The present disclosure generally to semiconductor devices, and more particularly to semiconductor devices having high-voltage transistors integrated on a semiconductor-on-insulator substrate and methods of forming the same. The present disclosure provides a semiconductor device including a bulk substrate having an upper surface, a semiconductor layer above the bulk substrate, an insulating layer between the semiconductor layer and the bulk substrate, a source region and a drain region on the bulk substrate, the source region and the drain region are raised above the upper surface of the bulk substrate, in which the source region and the drain region include an epitaxial semiconductor material, a gate dielectric between the source region and the drain region, the gate dielectric having a first portion on the bulk substrate and a second portion on the semiconductor layer, and a gate electrode above the gate dielectric.