In a method of manufacturing a semiconductor device, an isolation pattern may be formed on a substrate to define a plurality of active patterns. The active patterns may protrude from the isolation pattern. A preliminary polysilicon layer may be formed on the active patterns to fill a gap between adjacent ones of the active patterns. Ions having no conductivity may be implanted into the preliminary polysilicon layer to form a polysilicon layer having no void. The active patterns maintain their crystalline state during the implanting of the ions. The polysilicon layer may be patterned to form a dummy gate structure on the active pattern. A source/drain region may be formed at an upper portion of the active patterns adjacent to sides of the dummy gate structure.