A semiconductor structure includes an active area in a substrate, a device isolation region surrounding the active area, first and second bit line structures on the substrate, a conductive diffusion region in the active area between the first and the second bit line structures, and a contact hole between the first and the second bit line structures. The contact hole partially exposes the conductive diffusion region. A buried plug layer is disposed in the contact hole and in direct contact with the conductive diffusion region. A storage node contact layer is disposed on the buried plug layer within the contact hole. The storage node contact layer has a downwardly protruding portion surrounded by the buried plug layer. The buried plug layer has a U-shaped cross-sectional profile.