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1、2020/7/4,ElectronicCircuitsandApplications,ChapterThreePhysicalElectronicsOfTransistors,ContentsActivedevicesandControlElementsBipolarTransistorsasControlElementsField-EffectTransistorsasControlElements,3.0ActivedevicesandControlElements(1),1.Whatisanactivedevice有源器件?Anactivedeviceisadevicecapableof
2、controllingtheflowofelectricalenergyfromasourcetoaload.Ifanactivedeviceistobeusefulforincreasingthepowerlevelofsignal,thenthepowerrequiredatthecontrolinputsmustbemuchlessthanthepowerdeliveredtotheload,thebalancecomingfromthedcpowersources.Thecombinationoftheactivedeviceanditsassociatedpowersourcethe
3、nfunctionsasanamplifier,andissaidtohavepowergain.Theop-ampisanactivedevice.2.Whatisacontrolelement控制器件?Acontrolelementisanetworkelementthatcanbeusedforthecontrolofpowerflow.Acontrolelementnormallyrequiresatleastthreeelectricalterminals,withthev-icharacteristicattheoutputterminalpairbeingdependentont
4、hevoltageorcurrentattheotherterminals.Thetransformisacontrolelement.,3.0ActivedevicesandControlElements(2),3.TransistorandclassificationThetransistorismadefromsemiconductor.Itisthemostwidelyusedactivedeviceinmodernelectroniccircuits.Transistorscanbedividedintotwogeneralcategories:Bipolartransistor(B
5、JT)双极性晶体管Field-effecttransistor(FET)场效应晶体管,3.1BipolarTransistorsasControlElements(2),Tounderstandhowthebipolartransistoroperatesasacontrolelement,wecandrawonourunderstandingofthep-njunctiondiodedevelopedinSection7.3.Inthediodeaforwardbiasproducesasignificantcurrentresultingfromanetflowofholesandelec
6、tronsfromtheregionswheretheyaremajoritycarrierstotheregionwheretheyareminoritycarriers.Thisinjectionprocessisreviewedschematicallyinfollowingfigure.,ThetotalterminalcurrentIisgivenbythesumofthetwocurrentcomponentsarisingfromtheholeflowandtheelectronflow.,3.1BipolarTransistorsasControlElements(3),Und
7、erreversebias,thep-njunctiondiodeischaracterizedbyasmallsaturationcurrent,arisingfromthecollectionofminoritycarriersfromtheirrespectiveregionsbytheelectricfieldinthespace-chargelayer.Themagnitudeofthisreversesaturationcurrentdependsontheavailableconcentrationofminoritycarriers,andissmallinthediode.,
8、Thebipolartransistorworksasacontrolelementbycombininginjectionatoneofitsp-njunctionswithcollectionattheotherp-njunction.,3.1BipolarTransistorsasControlElements(4),Inthenormalgainortheactivegainregion放大区ofoperation,theemitter-basejunctionismaintainedinforwardbias,andthecollector-basejunctionisheldinr
9、eversebias.Bydopingtheemitterregionmuchmoreheavilythanthebase,mostoftheinjectionofminoritycarriersismadetooccurintothebasesideofthejunction.Thus,underforwardbiasconditions,thereisalargebuildupofminoritycarriers(electronsinanNPNtransistor,holesinaPNPtransistor)onthebasesideoftheemitter-basejunction.W
10、hilethisbuildupofelectronconcentrationistakingplaceattheemitterendofthebaseregion,thereverse-biasedcollector-basejunctionkeepstheconcentrationofminorityelectronsverylowatthecollectorendofthebaseregion.Thiscombinationofaforwardbiasattheemitter-basejunctionandareversebiasatthecollector-basejunctionthu
11、sestablishesalargeconcentrationgradientofminoritycarriersacrossthebaseregion.Normalthermalmotions,therefore,produceadiffusiveflowofminorityelectronsthroughthebaseregion,fromtheemitterend,wheretheyareinexcess,tothecollectorend,wheretheyaresweptacrossthecollector-basejunctionintothecollectorregion.,3.
12、1BipolarTransistorsasControlElements(5),3.1BipolarTransistorsasControlElements(6),Abovefigureillustratesthisflowofelectronsfromtheemittertothebasebyinjection注入,acrossthebasebydiffusion扩散,andintothecollectorbycollection收集.Nearlyalltheelectronsenteringthebaseregionfromtheemitterreachthecollector.Asmal
13、lfraction,however,recombine复合withholestoformcompletecovalentbonds.Becauseofthisrecombinationprocess,andbecauseoftheinjectionofholesfromthebasetotheemitter,someholesmustbesuppliedtothebaseviathebaseterminal.Theseholescannotbesuppliedfromthecollectorbecausetheyaretheminoritycarrierthere,andarefewinnum
14、bers.Insummary,thecollector-basejunctionbehavesasareverse-biaseddiodewhosesaturationcurrentiscontrolledbytheinjectionofelectronsattheemitter-basejunction.Thecollectorcurrentisindependentofthecollector-basevoltage,providedthatareversebiasisestablishedatthecollector-basejunction.Thus,thebasicpropertyo
15、facontrolelement,inthiscasethedependenceoftheoutput(collector)currentonaninputvariable(emittercurrentoremitter-basevoltage)hasbeendemonstrated.,3.1BipolarTransistorsasControlElements(7),3.1.2CircuitSymbolsandTerminalVariablesforBipolarTransistors,1.NPNbipolartransistor,2.PNPbipolartransistor,Theonly
16、differencebetweenNPNandPNPdevicesymbolsisthedirectionofthearrowontheemitterlead,whichindicatestheactualdirectionofforwardcurrentintheemitter-basejunction.Thereferencedirectionsfortheterminalcurrentsarealldefinedenteringthedevice,irrespectiveofthedirectionofpositivecurrentflow.Thusatleastoneoftheterminalscurrentmustbealgebraicallynegative.,Thevoltagesubscriptsdenotetheterminalsbetweenwhichthevoltageismeasured,withthefirstsubscriptindicatingthepositivereferenceter
