The present disclosure provides a method of fabricating a semiconductor device. The method includes forming a buffer layer over a substrate, the buffer layer containing a first compound semiconductor that includes elements from one of: III-V families of a periodic table; and II-VI families of the periodic table. The method includes forming a channel layer over the buffer layer. The channel layer contains a second compound semiconductor that includes elements from the III-V families of the periodic table. The method includes forming a gate over the channel layer. The method includes depositing impurities on regions of the channel layer on either side of the gate. The method includes performing an annealing process to activate the impurities in the channel layer.