Electronic switch
Electronic switches are devices that can stop or start an electric current. Their invention was important in the history of computing for their use in the earliest electronic computers. Besides computers, electronic switches are also used in many other devices. Three generations of electronic switches have been used in the digital computer industry.
Electromechanical relays
The earliest digital switch was the electromechanical relay, consisting of a solenoid with mechanical contact points. Relays provide a physical switch that closes when electricity animates a magnet. Early relays were slow and prone to failure due to dust in the contacts or bending of moving metal parts. Modern relays are more resilient because they are encased in housings that prevent dirt or dust from clogging relays. However, they are still prone to failure over time due to mechanical stress.
Vacuum tubes
The vacuum tube, also called a triode, was invented in 1906 by American inventor Lee de Forest. The triode resulted from the introduction of a third electrode (called a grid) into the diode (an earlier form of vacuum tube similar to an incandescent light bulb). Triodes use no physical contacts to break or get dirty, and they are faster than electromechanical relays.
The triode could be used both as an amplifier (for radios) and a switch (for computers). Many of the early radio transmitters were built by de Forest using triodes, which revolutionized the field of broadcasting at the time. Radio transmission used vacuum tubes as analog devices, to modify (but not stop and stop) an electrical current. The ability of triode vacuum tubes to act as switches (on/off devices that stop or start) would later be important in the building of the first electronic computers.
Transistors
From the 1950's to the present, the prevalent electronic switch has been the transistor, invented at AT&T Bell Laboratories in 1948 by John Bardeen, Walter Brattain, and William Shockley (who won the Nobel prize in 1956 for its invention). Transistors are equivalent in operation to vacuum tubes, but are much smaller, require less power, and obtain faster switching times. The first transistor was made from materials that included a paper clip and a razor blade.
Although transistors can be manufactured as stand-alone devices, more often they are encapsulated inside small, silicon chips (call integrated circuits, or simply IC's). When technology advanced to the point where millions of transistors could be packed onto an IC the size of a fingernail, the density of transistors on the chip came to be called VLSI (Very Large Scale Integration). The density of transistors in silicon chips has been predicted for decades by Moore's law.