Methods of selectively forming epitaxial semiconductor layer on single crystalline semiconductor and semiconductor devices fabricated using the same

A method of fabricating a metal oxide semiconductor (MOS) transistor, the method comprising:

• forming an isolation layer at a predetermined region of a single crystalline semiconductor substrate to define an active region;
• forming an insulated gate electrode on the active region, the gate electrode being formed of a non-single crystalline semiconductor layer;
• forming an insulating gate spacer on sidewalls of the non-single crystalline gate electrode;
• loading the substrate having the gate spacer into a reaction chamber;
• injecting a main semiconductor source gas and a main etching gas into the reaction chamber to selectively form a non-single crystalline epitaxial semiconductor layer on the gate electrode formed of the non-single crystalline semiconductor layer and to selectively form a single crystalline epitaxial semiconductor layer on the active region;
• and
• injecting a selective etching gas into the reaction chamber to selectively remove the non- single crystalline epitaxial semiconductor layer on the gate electrode, wherein the main gases and the selective etching gas are alternately and repeatedly injected at least two times to selectively form elevated single crystalline source/drain regions having a desired thickness only on the active regions adjacent to the gate electrode.