BJT双极型晶体管.ppt

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1、Slide 8-28.1 Introduction to the BJTIC is an exponential function of forward VBE and independent of reverse VCB.Modern Semiconductor Devices for Integrated Circuits (C. Hu)N+ PNemittercollector base VBEVCB-EfnEfpVCBVBE(a)(b)(c)VBE IC0VCBBCEEcEvEfnNPN BJT:N+ PNECB VBEVCBEmitterBaseCollectorSlide 8-3C

2、ommon-Emitter ConfigurationQuestion: Why is IB often preferred as a parameter over VBE? Modern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-4Semiconductor Devices for Integrated Circuits (C. Hu)8.2 Collector CurrentBBBBDLLndxnd222 B : base recombination lifetime DB : base minority ca

3、rrier (electron) diffusion constant Boundary conditions :) 1()0(/0kTqVBBEenn0) 1()(0/0BkTqVBBnenWnBCN+ PN emitter basecollectorx0 W depletion layersBSlide 8-5BBBBkTqVBLWLxWenxnBE/sinhsinh) 1()(/0) 1(/2kTqVBiBBBEBECBEeNnWDqAdxdnqDAI) 1(/kTqVSCBEeIIBBEWBiBiBkTqVBiECdxDpnnGeGqnAI022/2) 1(It can be show

4、nGB (scm4) is the base Gummel number8.2 Collector Currentni2NB-eqVBEkT1nn 0-)0(/ )(nxn011) 1()(/2kTqVBiBBEeNnxn x/x/WB)/1)(1()/1)(0()(/2BkTqVBiBBWxeNnWxnxnBEModern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-6At low-level injection, inverse slope is 60 mV/decadeHigh-level injection

5、effect :8.2.1 High Level Injection Effect00.2 0.4 0.6 0.8 1.0 10-12 10-10 10-8 10-6 10-410-2 VBEIC (A)IkF60 mV/decadeAt large VBE, BNpnpnpnkTqViBEenpn2/kTqViBBEenpG2/kTqViBEenI2/CWhen p NB , inverse slope is 120mV/decade.kTqVikTEEqiBEFpFnenennp/2/ )(2Modern Semiconductor Devices for Integrated Circu

6、its (C. Hu)Slide 8-78.3 Base CurrentSome holes are injected from the P-type base into the N+ emitter.The holes are provided by the base current, IB .pE nBWE WB(b) emitter basecollectorcontact IE ICelectron flow +hole flowIB(a)contactModern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8

7、-8EBEWEiEiEkTqVEiEBdxDnnnGeGqnAI022/2) 1(Is a large IB desirable? Why?8.3 Base Current emitter basecollectorcontact IE ICelectron flow +hole flowIB(a)contactModern Semiconductor Devices for Integrated Circuits (C. Hu) 1(/2kTqVEEiEEEBBEeNWnDqAIFor a uniform emitter,Slide 8-98.4 Current GainBCFIIHow c

8、an F be maximized?Common-emitter current gain, F :Common-base current gain:FFBCBCCBCECFEFCIIIIIIIIIII1/1/It can be shown that FFF122iEBBEiBEEBBEFnNWDnNWDGGModern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-10EXAMPLE: Current GainA BJT has IC = 1 mA and IB = 10 mA. What are IE, F and

9、 F?Solution: 9901. 0mA 01. 1/mA 1/100A 10/mA 1/mA 01. 1A 10mA 1ECFBCFBCEIIIIIIIWe can confirm FFF1FFF1andModern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-118.4.1 Emitter Bandgap NarrowingEmitter bandgap narrowing makes it difficult to raise F by doping the emitter very heavily.Mod

10、ern Semiconductor Devices for Integrated Circuits (C. Hu)22iEiBBEnnNNTo raise F, NE is typically very large. Unfortunately, large NE makes22iiEnn(heavy doping effect).kTEVCigeNNn/2Since ni is related to Eg , this effect is also known as band-gap narrowing. kTEiiEgEenn/22EgE is negligible for NE 1018

11、 cm-3, is 50 meV at 1019cm-3, 95 meV at 1020cm-3, and 140 meV at 1021 cm-3.Slide 8-1222iEiBBEnnNNTo further elevate F , we can raise niB by using an epitaxial Si1-hGeh base.With h = 0.2, EgB is reduced by 0.1eV and niE2 by 30 x.8.4.2 Narrow-Bandgap Base and Heterojuncion BJTModern Semiconductor Devi

