BJT Intro And Large Signal Model - Penn Engineering

ESE319 Introduction to MicroelectronicsBJT Intro and Large Signal Model2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)1

ESE319 Introduction to MicroelectronicsWhy BJT?What's the competition to BJT and bipolar technologies?What advantages does the competition have over BJT?What advantages does BJT and bipolar have over their competition?What circuit applications benefit from BJT and bipolar technologies?2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)2

ESE319 Introduction to MicroelectronicsWhy BJTWhat's the competition to BJT and bipolar technologies?MOSFET, in particular CMOS is the leading competitorWhat advantages does the competition have over BJT?Small size (die area), low cost and low power dissipationWhat advantages does BJT and bipolar have over their competition?High frequency operation, high current drive, high reliability insevere environmental conditions.What circuit applications benefit from BJT and bipolar technologies?RF analog and digital circuits,power electronics and power amplifiers, automobile electronics, radiation hardened electronics2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)3

ESE319 Introduction to MicroelectronicsBJT Physical ConfigurationNPNPNPCloser to actual layoutEach transistor looks liketwo back-to-back diodes,but each behaves much differently!2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)4

ESE319 Introduction to MicroelectronicsNPNBJT Symbols and ConventionsPNPNote reversal in current directions and voltage signs for PNP vs. NPN!2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)5

ESE319 Introduction to MicroelectronicsNPN BJT Modes of OperationForward-Active ModeEBJ forward bias (VBE 0)CBJ reverse bias (VBC 0)iE iC iBvCE vCB vBEModeForward-ActiveVBEVBC 0 0Reverse-Active 0 0 0 0 0 0CutoffSaturation2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)VBC -VCBNot Useful!6

ESE319 Introduction to MicroelectronicsNPN BJT Forward-Active Mode Basic ModelCollector-base diodeis reverse biasedV CB 0( or VBC 0)Base-emitter diode isforward biasedsaturation currentV BE 0.7AEWNADnniA E q D n niI s N AW2Area of base-emitter junctionWidth of base regionDoping concentration in baseElectron diffusion constantIntrinsic carrier concentration f(T)iC I S ev BEVTVT kTo 25 mV @ 25 CqiCi B i E i B i C 1 i B common emitter current gain2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)7

ESE319 Introduction to MicroelectronicsNote that the iC equation looks like that of a forward-biaseddiode.Is it?2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)8

ESE319 Introduction to MicroelectronicsNote that the iC equation looks like that of a forward-biaseddiode. Is it?v BEVTi C I S e 1 By using the other two equations the question can be answered 11i E i B i C 1 i C iC common emitter current gain common base current gainand write:vv 11VVi E I S e 1 I S e 1 BEBETTAhHa! This iE equation describes a forward-biased emitter-base“diode”.2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)9

ESE319 Introduction to MicroelectronicsSo the new set ofequations is:v BEVTISi E e 1 v BEVTi C I S e 1 i EiCi B A E q D n ni 2I s N AWWhere: 1Typically:50 20010 18 I S 10 12 A.Is is strongly temperaturedependent, doubling for a5 degree Celsius increasein ambient temperature!2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)10

ESE319 Introduction to MicroelectronicsTwo equivalent large signal circuit models forthe forward-active mode NPN BJT:Nonlinear VCCSNonlinear CCCSKey Eqs. Fv BEVTiC I S e F i EISi E e FV BEVT2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)11

ESE319 Introduction to MicroelectronicsYet another NPN BJT large signal modeli C i B I S ev BEVTIS iB e v BEVTiBThis “looks like” a diodebetween base and emitterand the equivalent circuitbecomes:iCiENote that in this model, the diode current is represented interms of the base current. In the previous ones, it was represented in terms of the emitter current.2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)12

ESE319 Introduction to MicroelectronicsNPN BJT Operating in the Reverse-Active ModeRecall for NPN Reverse-Active Mode VBE 0 & VBC 0 We also have transistor action if we: Forward bias the base-collector junction and Reverse bias the base emitter junctionThe physical construction of the transistor, however, leads to betas on the order of 0.01 to 1 andcorrespondingly smaller values of alpha, e. g.: R0.1 R 0.091 R 1 1.1for R 0.12008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)13

ESE319 Introduction to MicroelectronicsThe equivalent large signal circuit model forthe reverse-active mode NPN BJT:RVRSActiveFWDActiveiCSat. or Scale Current Eq. F I SE R I SC I SiBiEBJT is non-symmetricalKey Eqs. I SiC e Ri E I S ev BCVTv BCVT2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL) R F R FAC A E14

ESE319 Introduction to MicroelectronicsThe Ebers-Moll Large Signal ModelThe E-M model combinesthe FWD & RVRS Activeequivalent circuits:Note that the lower leftdiode and the upper rightcontrolled current sourceform the forward-activemode model, while the upper left diode and the lowerright source represent thereverse-active mode model.i C F i DE i DCi E i DE R i DCi b 1 F I DE 1 R I DC2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)i DE I SE ev BEVT 1 v BCVTi DC I SC e 1 F I SE R I SC I S15

ESE319 Introduction to MicroelectronicsOperation in the Saturation ModeRecall for Saturation Mode VBE 0 & VBC 0 (or VCB 0)Consider the E-M model for collector current. We willinclude the “-IS” term in the ideal diode equation.i C F i DE i DCv BEVTISi C I S e 1 e Rv BCVT 1 The first term is the forwardv mode collector current:BEVT F i DE I S e 1 The second is the reverse mode collector current:vIS Vi DC e 1 RBCT2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)16

