A recessed-gate transistor device includes a gate electrode embedded in a gate trench formed in a semiconductor substrate, wherein the gate trench includes a vertical sidewall and a U-shaped bottom. A source region is provided at one side of the gate trench within the semiconductor substrate. A drain region is provided at the other side thereof. An asymmetric gate dielectric layer is formed between the gate electrode and the semiconductor substrate. The asymmetric gate dielectric layer has a first thickness between the gate electrode and the drain region and a second thickness between the gate electrode and the source region, wherein the first thickness is thicker than the second thickness.