PURPOSE: To whiten the surface of source wirings for higher surface qualities by a method wherein chemical etching is employed to make rugged the surface of a source wiring of Al or Al alloy forming a gate wiring together with polycrystalline Si, when a thin film transistor is built on a transparent substrate made of quartz glass or the like.

CONSTITUTION: An island of polycrystalline Si film is built on a transparent substrate 21 of quartz glass or soda glass. The surface exposed thereon is covered with an oxide gate insulating film 23, and a film 24 is formed of polycrystalline Si to serve as a gate electrode. On both sides of the film 24, source/drain diffused regions 25 are formed by means of ion implantation. The entire surface is then coated with an interlayer insulating film 26 wherein contact holes are provided for the regions 25, and then an Al or Al alloy layer 27 is laid down in contact with the regions 25. An etchant with H3PO4 as its principal component is applied for making the surface of the layer 27 rough and non-specular. The non-specular surface is then removed but on the source region 25 and a transparent electrode 28 is provided on the drain region 25.