A semiconductor device includes a device isolation pattern in which a polysilicon layer pattern doped with oxygen, carbon or nitrogen is interposed between an inner wall of a trench and a nitride liner. The semiconductor device includes a semiconductor substrate including a trench, a polysilicon layer pattern on a surface of the trench, a nitride layer pattern on the polysilicon layer pattern, and an insulation layer pattern on the nitride layer pattern and filling the trench. The polysilicon layer pattern may be doped with oxygen, carbon and/or nitrogen. Related manufacturing methods are also disclosed.