This paper presents a study of threshold voltage (Vth) fluctuation in n-MOSFETs having both gate lengths (Lg) and widths (W) as small as 0.15 micrometer for giga-bit DRAM cell transistors. The effects of varying Lg, W, and the gate oxide thickness (Tox) and fluctuation in the concentration of channel impurities (Na) were studied in detail. We fabricated and evaluated n-MOSFETs having various sizes of Lg and W. The Vth and the Vth fluctuation of these devices, when Lg/W equals 0.14/0.15 micrometer and the channel implantation condition was B+, 10 keV, and 9E12 cm-2, were 0.33 V and 47 mV, respectively. A short- channel effect and a slight inverse narrow-width effect were also observed. By determining the origin of the Vth fluctuation, we found that the short- and narrow-channel effect (combined with variations in device dimensions) had only a minor effect on Vth fluctuations at higher channel impurity concentrations, and that the channel impurity fluctuation had the biggest influence. This channel impurity fluctuation cannot be explained simply by the statistical dopant fluctuation. We presume that anomalous impurity diffusion, such as transient enhanced diffusion (TED), during the gate oxidation process may be the dominant factor.