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ses 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 been passing in an srd and the current is interrupted or reversed the reverse conduction will cease very abruptly as in a step waveform srds can therefore provide very fast voltage transitions by the very sudden disappearance of the charge carriers stabistors or forward reference diodes the term stabistor refers to a special type of diodes featuring extremely stable forward voltage characteristics these devices are specially designed for low voltage stabilization applications requiring a guaranteed voltage over a wide current range and highly stable over temperature transient voltage suppression diode tvs these are avalanche diodes designed specifically to protect other semiconductor devices from high voltage transients 59 their p n junctions have a much larger cross sectional area than those of a normal diode allowing them to conduct large currents to ground without sustaining damage tunnel diodes or esaki diodes these have a region of operation showing negative resistance caused by quantum tunneling 60 allowing amplification of signals and very simple bistable circuits because of the high carrier concentration tunnel diodes are very fast may be used at low mk temperatures high magnetic fields and in high radiation environments 61 because of these properties they are often used in spacecraft varicap or varactor diodes these are used as voltage controlled capacitors these are important in pll phase locked loop and fll frequency locked loop circuits allowing tuning circuits such as those in television receivers to lock quickly on to the frequency they also enabled tunable oscillators in the early discrete tuning of radios where a cheap and stable but fixed frequency crystal oscillator provided the reference frequency for a voltage controlled oscillator zener diodes these can be made to conduct in reverse bias backward and are correctly termed reverse breakdown diodes this effect called zener breakdown occurs at a precisely defined voltage allowing the diode to be used as a precision voltage reference the term zener diodes is colloquially applied to several types of breakdown diodes but strictly speaking zener diodes have a breakdown voltage of below 5 volts whilst avalanche diodes are used for breakdown voltages above that value in practical voltage reference circuits zener and switching diodes are connected in series and opposite directions to balance the temperature coefficient response of the diodes to near zero some devices labeled as high voltage zener diodes are actually avalanche diodes see above two equivalent zeners in series and in reverse order in the same package constitute a transient absorber or transorb a registered trademark graphic symbols edit main article electronic symbol the symbol used to represent a particular type of diode in a circuit diagram conveys the general electrical function to the reader there are alternative symbols for some types of diodes though the differences are minor the triangle in the symbols points to the forward direction i e in the direction of conventional current flow diode light emitting diode led photodiode schottky diode transient voltage suppression diode tvs tunnel diode varicap zener diode typical diode packages in same alignment as diode symbol the thin bar depicts the cathode numbering and coding schemes edit there are a number of common standard and manufacturer driven numbering and coding schemes for diodes the two most common being the eia jedec standard and the european pro electron standard eia jedec edit the standardized 1n series numbering eia370 system was introduced in the us by eia jedec joint electron device engineering council about 1960 most diodes have a 1 prefix designation e g 1n4003 among the most popular in this series were 1n34a 1n270 germanium signal 1n914 1n4148 silicon signal 1n400x silicon 1a power rectifier and 1n580x silicon 3a power rectifier 62 63 self published source 64 failed verification jis edit the jis semiconductor designation system has all semiconductor diode designations starting with 1s pro electron edit the european pro electron coding system for active components was introduced in 1966 and comprises two letters followed by the part code the first letter represents the semiconductor material used for the component a germanium and b silicon and the second letter represents the general function of the part for diodes a low power signal b variable capacitance x multiplier y rectifier and z voltage reference for example aa series germanium low power signal diodes e g aa119 ba series silicon low power signal diodes e g bat18 silicon rf switching diode by series silicon rectifier diodes e g by127 1250v 1a rectifier diode bz series silicon zener diodes e g bzy88c4v7 4 7v zener diode other common numbering coding systems generally manufacturer driven include gd series germanium diodes e g gd9 this is a very old coding system oa series germanium diodes e g oa47 a coding sequence developed by mullard a uk company related devices edit rectifier transistor thyristor or silicon controlled rectifier scr triac diac varistor in optics an equivalent device for the diode but with laser light would be the optical isolator also known as an optical diode that allows light to only pass in one direction it uses a faraday rotator as the main component 65 applications edit radio demodulation edit a simple envelope demodulator circuit the first use for the diode was the demodulation of amplitude modulated am radio broadcasts an am signal consists of alternating positive and negative peaks of a radio carrier wave whose amplitude or envelope is proportional to the original audio signal the diode rectifies the am radio frequency signal leaving only the positive peaks of the carrier wave the audio is then extracted from the rectified carrier wave using a simple filter and fed into an audio amplifier or transducer which generates sound waves via audio speaker in microwave and millimeter wave technology beginning in the 1930s researchers improved and miniaturized the crystal detector point contact diodes crystal diodes and schottky diodes are used in radar microwave and millimeter wave detectors 41 power conversion edit main article rectifier schematic of basic ac to dc power supply rectifiers are constructed from diodes where they are used to convert alternating current ac electricity into direct current dc automotive alternators are a common example where the diode which rectifies the ac into dc provides better performance than the commutator or earlier dynamo similarly diodes are also used in cockcroft walton voltage multipliers to convert ac into higher dc voltages reverse voltage protection edit since most electronic circuits can be damaged when the polarity of their power supply inputs are reversed a series diode is sometimes used to protect against such situations this concept is known by multiple naming variations that mean the same thing reverse voltage protection reverse polarity protection and reverse battery protection over voltage protection edit diodes are frequently used to conduct damaging high voltages away from sensitive electronic devices they are usually reverse biased non conducting under normal circumstances when the voltage rises above the normal range the diodes become forward biased conducting for example diodes are used in stepper motor and h bridge motor controller and relay circuits to de energize coils rapidly without the damaging voltage spikes that would otherwise occur a diode used in such an application is called a flyback diode many integrated circuits also incorporate diodes on the connec...
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