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e of silicon but other semiconducting materials such as gallium arsenide and germanium are also used 6 the obsolete thermionic diode is a vacuum tube with two electrodes a heated cathode and a plate in which electrons can flow in only one direction from the cathode to the plate among many uses diodes are found in rectifiers to convert alternating current ac power to direct current dc demodulation in radio receivers and can even be used for logic or as temperature sensors a common variant of a diode is a light emitting diode which is used as electric lighting and status indicators on electronic devices main functions edit unidirectional current flow edit the most common function of a diode is to allow an electric current to pass in one direction called the diode s forward direction while blocking it in the opposite direction the reverse direction its hydraulic analog is a check valve this unidirectional behavior can convert alternating current ac to direct current dc a process called rectification as rectifiers diodes can be used for such tasks as extracting modulation from radio signals in radio receivers 7 threshold voltage edit forward current voltage curve of 4 common semiconductor diodes thermionic diode current voltage curve a diode s behavior is often simplified as having a forward threshold voltage or turn on voltage or cut in voltage above which there is significant current and below which there is almost no current which depends on a diode s composition forward threshold voltage for various semiconductor diodes diode type forward threshold voltage silicon schottky 0 15 v to 0 45 v germanium p n 0 25 v to 0 3 v silicon p n 0 6 v to 0 7 v infrared gaas p n 1 2 v light emitting diodes leds 1 6 v red to 4 v violet light emitting diode physics materials has a complete list this voltage may loosely be referred to simply as the diode s forward voltage drop or just voltage drop since a consequence of the steepness of the exponential is that a diode s voltage drop will not significantly exceed the threshold voltage under normal forward bias operating conditions datasheets typically quote a typical or maximum forward voltage v f for a specified current and temperature e g 20 ma and 25 c for leds 8 so the user has a guarantee about when a certain amount of current will kick in at higher currents the forward voltage drop of the diode increases for instance a drop of 1 v to 1 5 v is typical at full rated current for silicon power diodes see also rectifier rectifier voltage drop however a semiconductor diode s exponential current voltage characteristic is really more gradual than this simple on off action 9 although the graph of the function may appear to have a definite knee around this threshold this is merely reflective of its exponential nature there is nothing unique about the region that differs from any other parts of the graph which can be seen by using a semi log plot with current being represented on a logarithmic scale and voltage on a linear scale since a diode s forward voltage drop varies only a little with the current and is more so a function of temperature this effect can be used as a temperature sensor or as a somewhat imprecise voltage reference reverse breakdown edit a diode s high resistance to current flowing in the reverse direction suddenly drops to a low resistance when the reverse voltage across the diode reaches a value called the breakdown voltage this effect is used to regulate voltage zener diodes or to protect circuits from high voltage surges avalanche diodes other functions edit a semiconductor diode s current voltage characteristic can be tailored by selecting the semiconductor materials and the doping impurities introduced into the materials during manufacture 9 these techniques are used to create special purpose diodes that perform many different functions 9 for example to electronically tune radio and tv receivers varactor diodes to generate radio frequency oscillations tunnel diodes gunn diodes impatt diodes and to produce light light emitting diodes tunnel gunn and impatt diodes exhibit negative resistance which is useful in microwave and switching circuits diodes both vacuum and semiconductor can be used as shot noise generators history edit thermionic vacuum tube diodes and solid state semiconductor diodes were developed separately at approximately the same time in the early 1900s as radio receiver detectors 10 until the 1950s vacuum diodes were used more frequently in radios because the early point contact semiconductor diodes were less stable in addition most receiving sets had vacuum tubes for amplification that could easily have the thermionic diodes included in the tube for example the 12sq7 double diode triode 11 and vacuum tube rectifiers and gas filled rectifiers were capable of handling some high voltage high current rectification tasks better than the semiconductor diodes such as selenium rectifiers that were available at that time thermionic edit structure of a vacuum tube diode the filament itself may be the cathode or more commonly as shown here used to heat a separate metal tube which serves as the cathode further information vacuum tube history and development in 1873 frederick guthrie observed that a grounded white hot metal ball brought close to an electroscope would discharge a positively charged electroscope but not a negatively charged electroscope 12 13 in 1880 thomas edison observed unidirectional current between heated and unheated elements in a bulb later called edison effect and was granted a patent on application of the phenomenon for use in a dc voltmeter 14 15 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 radio detector fleming patented the first true thermionic diode the fleming valve in britain on 16 november 1904 16 followed by u s patent 803 684 in november 1905 throughout the vacuum tube era valve diodes were used in almost all electronics such as radios televisions sound systems and instrumentation they slowly lost market share beginning in the late 1940s due to selenium rectifier technology and then to semiconductor diodes during the 1960s today they are still used in a few high power applications where their ability to withstand transient voltages and their robustness gives them an advantage over semiconductor devices and in musical instrument and audiophile applications semiconductor edit bose s 1901 patent 17 in 1874 german scientist karl ferdinand braun discovered the unilateral conduction across a contact between a metal and a mineral 18 19 c e fitts made a selenium current rectifier c 1886 but his work did not result in any practical devices until the 1930s 20 indian scientist jagadish chandra bose was the first to use a crystal for detecting radio waves in 1894 21 and he filed a u s patent for detecting radio signals with a galena crystal point contact semiconductor diode in 1901 20 the crystal detector was developed into a practical device for wireless telegraphy by greenleaf whittier pickard who invented a silicon crystal detector in 1903 and received a patent for it on 20 november 1906 22 he subsequently founded a company to market cat s whisker crystal radio detectors probably the first to make and sell commercial silicon semiconductor devices 20 other experimenters tried a variety of other minerals as detectors e g henry dunwoody received a patent using silicon carbide later in 1906 and wichi torikata earned a patent