Titanium oxide rectifiers

The first successful metallic rectifier was the cuprous oxide type first announced in 1920. In spite of the fact that several other types have been discovered since, there have been no other oxide rectifiers reported, and only a few patents have even referred to possibilities. The reasons for this presumably lie in the stringent requirements for a rectifier. First, the oxide must be formed as a thin, tightly adhering film; the oxide composition must be controllable so that it is a semiconductor with a suitable relatively high conductivity; and finally, in some way, an asymmetric potential barrier must be formed at a semiconductor-metal interface, in the Mott picture a thin insulating layer of different composition. It was believed that these conditions might be satisfied with titanium metal and its oxide.