EC1265– ELECTRONICS AND MICROPROCESSORS
UNITI: SEMICONDUCTORS ANDRECTIFIERS
Classification ofsolids based on energyband theory–Intrinsicsemiconductors– Extrinsic semiconductors – P-typeand N-type– PN junction– Zenereffect –Zenerdiode characteristics
–Half wave and fullwave rectifiers– Voltageregulation
CLASSIFICATION OF SOLID BASED ON ENERGY BAND THEORY:
- To know about the basic concept of semiconductor, diodes and rectifier.
- At the end of session the learners will able to derive and understand the concept rectifier types and semiconductor types.
Amodernsemiconductordiodeismadeofacrystalofsemiconductor likesiliconthathas impurities added to it to create a region on one side that contains negative charge carriers (electrons),calledn-typesemiconductor,andaregionontheothersidethatcontains positive charge carriers (holes), calledp-type semiconductor. The diode’sterminalsareattachedtoeachoftheseregions.Theboundarywithinthecrystalbetweenthesetworegions, called a PN junction, is where the action of the diode takes place. The crystal conductsconventionalcurrentinadirectionfromthep-typeside(calledtheanode)tothen- typeside(called the cathode), but not in the oppositedirection. Anothertypeofsemiconductordiode,theSchottkydiode,isformedfromthecontactbetween ametal and asemiconductor rather than byap-n junction. There are three types, they are insulators, conductor and semiconductor.
- To know about the basic concept of diodes
- At the end of session the learners will able to derive and understand the concept of PN diodes.
Asemiconductordiode‘sbehaviorinacircuitisgivenbyitscurrent–voltagecharacteristic,or I–Vgraph(seegraphbelow).Theshapeofthecurveisdeterminedbythetransportofcharge carriers throughtheso-called depletion layerordepletion regionthat existsatthep-n junction betweendifferingsemiconductors.Whenap-njunctionisfirstcreated,conduction band(mobile)electronsfromtheN-doped regiondiffuseintotheP-doped regionwherethere is a large population of holes (vacant places for electrons) with which the electrons ―recombine.Whenamobileelectronrecombineswithahole,bothholeandelectronvanish, leavingbehindanimmobilepositivelychargeddonor(dopant)ontheN-sideandnegatively chargedacceptor(dopant)ontheP-side.Theregionaroundthe p-njunctionbecomesdepleted of chargecarriers and thus behaves asaninsulator.
However,thewidthofthedepletionregion(calledthedepletionwidth)cannotgrowwithout limit. For eachelectron-hole pairthat recombines, apositivelychargeddopant ion isleft behindintheN-dopedregion,and anegativelychargeddopantionisleftbehindintheP- dopedregion.Asrecombinationproceedsmoreionsarecreated,anincreasingelectricfield developsthroughthedepletionzonewhichactstoslowandthenfinallystoprecombination. At this point,thereis a built-in potential acrossthe depletion zone.
If an external voltage is placed across the diode with the same polarityas the built-in potential, the depletion zonecontinues to act as an insulator, preventinganysignificant electriccurrentflow(unlesselectron/holepairsareactivelybeingcreatedinthejunctionby, forinstance,light.seephotodiode).Thisis thereversebiasphenomenon.However,ifthe polarityoftheexternalvoltageopposesthebuilt-inpotential,
At very large reverse bias, beyond thepeak inverse voltageor PIV, a process called reversebreakdown occurswhichcausesalargeincreaseincurrent(i.e.alargenumberof electronsandholesarecreatedat,andmoveawayfromthepnjunction)thatusuallydamages thedevicepermanently.Theavalanchediode isdeliberatelydesignedforuseintheavalanche region.Inthe zenerdiode,theconceptofPIVisnot applicable.Azenerdiodecontainsa heavilydopedp-njunctionallowingelectronstotunnelfromthevalencebandofthep-type material tothe conduction band ofthe n-type material, suchthat the reversevoltage is ―clampedtoaknownvalue(calledthezenervoltage),andavalanchedoesnotoccur.Both devices,however,dohavealimittothemaximumcurrentandpowerintheclampedreverse voltageregion.Also,followingtheendofforwardconductioninanydiode,thereisreverse currentforashorttime.Thedevicedoesnotattainitsfullblockingcapabilityuntilthereverse currentceases.
