A high voltage (HV) transistor is integrated on a silicon-on-insulator (SOI) substrate. The HV transistor is disposed in a HV device region disposed on a bulk substrate of the SOI substrate. The HV device region includes a top field oxide which includes at least a part of a buried oxide (BOX) of the SOI substrate. A HV gate is disposed in HV region overlapping the HV top field oxide and includes first and second HV gate sidewalls. A drain is disposed on the bulk substrate and displaced from the first HV gate sidewall by the HV top field oxide. A source is disposed on the bulk substrate adjacent to the side of the second HV gate sidewall.