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ficiently 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 they have temperature coefficients of opposite polarities constant current diodes these are actually jfets 48 with the gate shorted to the source and function like a two terminal current limiting analog to the voltage limiting zener diode they allow a current through them to rise to a certain value and then level off at a specific value also called clds constant current diodes diode connected transistors or current regulating diodes crystal rectifiers or crystal diodes these are point contact diodes 33 the 1n21 series and others are used in mixer and detector applications in radar and microwave receivers 49 50 51 the 1n34a is another example of a crystal diode 52 gunn diodes these are similar to tunnel diodes in that they are made of materials such as gaas or inp that exhibit a region of negative differential resistance with appropriate biasing dipole domains form and travel across the diode allowing high frequency microwave oscillators to be built light emitting diodes leds in a diode formed from a direct band gap semiconductor such as gallium arsenide charge carriers that cross the junction emit photons when they recombine with the majority carrier on the other side depending on the material wavelengths or colors 53 from the infrared to the near ultraviolet may be produced 54 the first leds were red and yellow and higher frequency diodes have been developed over time all leds produce incoherent narrow spectrum light white leds are actually a blue led with a yellow scintillator coating or combinations of three leds of a different color leds can also be used as low efficiency photodiodes in signal applications an led may be paired with a photodiode or phototransistor in the same package to form an opto isolator laser diodes when an led like structure is contained in a resonant cavity formed by polishing the parallel end faces a laser can be formed laser diodes are commonly used in optical storage devices and for high speed optical communication thermal diodes this term is used both for conventional p n diodes used to monitor temperature because of their varying forward voltage with temperature and for peltier heat pumps for thermoelectric heating and cooling peltier heat pumps may be made from semiconductors though they do not have any rectifying junctions they use the differing behavior of charge carriers in n and p type semiconductor to move heat photodiodes all semiconductors are subject to optical charge carrier generation this is typically an undesired effect so most semiconductors are packaged in light blocking material photodiodes are intended to sense light photodetector so they are packaged in materials that allow light to pass and are usually pin the kind of diode most sensitive to light 55 a photodiode can be used in solar cells in photometry or in optical communications multiple photodiodes may be packaged in a single device either as a linear array or as a two dimensional array these arrays should not be confused with charge coupled devices pin diodes a pin diode has a central un doped or intrinsic layer forming a p type intrinsic n type structure 56 they are used as radio frequency switches and attenuators they are also used as large volume ionizing radiation detectors and as photodetectors pin diodes are also used in power electronics as their central layer can withstand high voltages furthermore the pin structure can be found in many power semiconductor devices such as igbts power mosfets and thyristors schottky diodes schottky diodes are constructed from metal to semiconductor contact they have a lower forward voltage drop than p n junction diodes their forward voltage drop at forward currents of about 1 ma is in the range 0 15 v to 0 45 v which makes them useful in voltage clamping applications and prevention of transistor saturation they can also be used as low loss rectifiers although their reverse leakage current is in general higher than that of other diodes schottky diodes are majority carrier devices and so do not suffer from minority carrier storage problems that slow down many other diodes so they have a faster reverse recovery than p n junction diodes they also tend to have much lower junction capacitance than p n diodes which provides for high switching speeds and their use in high speed circuitry and rf devices such as switched mode power supply mixers and detectors super barrier diodes super barrier diodes are rectifier diodes that incorporate the low forward voltage drop of the schottky diode with the surge handling capability and low reverse leakage current of a normal p n junction diode gold doped diodes as a dopant gold or platinum acts as recombination centers which helps the fast recombination of minority carriers this allows the diode to operate at higher signal frequencies at the expense of a higher forward voltage drop gold doped diodes are faster than other p n diodes but not as fast as schottky diodes they also have less reverse current leakage than schottky diodes but not as good as other p n diodes 57 58 a typical example is the 1n914 snap off or step recovery diodes the term step recovery relates to the form of the reverse recovery characteristic of these devices after a forward current has 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