A semiconductor element may include a substrate including source and drain regions formed in the substrate apart from each other by a trench, a gate insulating layer covering a bottom surface and a sidewall of the trench, a gate electrode including lower and upper buried portions. The lower buried portion may be in the trench with the gate insulating layer therearound and fill a lower region of the trench. The upper buried portion may be on the lower buried portion with the gate insulating layer therearound and fill an upper region of the trench. The upper buried portion may include a two-dimensional material layer in the trench on an upper surface of the first conductive layer and an upper region of the sidewall of the gate insulating layer, and a second conductive layer in the upper region of the trench and surrounded by the two-dimensional material layer.