Some ceramic semiconductors being produced at high volume

Non-ohmic semiconducting devices called varistors have been used in large numbers in telephone systems for many years. Recently, the development of a telephone set having improved electrical characteristics and suited to a lower manufacturing cost initiated a large-scale development program on the silicon carbide and oxidized thallium-copper varistors. The purposes of this development were to make varistors to new and closer electrical tolerances than had been heretofore specified and to obtain high yields and long-time stability at low cost. To meet these demands, raw materials were investigated, new process methods were evaluated, and statistically designed experiments were carried out to determine the effects of the many parametric relationships involved in their manufacture. At present, twenty million of these semiconductor devices are being produced annually for Bell System use. Approximately forty-five million are in service, some for as long as eight years, without any reported field failures.