BJT Intro and Large Signal Model
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ESE319 Introduction to Microelectronics BJT Intro and Large Signal Model 2010 by Kenneth R. Laker, update 15Sep10 KRL 1 ESE319 Introduction to Microelectronics VLSI Chip Manufacturing Process 2010 by Kenneth R. Laker, update 15Sep10 KRL 2 ESE319 Introduction to Microelectronics 0.35 mm SiGe BiCMOS Layout for RF (3.5 GHz) Two-Stage Power Amplifier Each transistor above is realized as net of four heterojunction bipolar transistors (HBT) 2010 by Kenneth R. Laker, update 15Sep10 KRL 3 ESE319 Introduction to Microelectronics Why BJT? What's the competition to BJT and bipolar technologies? What advantages does the competition have over BJT? What advantages does BJT and bipolar technologies have over their competition? What circuit applications benefit from BJT and bipolar technologies? 2010 by Kenneth R. Laker, update 15Sep10 KRL 4 ESE319 Introduction to Microelectronics Why BJT What's the competition to BJT and bipolar technologies? MOSFET, in particular CMOS is the leading competitor What advantages does the competition have over BJT? Small size (die area), low cost and low power dissipation What advantages does BJT and bipolar technologies have over their competition? High frequency operation, high current drive, high reliability in severe 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 electronics. 2010 by Kenneth R. Laker, update 15Sep10 KRL 5 ESE319 Introduction to Microelectronics BJT Physical Configuration NPN PNP Closer to actual layout Each transistor looks like two back-to-back diodes, but each behaves much differently! 2010 by Kenneth R. Laker, update 15Sep10 KRL 6 ESE319 Introduction to Microelectronics NPN BJT Symbols and Conventions PNP Metal contact IC IE IE IB IE = IC + IB IC IB Note reversal in current directions and voltage signs for PNP vs. NPN! 2010 by Kenneth R. Laker, update 15Sep10 KRL 7 ESE319 Introduction to Microelectronics NPN BJT Modes of Operation i E = iC + iB vCE = vCB + vBE vXY = VXY + vxy large signal dc bias ac signal Forward-Active Mode EBJ forward bias (VBE > 0) CBJ reverse bias (VBC < 0) VCB VBE Mode Forward-Active VBE VBC >0 <0 Reverse-Active <0 <0 >0 >0 <0 >0 Cutoff Saturation 2010 by Kenneth R. Laker, update 15Sep10 KRL VBC = -VCB Not Useful! 8 ESE319 Introduction to Microelectronics NPN BJT Modes of Operation VBE Saturation region iC iC VCE -VA 0 2010 by Kenneth R. Laker, update 15Sep10 KRL Forward-Active region vBE4 vBE3 vBE2 vBE1 vCE 9 ESE319 Introduction to Microelectronics NPN BJT Forward-Active Current Flow Forward-biased p n E iE iE iE Reverse-biased n Injected Injected Collected i C electrons electrons electrons Injected holes (iB1) Recombined electrons (iB2) iB i B =i B1 i B2 i vBE VBE 2010 by Kenneth R. Laker, update 15Sep10 KRL B iE iC B i E =i C i B vCB C iC iC VCB 10 ESE319 Introduction to Microelectronics NPN BJT Forward-Active Mode Basic Model Collector-base diode is reverse biased V CB0 ( or VBC < 0) 1 Base-emitter diode is forward biased saturation current V BE ≈0.7 AE W NA Dn ni A E q D n ni I s= N AW 2 Area of base-emitter junction Width of base region Doping concentration in base Electron diffusion constant Intrinsic carrier concentration = f(T) 2010 by Kenneth R. Laker, update 15Sep10 KRL iC ≈ I S e v BE VT VT= kT o ≈ 25 mV @ 25 C q iC i B= i E =i B i C =1i B =common−emitter current gain 11 ESE319 Introduction to Microelectronics NPN BJT Forward-Active Beta (β) AE -> Area of base-emitter junction W -> Width of base region NA -> Doping concentration in base ND -> Doping concentration in emitter Dn -> Electron diffusion constant Dp -> Hole diffusion constant Lp -> Hole diffusion length in emitter τb -> Minority-carrier lifetime ni -> Intrinsic carrier concentration = f(T) 2010 by Kenneth R. Laker, update 15Sep10 KRL 1 = Dp N A W 1 W 2 D n N D L p 2 D n b Large β => NA -> small ND -> large W -> small 12 ESE319 Introduction to Microelectronics Note that the iC equation looks like that of a forward-biased diode collector-base. Is it? Using Eqs. iE = iB + iC and iC = βiB we can answer this question, i.e. 1 1 i E =i B i C = 1i C = iC =common−emitter current gain =common−base current gain and write: v v iC 1 1 V V iE= I S e −1= I S e −1= = where 1 BE BE T T AhHa! This iE equation describes a forward-biased emitter-base “diode”. 