MOS transistors have an active region defined in a portion of a semiconductor substrate, a gate electrode on the active region, and drain and source regions in the substrate. First and second lateral protrusions extend from the lower portions of respective sidewalls of the gate electrode. The drain region has a first lightly-doped drain region under the first lateral protrusion, a second lightly-doped drain region adjacent the first lightly-doped drain region, and a heavily-doped drain region adjacent to the second lightly-doped drain region. The source region similarly has a first lightly-doped source region under the second lateral protrusion, a second lightly-doped source region adjacent the first lightly-doped source region, and a heavily-doped source region adjacent to the second lightly-doped source region. The second lightly-doped regions are deeper than the first lightly-doped regions, and the gate electrode may have an inverted T-shape.