A semiconductor device and fabricating method are provided, by which device drivability can be increased by forming second LDD regions after isolating first LDD regions from source/drain regions to prevent heavily doped impurities therein from diffusing into the first LDD regions and to provide stepped densities within the LDD regions. The method includes the steps of stacking oxide and conductive layers on a semiconductor substrate, forming a gate electrode by patterning the conductive layer, etching the exposed substrate to a first depth, forming a first LDD region in the etched substrate, forming a spacer on a sidewall of the gate electrode, forming a source/drain region in the substrate having the spacer, etching the substrate having the source/drain region to a second depth, and forming a second LDD region between the first LDD region and the source/drain region of the etched substrate.