There is provided an improved thin-film transistor of which a principal semiconducting layer comprises a layer composed of an amorphous material prepared by (a) introducing (i) a gaseous substance containing atoms capable of becoming constituents for said layer into a film forming chamber having a substrate for thin-film transistor through a transporting conduit for the gaseous substance and (ii) a gaseous halogen series substance having a property to oxidize the gaseous substance into the film forming chamber through a transporting conduit for the gaseous halogen series oxidizing agent, (b) chemically reacting the gaseous substance and the gaseous halogen series agent in the film forming chamber in the absence of a plasma to generate plural kinds of precursors containing exited precursors and (c) forming said layer on the substrate with utilizing at least one kind of those precursors as a supplier.