ESE319 Introduction to Microelectronics BJT Intro and Large Signal Model

Transcription

1 BJT Intro and Large Signal Model 1

2 VLSI Chip Manufacturing Process 2

3 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) 3

4 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? 4

5 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. 5

6 BJT Physical Configuration NPN PNP Closer to actual layout Each transistor looks like two back-to-back diodes, but each behaves much differently! 6

7 I E NPN BJT Symbols and Conventions Metal contact I C PNP I E I C I B I E = I C + I B I B Note reversal in current directions and voltage signs for PNP vs. NPN! 7

8 NPN BJT Modes of Operation i E = + i B v CE = v CB + V CB Forward-Active Mode EBJ forward bias (V BE > 0) CBJ reverse bias (V BC < 0) v XY = V XY + v xy large signal dc bias ac signal V BE Mode Forward-Active Reverse-Active Cutoff Saturation V BE V BC > 0 < 0 < 0 > 0 < 0 < 0 > 0 > 0 V BC = -V CB Not Useful! 8

9 NPN BJT Modes of Operation V BE V CE Saturation region Forward-Active region V A 1 0 v CE 9

10 i E ESE319 Introduction to Microelectronics NPN BJT Forward-Active Current Flow E Forward-biased i E n Injected electrons p Injected electrons Injected holes (i B1 ) i B Reverse-biased n Collected electrons Recombined electrons (i B2 ) C i B i B =i B1 i B2 v CB i E i E V BE B i E = i B V CB 10

11 NPN BJT Forward-Active Mode Basic Model Collector-base diode is reverse biased V CB 0 ( or V BC < 0) Base-emitter diode is forward biased saturation current V BE 0.7 A E W N A D n n i I s = A E q D n n i N A W Area of base-emitter junction Width of base region Doping concentration in base Electron diffusion constant Intrinsic carrier concentration = f(t) 2 I S e = kt q 25o C i B = i E =i B = 1 i B =common emitter current gain 1 11

12 NPN BJT Forward-Active Beta (β) A E -> Area of base-emitter junction W -> Width of base region N A -> Doping concentration in base N D -> Doping concentration in emitter D n -> Electron diffusion constant D p -> Hole diffusion constant L p -> Hole diffusion length in emitter τ b -> Minority-carrier lifetime n i -> Intrinsic carrier concentration = f(t) = D p D n 1 N A W 1 N D L p 2 Large β => N A -> small N D -> large W -> small W 2 D n b 12

13 Note that the equation looks like that of a forward-biased diode collector-base. Is it? Using Eqs. i E = i B + and = βi B we can answer this question, i.e. i E =i B = 1 1 = 1 and write: i E = 1 I S e 1 = 1 I V S e T 1 = AhHa! This i E equation describes a forward-biased emitter-base diode. =common emitter current gain =common base current gain where = 1 13

14 So the new set of equations are: i E = I S e 1 V =I S e T 1 = i E Where: = 1 Typically: => i B = I s = A 2 E q D n n i N A W I S A. I s is strongly temperaturedependent, doubling for a 5 degree Celsius increase in ambient temperature! 14

15 Two equivalent large signal circuit models for the forward-active mode NPN BJT: = I S e Nonlinear VCCS 1 I S e Nonlinear CCCS Key Eqs. V I S e T = F i E = F i E I S e I F SE i B =i E = 1 15

16 Yet another NPN BJT large signal model = i B I S e i B I S e i B This looks like a diode between base and emitter and the equivalent circuit becomes: i B = I S e i E i B 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. 16

17 NPN BJT Operating in the Reverse-Active Mode Recall for NPN Reverse-Active Mode V BE < 0 & V BC > 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. = for =0.1 17

18 The equivalent large signal circuit model for the reverse-active mode NPN BJT: RVRS Active collector and emitter reverse roles FWD Active i B i B i B i E V I S e T = F i E i E i E Note that the directions of the reverse-active currents are the reverse of the forward-active currents; hence the minus signs. Key Eqs. I S e i E I S e v BC v BC V = I SC e T v BC = BJT is non-symmetrical F F 18

