A device isolation region is formed, delimiting an active region in a substrate. A word line is formed, extending across the active region and the device isolation region and buried therein. A bit line is formed crossing the word line on the substrate. A channel is formed adjacent the word line, the channel having a retrograde doping profile having a doping concentration that increases away from a top surface of the active region. Formation of the channel includes performing a field ion implantation in the active region having a projected range near a bottom of the device isolation region.