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.