Lecture #25Base-Width ModulationSlide 3Slide 4Early Voltage, VADerivation of Formula for VASlide 7BJT Breakdown MechanismsPunch-ThroughAvalanche MultiplicationNon-Ideal Effects at Low VEBNon-Ideal Effects at High VEBGummel Plot and bdc vs. ICGummel NumbersSlide 15Summary: BJT Performance RequirementsReview: Modes of OperationEE130 Lecture 25, Slide 1Spring 2007Lecture #25OUTLINE •BJT: Deviations from the Ideal–Base-width modulation, Early voltage–Punch-through–Non-ideal effects at low |VEB|, high |VEB|•Gummel plotReading: Chapter 11.2Measured BJTCommon-EmitterOutput Characteristics:EE130 Lecture 25, Slide 2Spring 2007 WNDnLNDnIIBEiEEEBiBLWLWNNDDnndcBCBEEBBEiBiE22221 122Base-Width ModulationP+ N PWW(VBC)xpB(x) 1/0kTqVBEBep(VCB=0)0+VEBIEICCommon-Emitter Configuration, Active Mode OperationVECICEE130 Lecture 25, Slide 3Spring 2007EE130 Lecture 25, Slide 4Spring 2007The base-width modulation effect is reduced if we(a) increase the base width, W, or(b) increase the base dopant concentration, NB, or(c) decrease the collector dopant concentration, NC .Which of the above is the most acceptable action?EE130 Lecture 25, Slide 5Spring 2007Output resistance:CAECCIVVIr 10A large VA (i.e. a large ro ) is desirableIB3ICVEC0IB2IB1Early Voltage, VAVAEE130 Lecture 25, Slide 6Spring 2007Derivation of Formula for VA00 gIVVIdVdIgCAACECCOutput conductance:for fixed VEBBCCECCoBCEBECdVdIdVdIgVVV so --BCnCCBCCodVdxdWdIdVdWdWdIgwhere xnC is the width of the collector-junction depletion region on the base sideP+ N PxnCEE130 Lecture 25, Slide 7Spring 2007 WIeNWDqAndWdIeWNDqAnICkTqVBBiCkTqVBBiCEBEB11/22/2BJCBCnCBCnCBBCnCBBCdepCJCqNCdVdxdVdxqNdVxqNddVdQC)( 1)1(/)/sinh(/cosh(00/)/sinh(10)kTqVLWLWBLDCLDkTqVLWBLDCCBBBBBCCEBBBBepnepqAIJCBBJCCCBCnCCCCACWqNqNCWIIdVdxdWdIIgIV --0EE130 Lecture 25, Slide 8Spring 2007BJT Breakdown Mechanisms•In the common-emitter configuration, for high output voltage VCE, the output current IC will increase rapidly due to one of two mechanisms:–punch-through–avalancheEE130 Lecture 25, Slide 9Spring 2007Punch-ThroughE-B and E-B depletion regions in the base touch, so that W = 0As |VCB| increases, the potential barrierto hole injection decreases and thereforeIC increasesEE130 Lecture 25, Slide 10Spring 2007Avalanche Multiplication•Holes are injected into the base [0], then collected by the B-C junction–Some holes in the B-C depletion region have enough energy to generate EHP [1]•The generated electrons are swept into the base [3], then injected into the emitter [4]–Each injected electron results in the injection of IEp/IEn holes from the emitter into the base [0]PNP BJT: For each EHP created in the C-B depletion region by impact ionization, (IEp/IEn)+1 > dc additional holes flow into the collector i.e. carrier multiplication in C-B depletion region is internally amplifiedmdcCBCEVV/100)1( where VCB0 = reverse breakdown voltage of the C-B junction62 mEE130 Lecture 25, Slide 11Spring 2007Non-Ideal Effects at Low VEB•In the ideal transistor analysis, thermal R-G currents in the emitter and collector junctions were neglected.•Under active-mode operation with small VEB, the thermal recombination current is likely to be a dominant component of the base current low emitter efficiency, hence lower gainThis limits the application of the BJT for amplification at low voltages.GREnEpEpIIIIEE130 Lecture 25, Slide 12Spring 2007Non-Ideal Effects at High VEB•Decrease in F at high IC is caused by:–high-level injection–series resistance–current crowding 1/2kTqVBBiCEBeWNDqAnIEE130 Lecture 25, Slide 13Spring 2007dcFrom top to bottom:VBC = 2V, 1V, 0V0.2 0.4 0.6 0.8 1.0 1.210-1210-1010-810-610-410-2 VBE IBICexcess base current due to R-G in depletion regionhigh level injection in baseGummel Plot and dc vs. ICEE130 Lecture 25, Slide 14Spring 2007Gummel NumbersFor a uniformly doped base with negligible band-gap narrowing, the base Gummel number isBBBDWNG (= total integrated “dose” (#/cm2) of majority carriers in the base, divided by DB)EBWWNNDDnGGEEBBEBiEin111122Emitter efficiency 11/2/2kTqVBikTqVBBiCEBEBeGqAneWNDqAnIGE is the emitter Gummel numberEE130 Lecture 25, Slide 15Spring 2007dxxDxNnnGBBWBiiB)()(022 BELWLWNNDDndcGGBEEBBEBiEin221221Notice thatIn real BJTs, NB and NE are not uniform, i.e. they are functions of x The more general formulas for the Gummel numbers aredxxDxNnnGEEWEiiE)()(022EE130 Lecture 25, Slide 16Spring 2007•High gain (dc >> 1) One-sided emitter junction, so emitter efficiency 1•Emitter doped much more heavily than base (NE >> NB) Narrow base, so base transport factor T 1•Quasi-neutral base width << minority-carrier diffusion length (W << LB)•IC determined only by IB (IC function of VCE,VCB) One-sided collector junction, so quasi-neutral base width W does not change drastically with changes in VCE (VCB)•Based doped more heavily than collector (NB > NC)(W = WB – xnEB – xnCB for PNP BJT)Summary: BJT Performance RequirementsEE130 Lecture 25, Slide 17Spring 2007Review: Modes of OperationCommon-emitter output characteristics (IC vs. VCE) Why?operation. mode active invertedfor lower is BCdcIIβ
View Full Document
Unlocking...