2010 by Kenneth R. Laker, update 15Sep10 KRL 13 ESE319 Introduction to Microelectronics So the new set of equations are: v BE VT IS i E = e −1 v BE VT i C =I S e −1= i E iC i B= A E q D n ni 2 I s= N AW 2010 by Kenneth R. Laker, update 15Sep10 KRL Where: = 1 Typically: 50200 => 0.9800.995 10−18 I S 10−12 A. Is is strongly temperaturedependent, doubling for a 5 degree Celsius increase in ambient temperature! 14 ESE319 Introduction to Microelectronics Two equivalent large signal circuit models for the forward-active mode NPN BJT: v BE VT iC = I S e −1≈ I S e Nonlinear VCCS v BE VT Nonlinear CCCS Key Eqs. v BE VT = F i C ≈ I S e = F i E IS i E≈ e F I v BE VT 1 i B =i E −i C = i C SE 2010 by Kenneth R. Laker, update 15Sep10 KRL 15 ESE319 Introduction to Microelectronics Yet another NPN BJT large signal model i C = i B ≈ I S e v BE VT IS ⇒ iB≈ e v BE VT This “looks like” a diode between base and emitter and the equivalent circuit becomes: iB iC iB v BE IS V i B= e T iE Note that in this model, the diode current is represented in terms of the base current. In the previous ones, it was represented in terms of the emitter current. 2010 by Kenneth R. Laker, update 15Sep10 KRL 16 ESE319 Introduction to Microelectronics NPN BJT Operating in the Reverse-Active Mode Recall for NPN Reverse-Active Mode VBE < 0 & VBC > 0 Weak transistor action if we: ● Forward bias the base-collector junction and Reverse bias the base-emitter junction Collector and emitter reverse roles ● ● ● The physical construction of the transistor results ● Weak reverse-active performance ● ● ● Small values of β on the order of 0.01 to 1 Correspondingly smaller values of α, e. g. R 0.1 R= ≈ ≈0.091 R1 1.1 2010 by Kenneth R. Laker, update 15Sep10 KRL for R =0.1 17 ESE319 Introduction to Microelectronics The equivalent large signal circuit model for the reverse-active mode NPN BJT: FWD iB Active iC collector and emitter reverse roles RVRS Active iC iB iC iB iE iE Note that the directions of the reverse-active currents are the reverse of the forward-active currents; hence the “minus” signs. iE v BE VT i C ≈ I S e = F i E Key Eqs. −I S iC ≈ e R v BC VT i E ≈−I S e 2010 by Kenneth R. Laker, update 15Sep10 KRL =−I SC e v BC VT v BC VT BJT is non-symmetrical R ≪ F R ≪ F = R i C 18 ESE319 Introduction to Microelectronics The Ebers-Moll Large Signal Model The E-M model combines the FWD & RVRS Active equivalent circuits: Note that the lower left diode and the upper right controlled current source form the forward-active mode model, while the upper left diode and the lower right source represent the reverse-active mode model. v BE i C = F i DE −i DC i E =i DE − R i DC i B =1− F I DE 1− R I DC 2010 by Kenneth R. Laker, update 15Sep10 KRL IS V i DE = e −1 F T v BC IS V i DC = e −1 R T 19 ESE319 Introduction to Microelectronics Operation in the Saturation Mode Recall for Saturation Mode vBE > 0 & vBC > 0 (or vCB < 0) Consider the E-M model for collector current. i C = F i DE −i DC v BE VT v BC IS V i C = I S e −1− e −1 R T The first term is the forward mode collector current: v BE VT F i DE = I S e −1 The second is the reverse mode collector current: v IS V i DC = e −1 R BC T 2010 by Kenneth R. Laker, update 15Sep10 KRL 20 ESE319 Introduction to Microelectronics Combining terms: Using typical values: v BE VT v BC IS V iC = I S e −1− e −1 R [ i C = e v BE VT R1 −1− e R We obtain: i C =[ e 40 v BE R =0.1 T −v CB VT 1 R 1 = R R −40 v CB −1−11e ] −1 I S I S =10−14 A V T =0.025V −1 ] 10−14 Let's plot iC vs. vBC (or vCB) with vBE = 0.7 V 2010 by Kenneth R. Laker, update 15Sep10 KRL 21 ESE319 Introduction to Microelectronics Scilab Saturation Mode Calculation //Calculate and plot npn BJT collector //current in saturation mode vBE=0.7; VsubT=0.025; VTinv=1/VsubT; v BE v BC betaR=0.1; I V S V IsubS=1E-14; i C =I S e T −1− e T −1 R 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 values plot(vCB,1000*iCplus); //Current in mA. 2010 by Kenneth R. Laker, update 15Sep10 KRL 22 ESE319 Introduction