ESE319 Introduction to MicroelectronicsCombining terms:Using typical values:v BEVTv BCIS ViC I S e 1 e 1 R[i C ev BEVT R 1 1 e RWe obtain:i C [ e40 v BE R 0.1T v CBVT1 R 1 R R 40 v CB 1 11 e] 1 I SI S 10 14A.V T 0.025 V. 1 ] 10 14Let's plot iC vs. vBC (or vCB) with vBE 0.7 V2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)17

ESE319 Introduction to MicroelectronicsScilab Saturation Mode Calculation//Calculate and plot npn BJT collector//current in saturation modevBE 0.7;VsubT 0.025;VTinv 1/VsubT;betaR 0.1;IsubS 1E-14;alphaR betaR/(betaR 1);alphaInv 1/alphaR;ForwardExp exp(VTinv*vBE)-1;vCB -0.7:0.001:-0.1;vBC -vCB;ReverseExp alphaInv*(exp(VTinv*vBC)-1);iC (ForwardExp-ReverseExp)*IsubS;signiC sign(iC);iCplus (iC signiC.*iC)/2; //Zero negative valuesplot(vCB,1000*iCplus); //Current in mA.2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)18

ESE319 Introduction to MicroelectronicsSaturation Mode PlotForward-activeRecall for Sat.ModeVBE 0&VBC 0(or VCB 0)SaturationA2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)19

ESE319 Introduction to MicroelectronicspnpnpnThe PNP TransistorModeForward-ActiveVEBVCB 0 0Reverse-Active 0 0 0 0 0 0CutoffSaturation2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)VCB -VBCNot Useful!20

ESE319 Introduction to MicroelectronicsPNP BJT Forward-Active Mode Basic ModelCollector-base diodeis reverse biasedV BC 0Emitter-base diode isforward biasedV EB 0.7Note reversal in voltagepolarity and in currentdirections!i C I S eiCi B v EBVT 1 i E i B i C 1 i B2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)21

ESE319 Introduction to MicroelectronicsPNP BJT Large Signal ModelFWD. Activei C I S eiCi B v EBVT 1 i E i B i C 1 i BSubstituting, as in thenpn case, we get:Note reversal in currentdirections!2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)v EBIS Vi E e 1 T22

ESE319 Introduction to MicroelectronicsYet another PNP BJT large signal modelv EBVTv EBVTISISi C i B I S e 1 i B e 1 e v EBVTThis “looks like” a diodebetween base and emitterand the equivalent circuitbecomes:iBiCAgain, in this model, the diode carries only base current,not emitter current.2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)23

ESE319 Introduction to MicroelectronicsScilab Plot of NPN Characteristic(iC vs. vCE and vBE)//Calculate and plot npn BJT collector//characteristic using Ebers-Moll modelVsubT 0.025;VTinv 1/VsubT;betaR 0.1;alphaInv (betaR 1)/betaR;IsubS 1E-14;for vBE 0.6:0.02:0.68ForwardExp exp(VTinv*vBE)-1;vCE -0:0.001:10;vBC vBE-vCE;ReverseExp alphaInv*(exp(VTinv*vBC)-1);iC (ForwardExp-ReverseExp)*IsubS;signiC sign(iC);iCplus (iC signiC.*iC)/2; //Zero negative valsplot(vCE,1000*iCplus); //Current in mA.end2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)i C F i DE i DCwherev BEVTi DE I SE e 1 i DC I SC ev BCVT 1 v BC v BE v CENo Early effect24

ESE319 Introduction to MicroelectronicsPlot Outputsaturation modeforward-active modev BE 0.68 VEarly effectnot included.v BE 0.66 Vv BE 0.64 V2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)25

ESE319 Introduction to MicroelectronicsMore on NPN SaturationThe base-collector diode has much largerarea than the base-emitter one. Therefore, with the same applied voltage, it willconduct a much larger forward current than willthe base-emitter diode.When the collector-emitter voltage drops belowthe base-emitter voltage, the base-collectordiode is forward biased and conducts heavily.v CB v CE v BEwhenv CE V CE sat 2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)26

ESE319 Introduction to MicroelectronicsIC Expansion Around Zero VCEIC (mA)IC 0VCE (V)Note that the collector current is zero at about VCE 0.06 V., not 0 V.!Also note the large reverse collector-base current below 0.06 V.2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)27

ESE319 Introduction to MicroelectronicsVoltageatZeroCollectorCurrentvvIS ViC I S e 1 e 1 RBEBCVTTv BEVT R e 1 e R 1 ev BEVT e v BE vCE VT v CEVTV BE 40 0.7 eVT IC 0 vvIS VV e 1 I S e 1 R R eBCBETTv BEVT 1 ev BCVT R evCEVT 1 ev BEVTv BEVT 1 vCE V T ln R v CE 0.025 ln 1 2008 by Kenneth R. Laker (based on P.V. Lopresti 2006) update 09Sep08 KRL)0.1 0.0599V.0.1 128

NPN BJT Forward-Active Mode Basic Model Collector-base diode is reverse biased V CB 0 Base-emitter diode is forward biased V BE 0.7 i C I S e v BE V T i B i C i E i B i C 1 i B V T kT q I 25m @ oC s A E qD n n i 2 N A W Area of base-emitter junction Width of base region Doping concentration in base Electron diffusion constant