12、ces for Integrated Circuits (C. Hu)Slide 8-13Assume DB = 3DE , WE = 3WB , NB = 1018 cm-3, and niB2 = ni2. What is F for (a) NE = 1019 cm-3, (b) NE = 1020 cm-3, and (c) NE = 1020 cm-3 and a SiGe base with EgB = 60 meV ?(a) At NE = 1019 cm-3, EgE 50 meV,(b) At NE = 1020 cm-3, EgE 95 meV(c) 292. 12meV

13、26/meV 502/228 . 6iiikTEiiEnenenenngE138 . 61010921821922iiiEBiEBEEBFnnnNnNWDWD2238iiEnn24F2meV 26/meV 602/2210iikTEiiBnenenngB237FEXAMPLE: Emitter Bandgap Narrowing and SiGe BaseModern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-14A high-performance BJT typically has a layer of As-

14、doped N+ poly-silicon film in the emitter. F is larger due to the large WE , mostly made of the N+ poly-silicon. (A deep diffused emitter junction tends to cause emitter-collector shorts.) N-collector P-baseSiO2 emitter N+-poly-Si8.4.3 Poly-Silicon EmitterModern Semiconductor Devices for Integrated

15、Circuits (C. Hu)Slide 8-15Why does one want to operate BJTs at low IC and high IC?Why is F a function of VBC in the right figure?FFrom top to bottom:VBC = 2V, 1V, 0V8.4.4 Gummel Plot and F Fall-off at High and Low IcHint: See Sec. 8.5 and Sec. 8.9.SCR BE currentModern Semiconductor Devices for Integ

16、rated Circuits (C. Hu)Slide 8-168.5 Base-Width Modulation by Collector VoltageOutput resistance :CACECIVVIr10Large VA (large ro ) is desirable for a large voltage gainIB3ICVCE0VAVA : Early VoltageIB2IB1Modern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-17How can we reduce the base-w

17、idth modulation effect?8.5 Base-Width Modulation by Collector VoltageN+ PN emitter basecollector VCE WB3WB2WB1 xn VCE1 VCE2VCE3 VBEModern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-18The base-width modulation effect is reduced if we(A) Increase the base width,(B) Increase the base

18、doping concentration, NB , or(C) Decrease the collector doping concentration, NC .Which of the above is the most acceptable action? 8.5 Base-Width Modulation by Collector VoltageN+ PN emitter basecollector VCE WB3WB2WB1 xn VCE 1 VCE2VCE3 VBEModern Semiconductor Devices for Integrated Circuits (C. Hu

19、)Slide 8-198.6 Ebers-Moll ModelThe Ebers-Moll model describes both the active and the saturation regions of BJT operation.Modern Semiconductor Devices for Integrated Circuits (C. Hu) IB IC0VCEsaturationregionactive regionSlide 8-20IC is driven by two two forces, VBE and VBC .When only VBE is present

20、 : ) 1() 1(/kTqVFSBkTqVSCBEBEeIIeIINow reverse the roles of emitter and collector.When only VBC is present :) 1)(11 () 1() 1(/kTqVRSBECkTqVRSBkTqVSEBCBCBCeIIIIeIIeIIR : reverse current gainF : forward current gain8.6 Ebers-Moll ModelICVBCVBEIB E BCModern Semiconductor Devices for Integrated Circuits

21、 (C. Hu)Slide 8-21) 1() 1() 1)(11 () 1(/kTqVFSkTqVFSBkTqVRSkTqVSCBCBEBCBEeIeIIeIeIIIn general, both VBE and VBC are present :In saturation, the BC junction becomes forward-biased, too. VBC causes a lot of holes to be injected into the collector. This uses up much of IB. As a result, IC drops.VCE (V)

22、8.6 Ebers-Moll ModelModern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-228.7 Transit Time and Charge Storage CFFIQWhen the BE junction is forward-biased, excess holes are stored in the emitter, the base, and even in the depletion layers. QF is all the stored excess hole charge F det

23、ermines the high-frequency limit of BJT operation. F is difficult to be predicted accurately but can be measured.Modern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-238.7.1 Base Charge Storage and Base Transit TimeLets analyze the excess hole charge and transit time in the base only.

