A semiconductor device and method of fabricating a semiconductor device are provided. The method includes forming a gate trench in a semiconductor substrate to define source/drain regions. The source/drain regions are separated from each other by the gate trench, and the semiconductor substrate is exposed through the gate trench. The semiconductor substrate has impurities of a first conductivity type. The source/drain regions have impurities of a second conductivity type different from the first conductivity type. The concentration of the second conductivity type impurities increases as the impurities approach the surfaces of the source/drain regions. A differential gate dielectric layer is formed along the surfaces of the source/drain regions and the semiconductor substrate exposed through the gate trench. A gate electrode filling the gate trench is formed. The differential gate dielectric layer has a first thickness between the gate electrode and the semiconductor substrate and has a second thickness greater than the first thickness between the gate electrode and the source/drain regions.