2020/7/4,ElectronicCircuitsandApplications,ChapterThreePhysicalElectronicsOfTransistors,ContentsActivedevicesandControlElementsBipolarTransistorsasControlElementsField-EffectTransistorsasControlElements,3.0ActivedevicesandControlElements(1),1.Whatisanactivedevice有源器件?Anactivedeviceisadevicecapableofcontrollingtheflowofelectricalenergyfromasourcetoaload.Ifanactivedeviceistobeusefulforincreasingthepowerlevelofsignal,thenthepowerrequiredatthecontrolinputsmustbemuchlessthanthepowerdeliveredtotheload,thebalancecomingfromthedcpowersources.Thecombinationoftheactivedeviceanditsassociatedpowersourcethenfunctionsasanamplifier,andissaidtohavepowergain.Theop-ampisanactivedevice.2.Whatisacontrolelement控制器件?Acontrolelementisanetworkelementthatcanbeusedforthecontrolofpowerflow.Acontrolelementnormallyrequiresatleastthreeelectricalterminals,withthev-icharacteristicattheoutputterminalpairbeingdependentonthevoltageorcurrentattheotherterminals.Thetransformisacontrolelement.,3.0ActivedevicesandControlElements(2),3.TransistorandclassificationThetransistorismadefromsemiconductor.Itisthemostwidelyusedactivedeviceinmodernelectroniccircuits.Transistorscanbedividedintotwogeneralcategories:Bipolartransistor(BJT)双极性晶体管Field-effecttransistor(FET)场效应晶体管,3.1BipolarTransistorsasControlElements(2),Tounderstandhowthebipolartransistoroperatesasacontrolelement,wecandrawonourunderstandingofthep-njunctiondiodedevelopedinSection7.3.Inthediodeaforwardbiasproducesasignificantcurrentresultingfromanetflowofholesandelectronsfromtheregionswheretheyaremajoritycarrierstotheregionwheretheyareminoritycarriers.Thisinjectionprocessisreviewedschematicallyinfollowingfigure.,ThetotalterminalcurrentIisgivenbythesumofthetwocurrentcomponentsarisingfromtheholeflowandtheelectronflow.,3.1BipolarTransistorsasControlElements(3),Underreversebias,thep-njunctiondiodeischaracterizedbyasmallsaturationcurrent,arisingfromthecollectionofminoritycarriersfromtheirrespectiveregionsbytheelectricfieldinthespace-chargelayer.Themagnitudeofthisreversesaturationcurrentdependsontheavailableconcentrationofminoritycarriers,andissmallinthediode.,Thebipolartransistorworksasacontrolelementbycombininginjectionatoneofitsp-njunctionswithcollectionattheotherp-njunction.,3.1BipolarTransistorsasControlElements(4),Inthenormalgainortheactivegainregion放大区ofoperation,theemitter-basejunctionismaintainedinforwardbias,andthecollector-basejunctionisheldinreversebias.Bydopingtheemitterregionmuchmoreheavilythanthebase,mostoftheinjectionofminoritycarriersismadetooccurintothebasesideofthejunction.Thus,underforwardbiasconditions,thereisalargebuildupofminoritycarriers(electronsinanNPNtransistor,holesinaPNPtransistor)onthebasesideoftheemitter-basejunction.Whilethisbuildupofelectronconcentrationistakingplaceattheemitterendofthebaseregion,thereverse-biasedcollector-basejunctionkeepstheconcentrationofminorityelectronsverylowatthecollectorendofthebaseregion.Thiscombinationofaforwardbiasattheemitter-basejunctionandareversebiasatthecollector-basejunctionthusestablishesalargeconcentrationgradientofminoritycarriersacrossthebaseregion.Normalthermalmotions,therefore,produceadiffusiveflowofminorityelectronsthroughthebaseregion,fromtheemitterend,wheretheyareinexcess,tothecollectorend,wheretheyaresweptacrossthecollector-basejunctionintothecollectorregion.,3.1BipolarTransistorsasControlElements(5),3.1BipolarTransistorsasControlElements(6),Abovefigureillustratesthisflowofelectronsfromtheemittertothebasebyinjection注入,acrossthebasebydiffusion扩散,andintothecollectorbycollection收集.Nearlyalltheelectronsenteringthebaseregionfromtheemitterreachthecollector.Asmallfraction,however,recombine复合withholestoformcompletecovalentbonds.Becauseofthisrecombinationprocess,andbecauseoftheinjectionofholesfromthebasetotheemitter,someholesmustbesuppliedtothebaseviathebaseterminal.Theseholescannotbesuppliedfromthecollectorbecausetheyaretheminoritycarrierthere,andarefewinnumbers.Insummary,thecollector-basejunctionbehavesasareverse-biaseddiodewhosesaturationcurrentiscontrolledbytheinjectionofelectronsattheemitter-basejunction.Thecollectorcurrentisindependentofthecollector-basevoltage,providedthatareversebiasisestablishedatthecollector-basejunction.Thus,thebasicpropertyofacontrolelement,inthiscasethedependenceoftheoutput(collector)currentonaninputvariable(emittercurrentoremitter-basevoltage)hasbeendemonstrated.,3.1BipolarTransistorsasControlElements(7),3.1.2CircuitSymbolsandTerminalVariablesforBipolarTransistors,1.NPNbipolartransistor,2.PNPbipolartransistor,TheonlydifferencebetweenNPNandPNPdevicesymbolsisthedirectionofthearrowontheemitterlead,whichindicatestheactualdirectionofforwardcurrentintheemitter-basejunction.Thereferencedirectionsfortheterminalcurrentsarealldefinedenteringthedevice,irrespectiveofthedirectionofpositivecurrentflow.Thusatleastoneoftheterminalscurrentmustbealgebraicallynegative.,Thevoltagesubscriptsdenotetheterminalsbetweenwhichthevoltageismeasured,withthefirstsubscriptindicatingthepositivereferenceter