6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 17-1Lecture 17 - The Bipolar Junction Transistor(I)Forward Active RegimeNovember 8, 2005Contents:1. BJT: structure and basic operation2. I-V characteristics in forward active regimeReading assignment:Howe and Sodini, Ch. 7, §§7.1, 7.2Announcements:Quiz 2: 11/16, 7:30-9:30 PM,open book, mustbring calculator; lectures #10-18.6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 17-2Key questions• What does a bipolar junction transistor look like?• How does a bipolar junction transistor operate?• What are the leading dependencies of the terminalcurrents of a BJT in the forward active regime?6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 17-31. BJT: structure and basic operationemittercontactbase contactbase contactcollectorcontactn+ emitterp basen collectorn+ buried layern+ plugbase-emitter junction (area AE) emitter-stripe widthemitter-stripe lengthbase-collector junctioncollector-substrate junctionp substrate"intrinsic" BJTUniqueness of BJT: high-current drivability per input ca-pacitance ⇒ fast ⇒ excellent for analog and front-endcommunications applications.6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 17-4Simplified one-dimensional model of intrinsic device:EmitterBase CollectorIEIBICnpnNaBWB-XBEWB+XBCWB+XBC+WC-WE-XBENdEVBEVBCNdC+-+- x0BJT = two neighbouring pn junctions back-to-back:• close enough for minority carriers to interact(can diffuse quickly through base)• far apart enough for depletion regions not to interact(prevent ”punchthrough”)Regimes of operation:saturationreversecut-offforwardactiveVBCVBCVCEVBEICVBE+-+-+-collectoremitterbaseIEIB6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 17-5Basic operation in forward-active regime:n-Emitterp-Basen-CollectorIE<0IB>0IC>0VBE > 0VBC < 0VBE> 0 ⇒ injection of electrons from E to Binjection of holes from B to EVBC< 0 ⇒ extraction of electrons from B to Cextraction of holes from C to BTransistor effect:electrons injected from E to B, extracted by C!6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 17-6• Carrier profiles in thermal equilibrium:log po, nopopononoNdENdCni2NdEni2NaBni2NdCx0NaBWB-XBEWB+XBC-WE-XBEWB+XBC+WCx• Carrier profiles in forward-active regime:log p, nnNdENdCni2NdEni2NaBni2NdCx0NaBWB-XBEWB+XBC-WE-XBEpWB+XBC+WCx6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 17-7Dominant current paths in forward active regime:n-Emitterp-Basen-CollectorIE<0IB>0IC>0VBE > 0VBC < 0IC: electron injection from E to B and collection into CIB: hole injection from B to EIE= −IC− IBKey dependencies (choose one):ICon VBE: eqVBE/kT,1/√VBE, none, otherICon VBC: eqVBC/kT,1/√VBC, none, otherIBon VBE: eqVBE/kT,1/√VBE, none, otherIBon VBC: eqVBC/kT,1/√VBC, none, otherICon IB: exponential, quadratic, none, other6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 17-8In forward-active regime:• VBEcontrols IC(”transistor effect”)• ICindependent of VBC(”isolation”)• price to pay for control: IBComparison with MOSFET:ideal MOSFET ideal BJTfeature in saturation in FARcontrolling terminal gate basecommon terminal source emittercontrolled terminal drain collectorfunctional dependenceof controlled current quadratic exponentialDC current incontrolling terminal 0 exponentialFigure of merit for BJT:-common-emitter current gain:βF=ICIB(want big enough, ' 100)6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 17-92. I-V characteristics in forward active regime2 Collector current: focus on electron diffusion in basenx0npB(0)JnBnpB(WB)=0npB(x)WBni2NaBBoundary conditions:npB(0) = npBoexpqVBEkT,npB(WB)=0Electron profile:npB(x)=npB(0)(1 −xWB)6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 17-10Electron current density:JnB= qDndnpBdx= −qDnnpB(0)WBCollector current scales with area of base-emitter junctionAE:IC AEEBBCCollector terminal current:IC= −JnBAE= qAEDnWBnpBoexpqVBEkTorIC= ISexpqVBEkTIS≡ collector saturation current [A]6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 17-112 Base current: focus on hole injection and recombina-tion in emitterpx-xBEpnE(x)pnE(-xBE)-WE-xBEni2NdEpnE(-WE-xBE)=ni2NdEBoundary conditions:pnE(−xBE)=pnEoexpqVBEkT,pnE(−WE−xBE)=pnEoHole profile:pnE(x)=[pnE(−xBE) − pnEo](1 +x + xBEWE)+pnEo6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 17-12Hole current density:JpE= −qDpdpnEdx= −qDppnE(−xBE) − pnEoWEBase current scales with area of base-emitter junction AE:IB AEEBBCBase terminal current:IB= −JpEAE= qAEDpWEpnEo(expqVBEkT− 1)Then:IB=ISβF(expqVBEkT− 1)For VBEkTq:IB'ICβF6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 17-13Gummel plot: semilog plot of ICand IBvs. VBE:VBElog IC, IBISICIBISβF60 mV/dec at 300 KVBC<06.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 17-142 Current gain:βF=ICIB=npBoDnWBpnEoDpWE=NdEDnWENaBDpWBTo maximize βF:• NdE NaB• WE WB• want npn, rather than pnp design because Dn>DpState-of-the-art IC BJT’s today: IC∼ 0.1−1 mA, βF∼50 − 300βFhard to control in manufacturing environment ⇒ needcircuit techniques that are insensitive to variations in βF6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 17-15βFdependence on IC:log ICβF6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 17-16Gummel plot of BJT (VCE=3V ):6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 17-17Key conclusionsnpn BJT in forward active regime:n-Emitterp-Basen-CollectorIE<0IB>0IC>0VBE > 0VBC < 0• Emitter ”injects” electrons into Base,Collector ”collects” electrons from Base.⇒ ICcontrolled by VBE, independent of VBC(transistor effect)IC∝ expqVBEkT• Base injects holes into Emitter ⇒ IBIB∝
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