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ctive 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 exponential current voltage i v relationship of diodes in moderate forward or reverse bias the article shockley diode equation provides details small signal behavior edit at forward voltages less than the saturation voltage the voltage versus current characteristic curve of most diodes is not a straight line the current can be approximated by i i s e v d n v t displaystyle i i_ text s e v_ text d nv_ text t as explained in the shockley diode equation article in detector and mixer applications the current can be estimated by a taylor s series 43 the odd terms can be omitted because they produce frequency components that are outside the pass band of the mixer or detector even terms beyond the second derivative usually need not be included because they are small compared to the second order term 43 the desired current component is approximately proportional to the square of the input voltage so the response is called square law in this region 33 p 3 reverse recovery effect edit following the end of forwarding conduction in a p n type diode a reverse current can flow for a short time the device does not attain its blocking capability until the mobile charge in the junction is depleted the effect can be significant when switching large currents very quickly 44 a certain amount of reverse recovery time t r on the order of tens of nanoseconds to a few microseconds may be required to remove the reverse recovery charge q r from the diode during this recovery time the diode can conduct in the reverse direction this might give rise to a large current in the reverse direction for a short time while the diode is reverse biased the magnitude of such a reverse current is determined by the operating circuit i e the series resistance and the diode is said to be in the storage phase 45 in certain real world cases it is important to consider the losses that are incurred by this non ideal diode effect 46 however when the slew rate of the current is not so severe e g line frequency the effect can be safely ignored for most applications the effect is also negligible for schottky diodes the reverse current ceases abruptly when the stored charge is depleted this abrupt stop is exploited in step recovery diodes for the generation of extremely short pulses types of semiconductor diode edit current voltage curves of several types of diodes normal p n diodes which operate as described above are usually made of doped silicon or germanium before the development of silicon power rectifier diodes cuprous oxide and later selenium was used their low efficiency required a much higher forward voltage to be applied typically 1 4 to 1 7 v per cell with multiple cells stacked so as to increase the peak inverse voltage rating for application in high voltage rectifiers and required a large heat sink often an extension of the diode s metal substrate much larger than the later silicon diode of the same current ratings would require the vast majority of all diodes are the p n diodes found in cmos integrated circuits 47 which include two diodes per pin and many other internal diodes avalanche diodes these are diodes that conduct in the reverse direction when the reverse bias voltage exceeds the breakdown voltage these are electrically very similar to zener diodes and are often mistakenly called zener diodes but break down by a different mechanism the avalanche effect this occurs when the reverse electric field applied across the p n junction causes a wave of ionization reminiscent of an avalanche leading to a large current avalanche diodes are designed to break down at a well defined reverse voltage without being destroyed the difference between the avalanche diode which has a reverse breakdown above about 6 2 v and the zener is that the channel length of the former exceeds the mean free path of the electrons resulting in many collisions between them on the way through the channel the only practical difference between the two types is th...
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