A device includes a buried oxide layer disposed on a substrate, a first region disposed on the buried oxide layer and a first ring region disposed in the first region. The first ring region includes a portion of a guardring. The device further includes a first terminal region disposed in the first ring region, a second ring region disposed in the first region and a second terminal region disposed in the second ring region. The first terminal region is connected to an anode and the second terminal region is connected to a cathode. The first region has a graded doping concentration. The first region, the second ring region and the second terminal region have a first conductivity type, and the first ring region and the first terminal region have a second conductivity type. The first conductivity type is different from the second conductivity type.