Diodes have two active electrodes between which the signal of interest may flow, and most are used for their unidirectional electric current property.
The unidirectionality most diodes exhibit is sometimes generically called the rectifying property. The most common function of a diode is to allow an electric current in one direction (called the forward biased condition) and to block the current in the opposite direction (the reverse biased condition). Thus, the diode can be thought of as an electronic version of a check valve.
Real diodes do not display such a perfect on-off directionality but have a more complex non-linear electrical characteristic, which depends on the particular type of diode technology. Diodes also have many other functions in which they are not designed to operate in this on-off manner.
Early diodes included “cat’s whisker” crystals and vacuum tube devices (also called thermionic valves). Today most diodes are made of silicon, but other semiconductors such as germanium are sometimes used.
History:-
Although the crystal (solid state) diode was popularized before the thermionic diode, thermionic and solid state diodes were developed in parallel.
The basic principle of operation of thermionic diodes was discovered by Frederick Guthrie in 1873.[1] Guthrie discovered that a positively-charged electroscope could be discharged by bringing a grounded piece of white-hot metal close to it (but not actually touching it). The same did not apply to a negatively charged electroscope, indicating that the current flow was only possible in one direction.
The principle was independently rediscovered by Thomas Edison on February 13, 1880. At the time Edison was carrying out research into why the filaments of his carbon-filament light bulbs nearly always burned out at the positive-connected end. He had a special bulb made with a metal plate sealed into the glass envelope, and he was able to confirm that an invisible current could be drawn from the glowing filament through the vacuum to the metal plate, but only when the plate was connected to the positive supply.
Edison devised a circuit where his modified light bulb more or less replaced the resistor in a DC voltmeter and on this basis was awarded a patent for it in 1883 (U.S. Patent 307,031). There was no apparent practical use for such device at the time, and the patent application was most likely simply a precaution in case someone else did find a use for the so-called “Edison Effect”.
About 20 years later, John Ambrose Fleming (scientific adviser to the Marconi Company and former Edison employee) realized that the Edison effect could be used as a precision radio detector. Fleming patented the first true thermionic diode in Britain [2] on November 16, 1904 (followed by U.S. Patent 803,684 in November 1905).
The principle of operation of crystal diodes was discovered in 1874 by the German scientist, Karl Ferdinand Braun.[3] Braun patented the crystal rectifier in 1899.[4] Braun’s discovery was further developed by Jagdish Chandra Bose into a useful device for radio detection.
The first actual radio receiver using a crystal diode was built by Greenleaf Whittier Pickard. Pickard received a patent for a silicon crystal detector on November 20, 1906[5] (U.S. Patent 836,531).
Other experimenters tried a variety of minerals and other substances, although by far the most popular was the Lead Sulfide mineral Galena. Although other substances offered slightly better performance, galena had the advantage of being cheap and easy to obtain, and was used almost exclusvely in home-built “crystal sets”, until the advent of inexpensive fixed germanium diodes in the 1950s.
At the time of their invention, such devices were known as rectifiers. In 1919, William Henry Eccles coined the term diode from Greek roots; dia means “through”, and ode (from ὅδος) means “path”.
Although the crystal (solid state) diode was popularized before the thermionic diode, thermionic and solid state diodes were developed in parallel.
The basic principle of operation of thermionic diodes was discovered by Frederick Guthrie in 1873.[1] Guthrie discovered that a positively-charged electroscope could be discharged by bringing a grounded piece of white-hot metal close to it (but not actually touching it). The same did not apply to a negatively charged electroscope, indicating that the current flow was only possible in one direction.
The principle was independently rediscovered by Thomas Edison on February 13, 1880. At the time Edison was carrying out research into why the filaments of his carbon-filament light bulbs nearly always burned out at the positive-connected end. He had a special bulb made with a metal plate sealed into the glass envelope, and he was able to confirm that an invisible current could be drawn from the glowing filament through the vacuum to the metal plate, but only when the plate was connected to the positive supply.
