This disclosed technology generally relates to a semiconductor device. One aspect relates to a method of fabricating a stacked semiconductor including forming a semiconductor structure protruding above the substrate and a gate structure extending across the semiconductor structure. The semiconductor structure includes a lower channel layer formed of a first material, an intermediate layer formed of a second material and an upper channel layer formed of a third material. The method additionally includes forming oxidized end portions defining second spacers on end surfaces of an upper layer. And forming the oxidized end portions comprises oxidizing end portions of the upper channel layer at opposite sides of the gate structure using an oxidization process adapted to cause a rate of oxidation of the third material which is greater than a rate of oxidation of the first material, while first spacers cover intermediate end surfaces.