A semiconductor device is manufactured using an expandable material. The method includes forming a first gate insulating layer on a substrate, forming first and second gate structures on the first gate insulating layer, the first and second gate structures being spaced apart from each other at a distance, forming an expandable material on sidewalls and upper surfaces of the first and second gate structures, forming a gap-fill layer on the expandable material between the first and second gate structures, and performing a heat-treatment process to increase the volume of the expandable material.