Edison devised a circuit where his modified light bulb more or less replaced the resistor in a DC voltmeter and on this basis was awarded a patent for it in 1883 (U.S. Patent 307,031). There was no apparent practical use for such device at the time, and the patent application was most likely simply a precaution in case someone else did find a use for the so-called “Edison Effect”.
About 20 years later, John Ambrose Fleming (scientific adviser to the Marconi Company and former Edison employee) realized that the Edison effect could be used as a precision radio detector. Fleming patented the first true thermionic diode in Britain [2] on November 16, 1904 (followed by U.S. Patent 803,684 in November 1905).
The principle of operation of crystal diodes was discovered in 1874 by the German scientist, Karl Ferdinand Braun.[3] Braun patented the crystal rectifier in 1899.[4] Braun’s discovery was further developed by Jagdish Chandra Bose into a useful device for radio detection.
The first actual radio receiver using a crystal diode was built by Greenleaf Whittier Pickard. Pickard received a patent for a silicon crystal detector on November 20, 1906[5] (U.S. Patent 836,531).
Other experimenters tried a variety of minerals and other substances, although by far the most popular was the Lead Sulfide mineral Galena. Although other substances offered slightly better performance, galena had the advantage of being cheap and easy to obtain, and was used almost exclusvely in home-built “crystal sets”, until the advent of inexpensive fixed germanium diodes in the 1950s.
At the time of their invention, such devices were known as rectifiers. In 1919, William Henry Eccles coined the term diode from Greek roots; dia means “through”, and ode (from ὅδος) means “path”.
Thermionic and gaseous stste diodes:-
Thermionic diodes are thermionic-valve devices (also known as vacuum tubes, tubes, or valves), which are arrangements of electrodes surrounded by a vacuum within a glass envelope. Early examples were fairly similar in appearance to incandescent light bulbs.
In thermionic valve diodes, a current through the heater filament indirectly heats the cathode, another internal electrode treated with a mixture of barium and strontium oxides, which are oxides of alkaline earth metals; these substances are chosen because they have a small work function. (Some valves use direct heating, in which a tungsten filament acts as both heater and cathode.) The heat causes thermionic emission of electrons into the vacuum. In forward operation, a surrounding metal electrode called the anode is positively charged so that it electrostatically attracts the emitted electrons. However, electrons are not easily released from the unheated anode surface when the voltage polarity is reversed. Hence, any reverse flow is negligible.
For much of the 20th century, thermionic valve diodes were used in analog signal applications, and as rectifiers in many power supplies. Today, valve diodes are only used in niche applicatio
ns such as rectifiers in electric guitar and high-end audio amplifiers as well as specialized high-voltage equipment.
Thermionic diodes are thermionic-valve devices (also known as vacuum tubes, tubes, or valves), which are arrangements of electrodes surrounded by a vacuum within a glass envelope. Early examples were fairly similar in appearance to incandescent light bulbs.
In thermionic valve diodes, a current through the heater filament indirectly heats the cathode, another internal electrode treated with a mixture of barium and strontium oxides, which are oxides of alkaline earth metals; these substances are chosen because they have a small work function. (Some valves use direct heating, in which a tungsten filament acts as both heater and cathode.) The heat causes thermionic emission of electrons into the vacuum. In forward operation, a surrounding metal electrode called the anode is positively charged so that it electrostatically attracts the emitted electrons. However, electrons are not easily released from the unheated anode surface when the voltage polarity is reversed. Hence, any reverse flow is negligible.
For much of the 20th century, thermionic valve diodes were used in analog signal applications, and as rectifiers in many power supplies. Today, valve diodes are only used in niche applicatio
ns such as rectifiers in electric guitar and high-end audio amplifiers as well as specialized high-voltage equipment.Semiconductor diodes:-
Most diodes today are based on semiconductor p-n junctions. In a p-n diode, conventional current is from the p-type side (the anode) to the n-type side (the cathode), but not in the opposite direction. Another type of semiconductor diode, the Schottky diode, is formed from the contact between a metal and a semiconductor rather than by a p-n junction.
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