1. A high-voltage transistor comprising: a drain region of a first conductivity type; a source region of the first conductivity type; a body region of a second conductivity type opposite to the first conductivity type, the body region adjoining the source region; a drift region of the first conductivity type extending in a first direction from the drain region to the body region, the drift region comprising first and second sections, each disposed below the body region and each having substantially different first and second doping concentration gradients, respectively; first and second field plate members respectively disposed on opposite sides of the drift region, the first and second field plate members being fully insulated from the drift region; and an insulated gate disposed adjacent the body region.