A semiconductor device includes a substrate having first and second active regions with a field insulating layer therebetween that contacts the first and second active regions, and a gate electrode on the substrate and traversing the first active region, the second active region, and the field insulating layer. The gate electrode includes a first portion over the first active region, a second portion over the second active region, and a third portion in contact with the first and second portions. The gate electrode includes an upper gate electrode having first through third thicknesses in the first through third portions, respectively, where the third thickness is greater than the first thickness, and smaller than the second thickness.