Semiconductor devices and methods of forming the same are provided. The methods may implanting dopants into a substrate to form a preliminary impurity region and heating the substrate to convert the preliminary impurity region into an impurity region. Heating the substrate may be performed at an ambient temperature of from about 800° C. to about 950° C. for from about 20 min to about 50 min. The method may also include forming first and second trenches in the impurity region to define an active fin and forming a first isolation layer and a second isolation layer in the first and second trenches, respectively. The first and second isolation layers may expose opposing sides of the active fin. The method may further include forming a gate insulation layer extending on the opposing sides and an upper surface of the active fin and forming a gate electrode traversing the active fin.