The diode is an electrical one-way valve with a two-path name
A diode lets current flow one way and all but blocks it the other, much like a check valve in plumbing. That humble trick turns household alternating current into the steady direct current that electronics need. The name arrived in 1919, when William Eccles joined the Greek words for two and path.
Two separate lines of discovery led here. In 1874 the German physicist Ferdinand Braun noticed that current crossed the contact between a metal and a crystalline mineral more easily in one direction. Six years later Thomas Edison saw current pass one way between a heated and an unheated element inside a bulb, an effect later named after him. About two decades on, John Ambrose Fleming, a former Edison employee advising the Marconi company, realised the effect could detect radio waves, and in 1904 he patented the Fleming valve, the first true vacuum-tube diode.
Crystals followed a parallel path. Jagadish Chandra Bose used a crystal to detect radio waves in 1894, and Greenleaf Whittier Pickard patented a silicon detector in 1906, then founded a company selling cat's whisker detectors, probably the first commercial silicon semiconductor devices. Nobody building them understood the physics. Through the 1950s vacuum diodes dominated radios because early crystal contacts were unreliable, yet wartime radar research at Bell Labs, MIT and elsewhere refined crystal diodes, and junction diodes appeared in the early 1950s.
Inside a vacuum diode, a cathode glows at roughly 800 to 1,000 degrees Celsius and boils off electrons, which a cold plate collects only when it is positive. Semiconductor diodes, usually silicon today, do the same job with a junction between two differently treated regions of crystal. Push a reverse voltage high enough and resistance suddenly collapses, a behaviour Zener diodes exploit to hold voltages steady and avalanche diodes use to absorb surges.
Tweaking materials yields a whole family: diodes that tune radios, generate microwaves, sense temperature or emit light. Old valve diodes still survive in some high-power equipment and in audiophile and musical gear, prized for their toughness.
Source: Diode