in 1908 20 current voltage characteristics of galena and perikon crystal rectifiers illustrating the nonlinear rectifying behavior used in early radio detectors from 1925 experimental wireless article semiconductor principles were unknown to the developers of these early rectifiers during the 1930s understanding of physics advanced and in the mid 1930s researchers at bell telephone laboratories recognized the potential of the crystal detector for application in microwave technology 23 researchers at bell labs western electric mit purdue and in the united kingdom intensively developed point contact diodes crystal rectifiers or crystal diodes during world war ii for application in radar 23 after world war ii at t used these in its microwave towers that criss crossed the united states and many radar sets use them even in the 21st century in 1946 sylvania began offering the 1n34 crystal diode 24 25 26 during the early 1950s junction diodes were developed etymology edit main article rectifier early asymmetric conduction devices were commonly referred to as rectifiers because their principal use was the conversion of alternating current to direct current in 1919 the year tetrodes were introduced william henry eccles coined the term diode from the greek di δί meaning two and hodos ὁδός meaning path the term described a vacuum tube having two electrodes 27 in modern usage the term diode refers generally to a two terminal device that exhibits asymmetric conduction the term rectifier is often used more specifically for devices intended for power supply conversion while diode is frequently used for small signal devices such as detectors or switching elements a similar distinction existed with thermionic devices small vacuum diodes were commonly used as radio detectors while larger two electrode valves designed for power conversion were typically referred to as rectifier valves or rectifiers vacuum tube diodes edit thermionic diode the image below has no alt text please help improve wikipedia by adding the alt parameter read about alt text a high power vacuum tube diode used in radio equipment as a rectifier component type active working principle thermionic emission inventor john ambrose fleming invention year 1904 pin names plate and cathode electronic symbol the symbol for an indirectly heated vacuum tube diode from top to bottom the element names are plate cathode and heater main article vacuum tube characteristics a thermionic diode is a vacuum tube consisting of a sealed evacuated glass or metal envelope containing two electrodes a cathode and a plate the cathode is either indirectly heated or directly heated if indirect heating is employed a heater is included in the envelope in operation the cathode is heated to red heat around 800 1 000 c 1 470 1 830 f a directly heated cathode is made of tungsten wire and is heated by a current passed through it from an external voltage source an indirectly heated cathode is heated by infrared radiation from a nearby heater that is formed of nichrome wire and supplied with current provided by an external voltage source a vacuum tube containing two power diodes the operating temperature of the cathode causes it to emit electrons into the vacuum by thermionic emission 28 in most receiving tubes the cathode is coated with oxides of alkaline earth metals typically barium and strontium oxides which possess a low work function and therefore emit electrons more readily than bare metal surfaces 29 the plate is not heated and does not normally emit electrons it serves as the electron collecting electrode when an alternating voltage is applied between cathode and plate the diode conducts only when the plate is positive with respect to the cathode in that condition the plate electrostatically attracts electrons emitted by the cathode and current flows from cathode to plate when the plate is negative with respect to the cathode it does not emit electrons and the electron stream from the cathode is repelled so no conduction occurs 30 under typical operating conditions the diode current is limited not by emission but by the space charge formed by electrons in transit between cathode and plate in this region the current density follows the child langmuir law varying approximately as the three halves power of the plate voltage ip ep 3 2 for fixed electrode spacing 31 other types of diodes include the mercury vapor diode the xenon gas diode the cold cathode rectifier and the magnetron 32 semiconductor diodes edit close up of an efd108 germanium point contact diode in do7 glass package showing the sharp metal wire cat whisker that forms the semiconductor junction point contact diodes edit until about the end of the 1950s diodes that relied on a small diameter metal wire point contact e g tungsten wire to make contact with a small semiconductor crystal were all called crystal detectors the contact could be a non welded contact type or a welded contact type non welded contact construction utilizes the schottky barrier principle the metal side is the pointed end of a small diameter wire that is in contact with the semiconductor crystal 33 in the welded contact type a small p region is formed in the otherwise n type crystal around the metal point during manufacture by momentarily passing a relatively large current through the device 34 35 with the availibility of vacuum tube diodes for receiver crystal detectors were mainly used for research and development between 1925 until 1940 the need for demodulator for radar applications and test equipment in the microwave bands up to 26 ghz led to the use of silicon for a crystal detector by bell laboratories with production starting in 1942 23 36 those crystal detectors had a ceramic case and needed manual adjustment of the point contact during production 37 for the typical radar bands at the time 1n25 1 ghz 1n21 3 ghz and later 1n21we 12 g 4 ghz 1n23 9 4 ghz and 1n26 24 ghz were available in february 1946 silvania electric introduced a crystal detector that used germanium instead of silicon the first models used also a ceramic case e g 1n34 while the later model 1n34a had a case made out of glass unlike the silicon type crystal detectors the 1n34 diode allowed also useas rectifiers for small currents 38 silvania electric called their diodes initially also crystal detectors 39 later in july 1946 changed it to crystal diode 40 and in november 1947 in data sheets to germanium crystal diodes 38 junction diodes edit p n junction diode edit main article p n diode a p n junction diode is made of a crystal of semiconductor usually silicon but germanium and gallium arsenide are also used impurities are added to it to create a region on one side that contains negative charge carriers electrons called an n type semiconductor and a region on the other side that contains positive charge carriers holes called a p type semiconductor when the n type and p type materials are attached together a momentary flow of electrons occurs from the n to the p side resulting in a third region between the two where no charge carriers are present this region is called the depletion region because there are no charge carriers neither electrons nor holes in it the diode s terminals are attached to the n type and p type regions the boundary between these two regions called a p n junction is where the action of the diode takes place when a sufficiently higher electrical potential is a...
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