An integrated circuit comprises a memory array including diffusion bit lines having composite impurity profiles in a substrate. A plurality of word lines overlies channel regions in the substrate between the diffusion bit lines, with data storage structures such as floating gate structures or dielectric charge trapping structures, at the cross-points. The composite impurity diffusion bit lines provide source/drain terminals on opposing sides of the channel regions that have high conductivity, good depth and steep doping profiles, even with channel region critical dimensions below 50 nanometers.