This paper investigates the random dopant fluctuation of multi-gate metal - oxide - semiconductor field-effect transistors (MOSFETs) using analytical solutions of three-dimensional (3D) Poisson's equation verified with device simulation. Especially, we analyze the impact of aspect ratio on the random dopant fluctuation in multi-gate devices. Our study indicates that with a given total width, lightly doped fin-type FET (FinFET) shows the smallest threshold voltage (Vth) dispersion because of its smaller Vth sensitivity to the channel doping. For heavily doped devices, quasi-planar shows smaller Vth, dispersion because of its larger volume. The V th, dispersion caused by random dopant fluctuation may still be significant in the lightly doped channel, especially for tri-gate and quasi-planar devices.