A semiconductor device includes a gate trench across an active region of a semiconductor substrate, a gate structure filling the gate trench, and source/drain regions formed in the active region at respective sides of the gate structure. The gate structure includes a sequentially stacked gate electrode and insulating capping pattern, and a gate dielectric layer between the gate electrode and the active region. The gate electrode is located at a lower level than an upper surface of the active region and includes a barrier conductive pattern and a gate conductive pattern. The gate conductive pattern includes a first part having a first width and a second part having a second width greater than the first width. The barrier conductive pattern is interposed between the first part of the gate conductive pattern and the gate dielectric layer.