19 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. = F i DE i DC i E =i DE i DC i B = 1 F I DE 1 I DC i DE = I S V e T 1 F i DC = I v BC S V e T 1 19

20 Operation in the Saturation Mode Recall for Saturation Mode > 0 & v BC > 0 (or v CB < 0) Consider the E-M model for collector current. = F i DE i DC = I S e 1 I v BC S V e T 1 The first term is the forward mode collector current: V F i DE =I S e T 1 The second is the reverse mode collector current: i DC = I v BC S V e T 1 20

21 Combining terms: Using typical values: =[ = I S e vbe e 1 1 e We obtain: 1 I v BC S V e T 1 v CB 1 = 1 1 ] I S =0.1 I S =10 14 A =0.025V =[ e e 40 v CB 1 ]10 14 Let's plot vs. v BC (or v CB ) with = 0.7 V 21

22 Scilab Saturation Mode Calculation //Calculate and plot npn BJT collector //current in saturation mode vbe=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; =I S e 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. 1 I v BC S V e T 1 22

23 (ma) Saturation Mode Plot Note: forward-active NPN operation continues for negative v CB down to - 0.5V; i.e. v CB > - 0.5V Forward-active Recall for Sat. Mode > 0 & v BC > 0 (or v CB < 0) Saturation A = 0.7V v CB (V) 23

24 ESE319 Introduction to Microelectronics Scilab Plot of NPN Characteristic ( vs. v CE and ) //Calculate and plot npn BJT collector //characteristic using Ebers-Moll model VsubT=0.025; VTinv=1/VsubT; betar=0.1; alphainv=(betar+1)/betar; IsubS=1E-14; for vbe=0.6:0.02:0.68 ForwardExp=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 vals plot(vce,1000*icplus); //Current in ma. end =I S e v CE = v CB + 1 I v BC S V e T 1 24

25 saturation mode (ma) Plot Output forward-active mode =0.68 V Early effect not included. =0.66V =0.64V =0.62 V v CE = v CB + v CE start of saturation V CEsat =v CE =v CB v B E 0.4V 0.7V =0.3V 25

26 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. = I S e 1 I v BC S V e T 1 where α << 1 R 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 CE V CEsat =v CB 0.3V 26

27 More on NPN Saturation - cont. In saturation the forward-biased the current through the collector-base junction increases i B and decreases as v BC increases. sat = forced = i B sat Test for saturation mode operation v CE = V CEsat = 0.1 to 0.3 V => collector-base junction is forward biased or Current ratio => collector-base junction is i forward biased B sat 27

28 Expansion Around Zero v CE (ma) = 0 forward-active mode = 0.68 V, v CE = 0.25 V, v BC = - v CE = 0.43 V Diode forward voltage v CE (V) Note that the collector current is zero at about v CE = 0.06 V, not 0 V! Also note the large reverse collector-base current for v CE < 0.06 V. 28

29 = I S e ESE319 Introduction to Microelectronics Voltage at Zero Collector Current 1 I S e v BC 1 e v CE 1 = e 1 1 e = e v CE e V BE e 1 = 0 => I S e e v BC 1 =I S e 1 v BC V 1 = e T 1 For β R = 0.1 => α R = 0.09 => v CE = V 29

30 pnp The PNP Transistor npn Mode Forward-Active Reverse-Active Cutoff Saturation V EB V CB > 0 < 0 < 0 > 0 < 0 < 0 > 0 > 0 V CB = -V BC Not Useful! 30

31 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! =I S e i B = v EB 1 i E =i B = 1 i B 31

32 PNP PNP BJT Large Signal Model FWD. Active rotate 180 o =I S e i B = v EB 1 i E =i B = 1 i B NPN i E Note reversal in current directions! Substituting, as in the npn case, we get: i E = I v EB S e 1 32

33 Yet another PNP BJT large signal model = i B =I S e v EB 1 i B = I v EB S e 1 I v EB S e This looks like a diode between base and emitter and the equivalent circuit becomes: i B Again, in this model, the diode carries only base current, not emitter current. 33