A high-voltage transistor device has a first well region with a first conductivity type in a semiconductor substrate, and a second well region with a second conductivity type in the semiconductor substrate substantially adjacent to the first well region. A field ring with the second conductivity type is formed on a portion of the first well region, and the top surface of the field ring has at least one curved recess. A field dielectric region is formed on the field ring and extends to a portion of the first well region. A gate structure is formed over a portion of the field dielectric region and extends to a portion of the second well region.