Thesecondregion,atreversebiasesmorepositivethanthePIV,hasonly averysmallreverse saturation current. InthereversebiasregionforanormalP-Nrectifierdiode,thecurrent throughthedeviceisvery low(intheµArange).However,thisistemperaturedependent,and atsufficientlyhightemperatures,asubstantialamountofreversecurrentcanbeobserved(mA ormore).
Thethird region is forward but smallbias, where onlyasmall forwardcurrent is conducted. Asthepotentialdifferenceisincreasedaboveanarbitrarilydefined―cut-involtageor―on- voltage or ―diodeforwardvoltagedrop(Vd),thediodecurrentbecomesappreciable(the levelofcurrentconsidered ―appreciableandthevalueofcut-involtagedependsonthe application), and the diode presents a very low resistance. The current–voltage curve is exponential.Inanormalsilicondiodeatratedcurrents,the arbitrarycut-involtageis definedas0.6to0.7volts.Thevalueisdifferentforotherdiodetypesschottkydiodes can beratedaslowas0.2 V,Germaniumdiodes0.25-0.3V,andredorbluelight-emitting diodes (LEDs)can havevalues of1.4 V and 4.0V respectively.
Athighercurrentstheforwardvoltagedropofthediodeincreases.Adropof1Vto1.5Vis typicalat full rated current forpower diodes.
Shockley diode equation
FIGURE: 1.1 Shockley diode equations
TheShockley idealdiodeequationor thediodelaw(namedafter transistorco- inventorWilliam Bradford Shockley, not to be confused withtetrodeinventor Walter H. Schottky)givestheI–Vcharacteristicofanidealdiodeineitherforwardorreversebias(orno bias). The equation is:
Iis thediode current,
ISisthe reversebiassaturation current(orscale current),
VDisthevoltageacross the diode,
nisthe idealityfactor,alsoknownasthequalityfactor orsometimesemissioncoefficient. The ideality factorn varies from 1 to 2 depending on the fabrication process and semiconductormaterialandinmanycasesisassumedtobeapproximately equalto1(thusthe notation n is omitted).
Thethermalvoltage VTisapproximately25.85mVat300K,atemperaturecloseto―room temperature‖commonlyusedindevicesimulationsoftware.Atanytemperatureitisaknown constant defined by:
wherekisthe Boltzmannconstant,Tistheabsolutetemperatureofthep-njunction,andq is the magnitudeof chargeon anelectron(the elementarycharge).
TheShockleyidealdiodeequationorthe diodelawisderivedwiththeassumptionthatthe onlyprocesses giving rise to the current in the diode are drift (due to electrical field), diffusion, and thermal recombination-generation. It also assumes that the recombination- generation(R-G)currentinthedepletionregionisinsignificant.ThismeansthattheShockley equationdoesn‘taccountfortheprocessesinvolvedinreversebreakdownandphoton-assisted R-G.Additionally,itdoesn‘tdescribethe―levelingoff‖oftheI–Vcurveathighforwardbias dueto internal resistance.
Underreversebiasvoltages(seeFigure5)theexponentialinthediodeequationisnegligible, andthecurrentisaconstant(negative)reversecurrentvalueof−IS.Thereverse breakdown region is notmodeled bythe Shockleydiode equation.
Foreven rathersmallforward biasvoltages (seeFigure5)the exponential is verylarge becausethe thermalvoltageisverysmall,sothesubtracted‗1‘inthediodeequationis negligible and the forward diode current is often approximated as
Theuse of the diode equation in circuit problems is illustrated in the article ondiode modeling.
Several types ofjunction diodes
Thereareseveraltypesofjunctiondiodes,whicheitheremphasizeadifferentphysicalaspect ofadiodeoftenbygeometricscaling,dopinglevel,choosingtherightelectrodes,arejustan applicationofadiodeinaspecialcircuit,orarereallydifferentdevicesliketheGunnand laser diode and theMOSFET:
Normal(p-n)diodes,whichoperateasdescribedabove,areusuallymadeofdopedsilicon or, more rarely,germanium. Before the development of modern silicon power rectifier diodes, cuprousoxide andlater selenium wasused;itslowefficiencygaveitamuchhigher forwardvoltagedrop(typically1.4–1.7 Vper―cell‖,withmultiplecellsstackedtoincrease thepeakinversevoltageratinginhighvoltagerectifiers),andrequiredalargeheatsink(often anextensionofthediode‘smetalsubstrate),muchlargerthanasilicondiode ofthesame current ratings would require. Thevast majorityof all diodes arethe p-n diodes found inCMOSintegrated circuits, which include two diodes perpin and manyother internal diodes.
