SEMICONDUCTOR MEMORY DEVICE

A semiconductor memory device is provided. The semiconductor device may include a semiconductor substrate having a device isolation trench defining active regions, a device isolation layer disposed in the device isolation trench, gate trenches extending in a first direction and crossing the active regions of the semiconductor substrate and the device isolation layer, word lines disposed in the gate trenches, respectively, each of the gate trenches may include first trench sections in the active regions and second trench sections in the device isolation layer, the first trench sections may have a first depth, and the second trench section may have a second depth greater than the first depth, the device isolation layer may include a lower portion positioned at a level lower than bottom surfaces of the first trench sections and an upper portion on the lower portion, and the lower portion may be formed of a dielectric material having a lower dielectric constant than that of the upper portion.