a substrate;
• a buried oxide layer in the substrate and near a surface of the substrate, wherein a surface of the buried oxide layer is flush with the surface of the substrate;
• a gate dielectric layer on the substrate and covering the buried oxide layer;
• a gate structure disposed on the gate dielectric layer and overlapping the buried oxide layer;
• and
• a source region and a drain region in the substrate and respectively at two sides of the gate structure, wherein a lower edge of the drain region is lower than a lower edge of the buried oxide layer, and the buried oxide layer directly contacts a side edge of the source region and a side edge of the drain region.