Most oxides are optically transparent, a favorite property for optical applications.
It has, however, been believed that active functions based on excited electrons, such as in crystalline semiconductor materials, are not possible for transparent oxides. In light of this history and expectations, Hideo Hosono and others have concentrated on the exploration of transparent conducting oxides following a working hypothesis established through a consideration of chemical bonding. As a consequence, more than ten kinds of novel transparent conducting oxides have been found. The most interesting materials among them are transparent amorphous conducting oxides and transparent p-type conducting oxides, which were created in 1996 and 1997, respectively. This was particularly interesting and important because most active functions of semiconductors originate from PN-junctions.