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 mistakenlycalledZener diodes,butbreakdownbyadifferentmechanism,theavalanche effect.Thisoccurswhenthereverseelectric fieldacrossthep-njunctioncausesawaveof ionization,reminiscentofanavalanche,leadingtoa largecurrent.Avalanchediodesare designed to break down at a well-defined reverse voltage without being destroyed. The differencebetweentheavalanchediode(whichhasareversebreakdownaboveabout6.2V) andtheZeneristhatthechannellengthoftheformerexceedsthe―meanfreepath‖ ofthe electrons,sotherearecollisionsbetweenthemon thewayout.Theonlypracticaldifferenceis that the two types havetemperaturecoefficients ofopposite polarities.
TheseareactuallyaJFET withthegateshortedtothesource,andfunctionlikeatwo- terminal current-limiteranalogtotheZenerdiode,whichislimitingvoltage.Theyallowa currentthroughthemto rise toacertainvalue,andthenleveloffataspecificvalue.Also called CLDs, constant-current diodes, diode-connected transistors, or current-regulating diodes.
Thesehavearegionofoperationshowingnegativeresistance causedbyquantumtunneling, thusallowingamplificationofsignalsandverysimplebistablecircuits.Thesediodesarealso the typemost resistant to nuclearradiation.
ThesearesimilartotunneldiodesinthattheyaremadeofmaterialssuchasGaAsorInPthat exhibitaregionofnegativedifferentialresistance.Withappropriatebiasing,dipoledomains form and travel across the diode, allowinghighfrequencymicrowave oscillators to be built.
Inadiodeformedfromadirectband-gap semiconductor,suchasgalliumarsenide,carriers thatcrossthe junctionemitphotonswhentheyrecombinewiththemajoritycarrieronthe other side. Depending on the material,wavelengths(or colors)from the infrared to the nearultravioletmaybeproduced.The forwardpotentialofthesediodesdependsonthe wavelengthoftheemittedphotons:1.2Vcorrespondstored,2.4Vtoviolet.ThefirstLEDs wereredandyellow,andhigher-frequencydiodeshavebeendevelopedovertime.AllLEDs produceincoherent,narrow-spectrumlight;―white‖LEDsareactuallycombinationsofthree LEDsofadifferentcolor,orablueLEDwithayellowscintillatorcoating.LEDscanalsobe usedaslow-efficiencyphotodiodesinsignalapplications.AnLEDmaybepairedwitha photodiodeor phototransistor in thesamepackage, to form anopto-isolator.
WhenanLED-likestructureiscontainedinaresonantcavityformedbypolishingtheparallel endfaces,a laser canbeformed.Laserdiodesarecommonlyusedinopticalstorage devices and for high speed optical communication.
ThistermisusedbothforconventionalPNdiodesusedtomonitortemperatureduetotheir varying forward voltage with temperature, and forPeltier heat pumpsforthermoelectric heatingandcooling..Peltierheatpumpsmaybemadefromsemiconductor,thoughtheydo nothaveanyrectifyingjunctions,theyusethedifferingbehaviorofchargecarriersinNand Ptypesemiconductor to moveheat.
Allsemiconductors aresubjecttoopticalcharge carriergeneration. This istypicallyan undesired effect, so most semiconductorsarepackaged in light blockingmaterial. Photodiodes are intendedtosenselight (photo detector),sothey are packagedinmaterialsthatallowlightto pass,andareusuallyPIN(thekindofdiodemostsensitivetolight).Aphotodiodecanbe usedinsolarcells,inphotometry,orinopticalcommunications.Multiplephotodiodesmaybe packagedinasingledevice,eitherasalineararrayorasatwo-dimensionalarray.These arrays should not beconfusedwithcharge-coupled devices.
These work the same as the junction semiconductor diodes described above, but their constructionis simpler.Ablockofn-typesemiconductorisbuilt,andaconductingsharp- pointcontactmadewithsome group-3metalisplacedincontactwiththesemiconductor. Somemetalmigratesintothesemiconductortomakeasmallregionofp-typesemiconductor nearthecontact.Thelong-popular1N34germaniumversionisstillusedinradioreceiversas adetectorand occasionallyin specialized analog electronics.
