Methods for forming semiconductor devices are provided. A semiconductor substrate is etched such that the semiconductor substrate defines a trench and a preliminary active pattern. The trench has a floor and a sidewall. An insulating layer is provided on the floor and the sidewall of the trench and a spacer is formed on the insulating layer such that the spacer is on the sidewall of the trench and on a portion of the floor of the trench. The insulating layer is removed on the floor of the trench and beneath the spacer such that a portion of the floor of the trench is at least partially exposed, the spacer is spaced apart from the floor of the trench and a portion of the preliminary active pattern is partially exposed. A portion of the exposed portion of the preliminary active pattern is partially removed to provide an active pattern that defines a recessed portion beneath the spacer. A buried insulating layer is formed in the recessed portion of the active pattern. Related devices are also provided.