A semiconductor device includes a gate structure on a channel region of a semiconductor substrate adjacent to a source/drain region therein and a surface insulation layer directly on the source/drain region of the substrate adjacent to the gate structure. The device further includes a spacer on a sidewall of the gate structure adjacent to the source/drain region. A portion of the surface insulation layer adjacent the gate structure is sandwiched between the substrate and the spacer. An interface between the surface insulation layer and the source/drain region includes a plurality of interfacial states. Portions of the source/drain region immediately adjacent the interface define a carrier accumulation layer having a greater carrier concentration than other portions thereof. The carrier accumulation layer extends along the interface under the spacer. Related methods are also discussed.