Semiconductor device

A semiconductor device includes an active region defined by a device isolation layer, a pad layer on the device isolation layer and a first region of the active region, a first separation layer penetrating through the pad layer and extending in a first direction, a second separation layer penetrating through the pad layer and extending in a second direction, a word line below the second separation layer, extending in the second direction, and embedded in a substrate, a bit line extending in the first direction and connected to a second region of the active region, a contact structure on a side surface of the bit line and connected to the pad layer, and a data storage structure on the contact structure and connected to the contact structure. The first separation layer includes an airgap or a material having a dielectric constant less than a dielectric constant of silicon nitride.