6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 18-1Lecture 18 - The Bipolar Junction Transistor(II)Regimes of OperationNovember 10, 2005Contents:1. Regimes of operation.2. Large-signal equivalent circuit model.3. Output characteristics.Reading assignment:Howe and Sodini, Ch. 7, §§7.3, 7.4Announcements: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 18-2Key questions• What other regimes of operation are there for theBJT?• What is unique about each regime?• How do equivalent circuit models for the BJT looklike?6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 18-31. Regimes of operationsaturationreversecut-offforwardactiveVBCVBCVCEVBEVBEBCE+-+-+-• forward active: device has good isolation and highgain; most useful regime;• saturation: device has no isolation and is flooded withminority carriers ⇒ takes time to get out of satura-tion; avoid• reverse: poor gain; not useful;• cut-off : negligible current: nearly an open circuit;useful.6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 18-42 Forward-active regime: VBE> 0, VBC< 0n-Emitterp-Basen-CollectorIE<0IB>0IC>0VBE > 0VBC < 0Minority carrier profiles ( not to scale):npBpnEpnCnpBopnEopnCo0WB-XBEWB+XBC-WE-XBEWB+XBC+WCxemitter basecollector6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 18-5• Emitter injects electrons into base, collector collectselectrons from base:IC= ISexpqVBEkT• Base injects holes into emitter, recombine at emittercontact:IB=ISβF(expqVBEkT− 1)• Emitter current:IE= −IC− IB= −ISexpqVBEkT−ISβF(expqVBEkT− 1)• State-of-the-art IC BJT’s today: IC∼ 0.1 − 1 mA,βF' 50 − 300.• βFhard to control tightly ⇒ circuit design techniquesrequired to be insensitive to variations in βF.6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 18-62 Reverse regime: VBE< 0, VBC> 0n-Emitterp-Basen-CollectorIE>0IB>0IC<0VBE < 0VBC > 0Minority carrier profiles:npBpnEpnCnpBopnEopnCo0WB-XBEWB+XBC-WE-XBEWB+XBC+WCxemitter basecollector6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 18-7• Collector injects electrons into base, emitter collectselectrons from base:IE= ISexpqVBCkT• Base injects holes into collector, recombine at collectorcontact and buried layer:IB=ISβR(expqVBCkT− 1)• Collector current:IC= −IE− IB= −ISexpqVBCkT−ISβR(expqVBCkT− 1)• Typically, βR' 0.1 − 5 βF.IBACEBBCIEAEEBBC6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 18-8Forward-active Gummel plot (VCE=3V ):Reverse Gummel (VEC=3V ):6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 18-92 Cut-off: VBE< 0, VBC< 0n-Emitterp-Basen-CollectorIE>0IB<0IC>0VBE < 0VBC < 0Minority carrier profiles:npBpnEpnCnpBopnEopnCo0WB-XBEWB+XBC-WE-XBEWB+XBC+WCxemitter basecollector6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 18-10• Base extracts holes from emitter:IB1= −ISβF= −IE• Base extracts holes from collector:IB2= −ISβR= −IC• These are tiny leakage currents (∼ 10−12A).6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 18-112 Saturation: VBE> 0, VBC> 0n-Emitterp-Basen-CollectorIEIB<0ICVBE > 0VBC > 0Minority carrier profiles:npBpnEpnCnpBopnEopnCo0WB-XBEWB+XBC-WE-XBEWB+XBC+WCxemitter basecollector6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 18-12Saturation is superposition of forward active + reverse:IC= IS(expqVBEkT− expqVBCkT) −ISβR(expqVBCkT− 1)IB=ISβF(expqVBEkT− 1) +ISβR(expqVBCkT− 1)IE= −ISβF(expqVBEkT− 1) − IS(expqVBEkT− expqVBCkT)• ICand IEcan have either sign, depending on relativemagnitude of VBEand VBC, and βFand βR.• In saturation, collector and base floo ded with excessminority carriers ⇒ takes lots of time to get transistorout of saturation.EBBCholeselectrons6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 18-132. Large-signal equivalent circuit modelSystem of equations that describes BJT operation:IC= IS(expqVBEkT− expqVBCkT) −ISβR(expqVBCkT− 1)IB=ISβF(expqVBEkT− 1) +ISβR(expqVBCkT− 1)IE= −ISβF(expqVBEkT− 1) − IS(expqVBEkT− expqVBCkT)Equivalent-circuit model representation:Non-Linear Hybrid- π ModelBCEIS(exp - exp ) qVBEkTISβFqVBE(exp kT-1)ISβRqVBC(exp kT-1)qVBCkTThree parameters in this model: IS, βF, and βR.Model equivalent to Ebers-Moll model in text.6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 18-14Simplifications of equivalent-circuit model:• Forward-active regime: VBE> 0, VBC< 0BCEBCEVBE,onIBISβFqVBE(exp kT-1)ISexp qVBEkTβFIBFor today’s technology: VBE,on' 0.7 V .IBdepends on outside circuit.• Reverse: VBE< 0, VBC> 0BCEEBCVBC,onISβRqVBC(exp kT-1)ISexp qVBCkTIBβRIBFor today’s technology: VBC,on' 0.5 V .IBalso depends on outside circuit.6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 18-15IBvs. VBEfor VCE=3V :IBvs. VBCfor VEC=3V :6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 18-16• Saturation: VBE> 0, VBC> 0BCEBCEVBE,onVBE,onVCE,satVBC,onVBC,onBCE+-Today’s technology: VCE,sat= VBE,on− VBC,on' 0.2 V .IBand ICdepend on outside circuit.• Cut-off: VBE< 0, VBC< 0BCEOnly negligible leakage currents.6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 18-173. Output characteristicsFirst, ICvs. VCBwith IBas parameter:VCBVBC,onICIBIB=000Next, common-emitter output characteristics(ICvs. VCEwith IBas parameter):VCE=VCB+VBEVCE,satICIBIB=0006.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 18-18ICvs. VCBfor 0 ≤ IB≤ 100 µA:ICvs. VCEfor 0 ≤ IB≤ 100 µA:6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 18-19ICvs. VCEfor 0 ≤ IB≤ 100 µA:6.012 - Microelectronic Devices and Circuits - Fall 2005 Lecture 18-20Key conclusions• Forward-active regime: most useful, device has gainand isolation. For bias calculations:BCEVBE,onIBβFIB• Saturation: device flooded with minority carriers. Notuseful. For bias calculations:VBE,onVCE,satVBC,onBCE+-• Cut-off: device open. Useful. For bias
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