Semiconductor devices are provided. A semiconductor device includes a substrate including a plurality of active regions that extend longitudinally in a direction and an isolation region that electrically isolates the plurality of active regions from each other. The semiconductor device includes a gate trench that extends across the plurality of active regions and the isolation region. The semiconductor device includes a gate structure that extends in the gate trench. The semiconductor device includes a gate dielectric layer that is between the gate trench and the gate structure, in each of the plurality of active regions. The gate structure has a first width in the direction in each of the plurality of active regions and has a second width in the direction in the isolation region that is different from the first width.