APINdiodehasacentralun-doped,or intrinsic,layer,formingap-type/intrinsic/n-type structure. Theyareusedasradiofrequencyswitchesandattenuators.Theyarealsousedas largevolumeionizing radiationdetectorsandasphoto detectors.PINdiodesarealsoused in powerelectronics,astheircentrallayercanwithstandhighvoltages.
- To know about the basic concept of Zener diodes
- At the end of session the learners will able to understand the Zener diodes..
Diodes that canbemadeto conduct backwards. This effect, calledZenerbreakdown,occurs at apreciselydefinedvoltage,allowingthediodetobeusedasaprecisionvoltagereference.In practicalvoltagereference circuitsZenerandswitchingdiodesareconnectedinseriesand oppositedirectionstobalancethetemperaturecoefficienttonearzero.Somedeviceslabeled ashigh-voltageZenerdiodesareactuallyavalanchediodes (see above).Two(equivalent) Zenersinseriesandinreverseorder,inthesamepackage,constitutea transientabsorber (orTransorb,aregisteredtrademark).
Zenerdiode isatypeofdiode thatpermitscurrentnotonlyintheforwarddirectionlikea normaldiode,but alsointhereversedirectionifthevoltageislargerthanthebreakdown voltageknown as "Zener knee voltage" or "Zener voltage". The devicewasnamedafterClarenceZener, who discovered this electrical property.
Aconventionalsolid-statediode willnotallowsignificantcurrentifitisreversebelowits reverse breakdownvoltage. When thereversebiasbreakdownvoltageisexceeded, aconventionaldiodeissubjecttohighcurrentduetoavalanchebreakdown.Unlessthiscurrent islimitedbycircuitry,thediodewillbepermanentlydamaged.Incaseoflargeforwardbias (currentinthedirectionofthearrow),thediodeexhibitsavoltagedropduetoitsjunction built-involtageandinternal resistance. Theamountofthevoltagedrop depends onthe semiconductor material and the dopingconcentrations.
AZenerdiodeexhibitsalmostthesameproperties,exceptthedeviceisspeciallydesignedso astohaveagreatlyreducedbreakdownvoltage,theso-calledZenervoltage.Bycontrastwith theconventionaldevice,a reverse-biasedZenerdiodewillexhibitacontrolledbreakdownand allow the current to keepthe voltage across theZener diode at theZener voltage.Forexample, adiodewithaZenerbreakdownvoltageof3.2Vwillexhibitavoltagedropof3.2Vevenif reversebiasvoltageappliedacrossitismorethanitsZenervoltage.The Zenerdiodeis therefore ideal for applications such as the generation of a reference voltage (e.g. foran amplifierstage), or asavoltagestabilizer forlow-current applications.
Anothermechanismthatproducesasimilareffectistheavalancheeffectasintheavalanche diode.Thetwo typesofdiodeareinfactconstructedthesamewayandbotheffectsare presentindiodesofthistype.Insilicondiodesuptoabout5.6volts,theZenereffectisthe predominanteffectandshowsamarkednegativetemperaturecoefficient.Above5.6volts, the avalancheeffectbecomespredominantandexhibitsapositive temperaturecoefficient.Ina5.6Vdiode,thetwoeffectsoccurtogetherandtheirtemperature coefficients neatly canceleachotherout,thusthe5.6Vdiodeisthecomponentofchoiceintemperature-critical applications.Modernmanufacturingtechniqueshaveproduceddeviceswithvoltages lower than 5.6 V with negligible temperature coefficients, but as higher voltage devices are encountered, thetemperaturecoefficientrisesdramatically.A75Vdiodehas10timesthe coefficient ofa12 V diode.
Allsuchdiodes,regardlessofbreakdownvoltage,areusuallymarketedundertheumbrella term of "Zener diode".
Current-voltagecharacteristicofaZenerdiodewithabreakdownvoltageof17volt.Notice thechangeofvoltagescalebetweentheforwardbiased(positive)directionandthereverse biased (negative) direction.
