A semiconductor device in which polysilicon is used to form source and drain regions in an initial process step so as to reduce resistance of bit lines and minimize a junction capacitance and thus improve its reliability, and a method for fabricating the same are disclosed, the semiconductor device including a semiconductor substrate, trenches formed in predetermined areas of the semiconductor substrate, an insulating layer formed in the trenches and beneath a surface of the substrate to have a recess, a polysilicon layer formed on the insulating layer in the trench, source and drain regions formed at both sides of the polysilicon layer beneath a surface of the semiconductor substrate, and gates formed over the semiconductor substrate.