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, 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, 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.