Embodiments for the present disclosure include a semiconductor device, an ultra-high voltage (UHV) laterally-diffused metal-oxide-semiconductor (LDMOS) transistor, and methods of forming the same. An embodiment includes a first well region of a first conductivity type in a top surface of a substrate, and a second well region of a second conductivity type in the top surface of the substrate. The second well region laterally separated from the first well region by a portion of the substrate. The embodiment further includes a third region of the second conductivity type in the first well region, and a first field oxide region in the first well region, a second field oxide region in the second well region, the second field oxide region having a second bottom surface, and the first field oxide region having a first bottom surface lower than the second bottom surface and on and directly contacting the third region.