to Microelectronics Saturation Mode Plot iC(ma) Note: forward-active NPN operation continues for negative vCB down to - 0.5V; i.e. vCB > - 0.5V Forward-active Recall for Sat. Mode vBE > 0 & vBC > 0 (or vCB < 0) Saturation A vBE = 0.7V vCB(V) 2010 by Kenneth R. Laker, update 15Sep10 KRL 23 ESE319 Introduction to Microelectronics Scilab Plot of NPN Characteristic (iC vs. vCE and vBE) //Calculate and plot npn BJT collector //characteristic using Ebers-Moll model VsubT=0.025; VTinv=1/VsubT; betaR=0.1; v BE v BC alphaInv=(betaR+1)/betaR; I S VT VT IsubS=1E-14; i C =I S e −1− e −1 R for vBE=0.6:0.02:0.68 ForwardExp=exp(VTinv*vBE)-1; vCE=-0:0.001:10; vCE = vCB + vBE vBC=vBE-vCE; ReverseExp=alphaInv*(exp(VTinv*vBC)-1); iC=(ForwardExp-ReverseExp)*IsubS; signiC=sign(iC); iCplus=(iC+signiC.*iC)/2; //Zero negative vals plot(vCE,1000*iCplus); //Current in mA. end 2010 by Kenneth R. Laker, update 15Sep10 KRL 24 ESE319 Introduction to Microelectronics Plot Output iC (ma) saturation mode forward-active mode v BE =0.68 V Early effect not included. v BE =0.66 V vCE = vCB + vBE v BE =0.64 V v BE =0.62 V vCE (V) @ start of saturation V CEs at =vCE =v CB v B E≈−0.4 V 0.7 V =0.3 V 2010 by Kenneth R. Laker, update 15Sep10 KRL 25 ESE319 Introduction to Microelectronics More on NPN Saturation The base-collector diode has much larger area than the base-emitter one. ● ● Therefore, with the same applied voltage, it will conduct a much larger forward current than will the base-emitter diode. v BE VT v BC VT IS i C = I S e −1− e −1 R ● where αR << 1 When the collector-emitter voltage drops below the base-emitter voltage, the base-collector diode is forward biased and conducts heavily. v CB =v CE −v BE 2010 by Kenneth R. Laker, update 15Sep10 KRL => v CE ≈V CEsat =v CB v BE ≈0.3 V 26 ESE319 Introduction to Microelectronics More on NPN Saturation - cont. In saturation the forward-biased the current through the collector-base junction increases iB and decreases iC as vBC increases. ● iC sat = forced = ≤ i B sat Test for saturation mode operation ● ● or ● vCE = VCEsat = 0.1 to 0.3 V => collector-base junction is forward biased Current ratio forward biased iC ≤ => collector-base junction is i B sat 2010 by Kenneth R. Laker, update 15Sep10 KRL 27 ESE319 Introduction to Microelectronics iC Expansion Around Zero vCE forward-active mode iC (mA) iC = 0 vBE = 0.68 V, vCE = 0.25 V, vBC= vBE - vCE= 0.43 V Diode forward voltage vCE (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 for vCE < 0.06 V. 2010 by Kenneth R. Laker, update 15Sep10 KRL 28 ESE319 Introduction to Microelectronics Voltage at Zero Collector Current v v IS V i C = I S e −1− e −1 R BE BC VT T R e v BE VT −1=e − R 1−e v BE VT v BE −vCE VT =e − v CE VT V BE ≈40∗0.7⇒ e VT − iC = 0 => v v IS V V e −1= I S e −1 R R e BC BE T T v BE VT −1=e v BC VT −1 − −e v BE VT v BE VT ≪1 For βR = 0.1 => αR = 0.09 => vCE = - 0.06 V −1 2010 by Kenneth R. Laker, update 15Sep10 KRL 29 ESE319 Introduction to Microelectronics pnp npn The PNP Transistor Mode Forward-Active VEB VCB >0 <0 Reverse-Active <0 <0 >0 >0 <0 >0 Cutoff Saturation 2010 by Kenneth R. Laker, update 15Sep10 KRL VCB = -VBC Not Useful! 30 ESE319 Introduction to Microelectronics PNP BJT Forward-Active Mode Basic Model Collector-base diode is reverse biased V BC 0 Emitter-base diode is forward biased V EB≈0.7 Note reversal in voltage polarity and in current directions! 2010 by Kenneth R. Laker, update 15Sep10 KRL i C =I S e iC i B= v EB VT −1 i E =i B i C =1i B 31 ESE319 Introduction to Microelectronics PNP BJT Large Signal Model FWD. Active NPN rotate 180o v EB VT PNP i C =I S e −1 iC i B= iE i E =i B i C =1i B Note reversal in current directions! 2010 by Kenneth R. Laker, update 15Sep10 KRL Substituting, as in the npn case, we get: v EB IS V i E = e −1 T 32 ESE319 Introduction to Microelectronics Yet another PNP BJT large signal model v EB VT v EB VT IS IS i C = i B = I S e −1⇒ i B = e −1≈ e This “looks like” a diode between base and emitter and the equivalent circuit becomes: v EB VT iB iC Again, in this model, the diode carries only base current, not emitter current. 2010 by Kenneth R. Laker, update 15Sep10 KRL 33
