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 semiconductor-on-insulator (SOI) substrate having a semiconductor layer, a bulk substrate and an insulating layer between the semiconductor layer and the bulk substrate, a source region and a drain region disposed on the bulk substrate, an isolation structure extending through the insulating layer and the semiconductor layer and terminates in the bulk substrate, and a gate structure between the source region and the drain region, the gate structure is disposed on the semiconductor layer.