We propose and demonstrate a novel self-aligning process for fabricating the tungsten (W) gate electrode of GaAs nanowire FETs by using selective-area metalorganic vapor phase epitaxy (SA-MOVPE) where SiO2/W composite films are used to mask the substrates. First, to study the growth process and its dependence on mask materials, GaAs wire structures were grown on masked substrates partially covered with a single W layer or SiO2/W Composite films. We found that lateral growth over the masked regions could be suppressed when a wire along the [110] direction and a SiO2/W composite mask were used. Using this composite mask, we fabricated GaAs narrow channel FETs using W as a Schottky gate electrode, and we were able to observe FET characteristics at room temperature. (C) 2004 Elsevier B.V. All rights reserved.