Methods of fabricating semiconductor devices are provided including providing a substrate having a first region and a second region, the substrate defining trenches in the first and second regions; forming active fins on the first and second regions, the active fins protruding from the trenches in the first and second regions; forming spacers on sidewalls of the active fins in the first and second regions; recessing floors of the trenches under the spacers to provide extensions of the active fins; implanting impurities of a first type in the extensions of the active fins in the first region; and implanting impurities of a second, type, different from the first type, in the extensions of the active fins in the second region.