Inthecaseofthissimplereference,thecurrentflowing inthediodeis determinedusing Ohms law and the known voltagedrop across the resistor R.IDiode=(UIN-UOUT)/ RΩ
Thevalue ofR mustsatisfytwoconditions:
1.R mustbesmallenoughthatthecurrentthroughDkeepsDinreversebreakdown.The valueofthis currentisgiveninthedatasheetforD.Forexample,thecommon BZX79C5V6device, a 5.6 V 0.5 W Zener diode, has a recommended reverse current of 5 mA. If insufficient current exists through D, then UOUTwill be unregulated,andlessthanthenominalbreakdown voltage (thisdifferstovoltage regulatortubeswheretheoutputvoltagewillbehigherthannominalandcouldriseas high as UIN).When calculatingR,allowancemustbemadeforanycurrent throughthe external load, not shown in this diagram, connected across UOUT.
2.R mustbelargeenoughthatthecurrentthroughDdoesnotdestroythedevice.Ifthe current through D isID, its breakdown voltage VB and its maximum power dissipationPMAX,then IDVB PMAX.
Aloadmaybeplacedacrossthediodeinthisreferencecircuit,andaslongasthezenerstays in reversebreakdown, thediode will provide astable voltagesourceto theload.
Shuntregulatorsaresimple,buttherequirementsthattheballastresistorbesmallenoughto avoidexcessivevoltagedropduringworst-caseoperation(lowinputvoltageconcurrentwith highloadcurrent)tendstoleave alotofcurrentflowinginthediodemuchofthetime, makingforafairlywastefulregulatorwithhighquiescentpowerdissipation,onlysuitablefor smaller loads.
Zenerdiodesinthisconfigurationareoftenusedasstablereferencesformoreadvanced voltageregulatorcircuits. These devices are also encountered, typically in series with a base-emitter junction, in transistorstageswhereselectivechoiceofadevicecenteredaroundtheavalanche/Zenerpoint canbeusedtointroducecompensatingtemperatureco-efficientbalancingofthetransistorPN junction.AnexampleofthiskindofusewouldbeaDCerroramplifierusedinaregulated power supplycircuitfeedback loop system. Zener diodesare also used insurgeprotectorsto limit transient voltagespikes.
INTRINSIC AND EXTRINSIC SEMICONDUCTOR
- To know about the basic concept of semiconductor and types of semiconductor
- At the end of session the learners will able to understand the concepts of the semiconductor types.
Although currents may be induced in pure, or intrinsic, semiconductor crystal due to the movement of free charges (the electron-hole pairs), these currents are too small to be of real use. This limitation is due to the relatively small number of electrons that obtain the required energy to jump the band gap (typically on the order of 1010 electrons/cm3 at room temperature for silicon). Now, while this may seem to be a very large number, you must keep in mind that silicon has on the order of 5x1022 atoms/cm3, so this effect is essentially nonexistent. What is important here is that, although the intrinsic concentration, Ni, is a function of band gap, temperature, and physical constants through
C is a material constant that depends on the effective density of states in the conduction and valence bands (C=5.4x1031 for silicon)
T is temperature in degrees Kelvin (oK)
Eg is the bandgap (Eg=1.12eV for silicon)
k is Boltzmann’s constant (k=8.62x10-5eV/oK)
For intrinisic silicon,
at room temperature (•300oK). Or, in words… the number of free electrons is equal to the number of free holes and is also equal to the intrinsic carrier concentration (since the only free carriers in an intrinsic material are due to EHP). Note that pn=ni2(we’ll use this in a couple of minutes). Finally, although the EHP generation is temperature dependent, free carrier generation through external excitation alone is a recipe for disaster To create practical, useful semiconductor devices, some really clever folks came up with a method of modifying the intrinsic crystal lattice. This process, called doping, involves introducing a specific number of atoms with a different valence number than the host semiconductor into the crystalline lattice. Pretty cool, huh? When the semiconductor crystal is doped such that its intrinsic nature is modified, it is termed extrinsic. In an extrinsic semiconductor, the equilibrium number of free electrons and holes are no longer equal since a tool other than EHP generation is used to create free carriers. Depending on the amount and type of impurity (or impurities) introduced, many electrical and optical properties of the semiconductor material may be modified or controlled to optimize desired behaviors or characteristics. We’re going to concentrate on silicon here and the creation of the two basic types of extrinsic (or doped) silicon, n-type and p-type - as illustrated in Figure 3.10 of your text (with a modified version of each type in the appropriate discussion below). Keep in mind that silicon is valence IV (four).
