High-voltage transistor devices with two-step field plate structures and method of fabricating the same

High-voltage transistor devices with two-step field plate structures and methods of fabricating the same are provided. An example high voltage transistor device includes: a gate electrode disposed over a substrate between a source region and a drain region, a first film laterally extending from over the gate electrode to over a drift region laterally arranged between the gate electrode and the drain region, a second film laterally extending over a portion of the drift region adjacent to the drain region and away from the gate electrode, and a field plate laterally extending from over the first film to over the second film. A first thickness vertically from a top surface of the gate electrode to a bottom surface of the field plate is smaller than a second thickness vertically from a top surface of the portion of the drift region to the bottom surface of the field plate.