A semiconductor device includes a first gate electrode structure having a first gate insulating layer on a substrate and a first gate electrode on the first gate insulating layer. A first spacer structure includes a first spacer and a second spacer on side walls of the first gate electrode structure. The first spacer is disposed between the second spacer and the first gate electrode. A source/drain region is disposed on opposite sides of the first gate electrode structure. The first gate electrode includes a lower part of the first gate electrode, an upper part of the first gate electrode disposed on the lower part of the first gate electrode, and the first spacer is disposed on the side wall of the upper pan of the first gate electrode and is not disposed on the side wall of the lower part of the first gate electrode.