When a silicon atom in the crystal lattice is replaced with a valence V (five) atom (such as phosphorous, arsenic or antimony) the four valence electrons of silicon are satisfied and there is an electron “left over.” The left over electron requires very little thermal energy to release it from its parent atom and easily becomes a free carrier. Introducing group five impurities into the silicon lattice is known as n-type dopingsince the concentration of free electrons (remember the symbol for electrons is n) is greater than that of the intrinsic material. The elements used for n-type doping are known as donorssince they “donate” electrons to the silicon host and their concentration in the host material is designated by ND. When the donor electrons move from their parent atoms, they leave behind a positively charged ion. These ions are fixed in the semiconductor lattice so they cannot contribute to current but, have no fear, they play an important role when we start combining materials to create devices
Not surprisingly, there is a similar discussion for the case where a silicon atom in the host lattice is replaced with a valence III (three) atom, most commonly boron. For this case, as illustrated below, there is one too few electrons in the impurity atom to satisfy the four valence electrons of silicon. This results in an unsatisfied covalent bond, or looking at it another way, a free hole. Introducing group three atoms into the silicon lattice is known as p-type doping since the concentration of free holes (remember the symbol for holes is p) is greater than that of the intrinsic material. The elements used for p-type doping are known as acceptors since they “accept” any free electrons that may roam by to satisfy the fourth covalent bond. NA designates the concentration of acceptors in the host material. When the acceptor atom “picks up its fourth,” it becomes a negatively charged ion. Analogous to the discussion of the donor ions above, these are immobile charged atoms whose effect we will see later.
Carrier Concentrations and Excess Carriers
It is important to note that, even when a semiconductor material is doped with impurities, it remains electrically neutral. This means that overall (macroscopically), the number of positive charges is equal to the number of negative charges. Conventional doping levels do not change the chemical or mechanical properties of a semiconductor, but do make a world of difference in the electrical properties!
For our purposes, we are going to assume that at room temperature all impurity atoms are ionized – all donors have contributed a free electron and all acceptors have contributed a free hole (captured an electron). Remember that, for an intrinsic material, all free carriers are due to electron-hole pair generation and that the number of free electrons is always equal to the number of free holes (and is equal to the intrinsic carrier concentration ni). Well… this doesn’t hold true for extrinsic semiconductors. When one type of impurity has been intentionally introduced into the host material, it is seen to dominate and we speak of majority carriers and minority carriers. Specifically,
- for n-type semiconductors: ND> NA, ND > ni, electrons (n) are the majority carriers and holes (p) are the minority carriers.
- for p-type semiconductors: NA > ND, NA > ni, holes (p) are the majority carriers and electrons (n) are the minority carriers.
Under thermal equilibrium conditions, the pn product remains constant, or
p0n0=ni2, where the subscript indicates equilibrium and niis still the intrinsic carrier concentration. For practical doping levels at room temperature, a valid approximation is that the number of free majority carriers is equal to the doping level and the minority carriers can be found from the above equation. Specifically, for n-type: nn0•ND and pn0•ni2/ND
p-type: pp0•NA and np0•ni2/NA
Again, keep in mind that the material – n-type or p-type - remains electrically neutral. Although free charges are moving in the material, creating ionized impurities in extrinsic material, the free carriers are neutralized by the bound charges associated with the ionized impurities.HALF-WAVE RECTIFIER
- To know about the basic concept of rectifier and types
- At the end of session the learners will able to understand the concepts of the rectifier
The voltage at point A does the opposite of that at point B.When A is increasing in a positive direction, B is increasing in a negative direction.It is rather like the two ends of a see-saw.During the first half cycle of the waveform shown on the left, A is positive and B is negative.
The diode is forward biased and current flows around the circuit formed by the diode, the transformer winding and the load.Since the current through the load, and the voltage across the load are in the same proportions, then the voltage across the load is as shown in the right hand diagram, during the first half cycle.During the second half cycle, A and the anode are negative, B andthe cathode are positive.The diode is reverse biased and no current flows.This is indicated by the horizontal line in the right hand diagram.The diode only conducts on every other half cycle.There is one pulse for every cycle in. i.e 50 pulses per second (in the UK).The diode only conducts during half the cycle.Hence, Half-Wave Rectification.The rectified voltage is DC (it is always positive in value).However, it is not a steady DC but PULSATING DC.
It needs to be smoothed before it becomes useful.If the diode is reversed then the output voltage is negative
FULL WAVE RECTIFIER