A transistor device employed in a support circuit of a DRAM includes a semiconductor substrate having thereon a gate trench, a recessed gate embedded in the gate trench, a source doping region disposed at one side of the recessed gate, a drain doping region disposed at the other side of the recessed gate, and a gate dielectric layer between the recessed gate and the semiconductor substrate. The gate dielectric layer has at least two thicknesses that render the high-voltage transistor device asymmetric. The thicker gate dielectric layer is between the recessed gate and the drain doping region, while the thinner gate dielectric layer is between the recessed gate and the source doping region.