A semiconductor device includes a substrate, a gate insulation layer, a gate structure, a gate spacer, and first and second impurity regions. The substrate has an active region defined by an isolation layer. The active region has a gate trench thereon. The gate insulation layer is formed on an inner wall of the gate trench. The gate structure is formed on the gate insulation layer to fill the gate trench. The gate structure has a width smaller than that of the gate trench, and has a recess at a first portion thereof. The gate spacer is formed on sidewalls of the gate structure. The first and second impurity regions are formed at upper portions of the active region adjacent to the gate structure. The first impurity region is closer to the recess than the second impurity region. Related methods are also provided.