Semiconductors have been used widely in signal-level or "brain" applications. Since their invention in 1948, transistors have revolutionized the electronics industry in computers, information processing, and communications. Now, however, semiconductors are being used more and more where consid­ erable "brawn" is required. Devices such as high-power bipolar junction tran­ sistors and power field-effect transistors, as well as SCRs, TRlACs, GTOs, and other semiconductor switching devices that use a p-n-p-n regenerative effect to achieve bistable action, are expanding the power-handling horizons of semicon­ ductors and finding increasing application in a wide range of products including regulated power supplies, lamp dimmers, motor drives, pulse modulators, and heat controls. HVDC and electric-vehicle propulsion are two additional areas of application which may have a very significant long range impact on the tech­ nology. The impact of solid-state devices capable of handling appreciable power levels has yet to be fully realized. Since it first became available in late 1957, the SCR or silicon-controlled rec­ tifier (also called the reverse blocking triode thyristor) has become the most popular member of the thyristor family. At present, SCRs are available from a large number of manufacturers in this country and abroad. SCR ratings range from less than one ampere to over three thousand amperes with voltage ratings in excess of three thousand volts.