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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 applied to the p side the anode than to the n side the cathode it allows electrons to flow through the depletion region from the n type side to the p type side the junction does not allow the flow of electrons in the opposite direction when the potential is applied in reverse creating in a sense an electrical check valve schottky diode edit main article schottky diode another type of junction diode the schottky diode is formed from a metal semiconductor junction rather than a p n junction which reduces capacitance and increases switching speed 41 42 current voltage characteristic edit a semiconductor diode s behavior in a circuit is given by its current voltage characteristic the shape of the curve is determined by the transport of charge carriers through the so called depletion layer or depletion region that exists at the p n junction between differing semiconductors when a p n junction is first created conduction band mobile electrons from the n doped region diffuse into the p doped region where there is a large population of holes vacant places for electrons with which the electrons recombine when a mobile electron recombines with a hole both hole and electron vanish leaving behind an immobile positively charged donor dopant on the n side and negatively charged acceptor dopant on the p side the region around the p n junction becomes depleted of charge carriers and thus behaves as an insulator however the width of the depletion region called the depletion width cannot grow without limit for each electron hole pair recombination made a positively charged dopant ion is left behind in the n doped region and a negatively charged dopant ion is created in the p doped region as recombination proceeds and more ions are created an increasing electric field develops through the depletion zone that acts to slow and then finally stop recombination at this point there is a built in potential across the depletion zone a p n junction diode in low forward bias mode the depletion width decreases as voltage increases both p and n junctions are doped at a 1e15 cm3 doping level leading to built in potential of 0 59v observe the different quasi fermi levels for conduction band and valence band in n and p regions red curves reverse bias edit see also p n diode reverse bias if an external voltage is placed across the diode with the same polarity as the built in potential the depletion zone continues to act as an insulator preventing any significant electric current flow unless electron hole pairs are actively being created in the junction by for instance light see photodiode forward bias edit see also p n diode forward bias however if the polarity of the external voltage opposes the built in potential recombination can once again proceed resulting in a substantial electric current through the p n junction i e substantial numbers of electrons and holes recombine at the junction that increases exponentially with voltage operating regions edit current voltage characteristic of a p n junction diode showing three regions breakdown reverse biased forward biased the exponential s knee is at v d the leveling off region which occurs at larger forward currents is not shown a diode s current voltage characteristic can be approximated by four operating regions from lower to higher bias voltages these are breakdown at very large reverse bias beyond the peak inverse voltage piv a process called reverse breakdown occurs that causes a large increase in current i e a large number of electrons and holes are created at and move away from the p n junction that usually damages the device permanently the avalanche diode is deliberately designed for use in that manner in the zener diode the concept of piv is not applicable a zener diode contains a heavily doped p n junction allowing electrons to tunnel from the valence band of the p type material to the conduction band of the n type material such that the reverse voltage is clamped to a known value called the zener voltage and avalanche does not occur both devices however do have a limit to the maximum current and power they can withstand in the clamped reverse voltage region also following the end of forwarding conduction in any diode there is reverse current for a short time the device does not attain its full blocking capability until the reverse current ceases reverse biased for a bias between breakdown and 0 v the reverse current is very small and asymptotically approaches i s for a normal p n rectifier diode the reverse current through the device is in the micro ampere μa range however this is temperature dependent and at sufficiently high temperatures a substantial amount of reverse current can be observed ma or more there is also a tiny surface leakage current caused by electrons simply going around the diode as though it were an imperfect insulator semi log i v logarithmic current vs linear voltage graph of various diodes forward biased the current voltage curve is exponential approximating the shockley diode equation when plotted using a linear current scale a smooth knee appears but no clear threshold voltage is visible on a semi log graph leveling off at larger forward currents the current voltage curve starts to be dominated by the ohmic resistance of the bulk semiconductor the curve is no longer exponential it is asymptotic to a straight line whose slope is the bulk resistance this region is particularly important for power diodes and can be modeled by a shockley ideal diode in series with a fixed resistor shockley diode equation edit main article shockley diode equation the shockley ideal diode equation or the diode law named after the bipolar junction transistor co inventor william bradford shockley models the exponent...
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