A method of fabricating a semiconductor device is provided as follows. A strain relaxed buffer (SRB) layer is formed on a substrate. The SRB layer is formed of a first silicon germanium alloy (SiGe) layer which has a first atomic percent of germanium (Ge) atoms. A heterogeneous channel layer is formed on the SRB layer. The heterogeneous channel layer includes a silicon layer on a first region of the SRB layer and a second SiGe layer on a second region of the SRB layer. The second SiGe layer includes a second atomic percent of germanium greater than the first atomic percent of germanium atoms. The silicon layer is in contact with the second SiGe layer.