Germanium and silicon power rectifiers

The special properties of the semi-conductors germanium and silicon have recently been used for power conversion; equipments have been made with germanium rectifiers rated at 300 and 1000kW, with conversion efficiencies exceeding 97%, and having other advantages over older types of convertor. The history of this development is first briefly given, followed by a short explanation of rectification phenomena in a single crystal having a p-n rectifying junction formed by the disposition of positive and negative current carriers in the body of the crystal. A distinction is made between two types of p-n junctions produced: one in growing the crystal and the other by a subsequent fusion process in the wafer of a single crystal. The latter is shown to be the preferred type for power rectifiers. Steps in the production of the rectifier, the various types made by one organization, the methods of rating and the electrical tests applied to low-, medium- and high-power units are then described. The latter half of the paper is devoted to the special features of the high-power unit, rated up to 2 kW. Tests are specified for series and parallel operation, methods of cooling and relative efficiencies are discussed and comparisons are made with other convertors. Typical installations are cited, one of which has been successfully operated for two years. An 18 MW installation in the course of construction is also mentioned. The paper is optimistic about the future of p-n junction devices. Data are given showing the unique properties of silicon, with its ability to operate at much higher temperatures than germanium. Thus, when more economically producible, silicon devices will be extensively used. A comprehensive list of references to earlier work is given, supplemented by Appendices. These include a fuller explanation and mathematical treatment of current flow in semi-conductors. The phenomena of inverse voltage breakdown and hole storage are also explained in greater detail, since the- se effects have special significance in operating the high-power fused-junction rectifier.