A method of fabricating a gallium nitride (GaN) thin layer, by which a high-quality GaN layer may be grown on a large-area substrate using an electrode layer suspended above a substrate, a GaN film structure fabricated using the method, and a semiconductor device including the GaN film structure. The method includes forming a sacrificial layer on a substrate, forming a first buffer layer on the sacrificial layer, forming an electrode layer on the first buffer layer, forming a second buffer layer on the electrode layer, partially etching the sacrificial layer to form at least two support members configured to support the first buffer layer and form at least one air cavity between the substrate and the first buffer layer, and forming a GaN thin layer on the second buffer layer.