24、 WB0n0 niB2NB- - eqVBEkT1= xp = n) 1()0(/2kTqVBiBBEeNnnpnBBFBCFBBEFBDWIQWnqAQ22/)0(2Modern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-24What is FB if WB = 70 nm and DB = 10 cm2/s?Answer: 2.5 ps is a very short time. Since light speed is 3108 m/s, light travels only 1.5 mm in 5 ps.E

25、XAMPLE: Base Transit Timeps 5 . 2s105 . 2/scm 102)cm 107(2122262BBFBDWModern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-25The base transit time can be reduced by building into the base a drift field that aids the flow of electrons. Two methods: Fixed EgB , NB decreases from emitter

26、 end to collector end. Fixed NB , EgB decreases from emitter end to collector end.dxdEqc1E E8.7.2 Drift TransistorBuilt-in Base FieldModern Semiconductor Devices for Integrated Circuits (C. Hu)-EBCEcEvEf-EBCEcEvEfSlide 8-268.7.3 Emitter-to-Collector Transit Time and Kirk Effect Top to bottom : VCE =

27、 0.5V, 0.8V, 1.5V, 3V. To reduce the total transit time, emitter and depletion layers must be thin, too. Kirk effect or base widening: At high IC the base widens into the collector. Wider base means larger F . Modern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-27 Base Widening at La

28、rge IcsatECqnvAIsatECCCvAIqNqnqNsdxdE Ee/ x E EbaseNcollectorN+collector basewidth depletionlayer x E EbaseNN+collector“basewidth” depletionlayercollectorModern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-288.8 Small-Signal ModelkTqVSCBEeII/Transconductance:)/()(/qkTIeIkTqeIdVddVdIg

29、CkTqVSkTqVSBEBECmBEBEAt 300 K, for example, gm=IC /26mV.)/(qkTIgCmvber gmvbeCE B EC +Modern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-29FmBECFBEBgdVdIdVdIr11mFCFBEBEFgIdVddVdQCThis is the charge-storage capacitance, better known as the diffusion capacitance.Add the depletion-layer

30、 capacitance, CdBE :dBEmFCgC8.8 Small-Signal ModelmFgr/vber gmvbeCE B EC +Modern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-30 EXAMPLE: Small-Signal Model ParametersA BJT is biased at IC = 1 mA and VCE = 3 V. F=90, F=5 ps, and T = 300 K. Find (a) gm , (b) r , (c) C .Solution: (a) (

31、b) (c) siemens)(milliqkTIgCm mS 39VmA39mV 26mA 1)/(k 3 . 2mS 3990/mFgrad)(femto fargCmF fF 19F109 . 1039. 01051412Modern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-31Once the model parameters are determined, one can analyze circuits with arbitrary source and load impedances.The par

32、ameters are routinely determined through comprehensivemeasurement of the BJT ACand DC characteristics.Modern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-328.9 Cutoff FrequencyThe load is a short circuit. The signal source is a current source,ib , at frequency, f. At what frequency d

33、oes the current gain fall to unity?)/(bciiCdBEFFmbcbemcbbbeqIkTCjjCjrgiivgiCjriiv/11/1)(, /1admittanceinput )/(21at 1CdBEFTqIkTCfvber gmvbe CE B EC +-SignalsourceLoaddBEmFCgCModern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-33fT is commonly used to compare the speed of transistors.

34、 Why does fT increase with increasing IC? Why does fT fall at high IC? fT = 1/2(F + CdBEkT/qIC)8.9 Cutoff FrequencyModern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-34 Poly-Si emitter Thin base Self-aligned poly-Si base contact Narrow emitter opening Lightly-doped collector Heavily

35、-doped epitaxial subcollector Shallow trench and deep trench for electrical isolationBJT Structure for Minimum Parasitics and High SpeedModern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-35In order to sustain an excess hole charge in the transistor, holes must be supplied through IB

36、 to susbtain recombination at the above rate.What if IB is larger than ? FFFQ/FFFBFQtIdtdQ)(Step 1: Solve it for any given IB(t) to find QF(t). 8.10 Charge Control ModelFor the DC condition,FFFFCBQII/Step 2: Can then find IC(t) through IC(t) = QF(t)/F .IC(t) = QF(t)/FModern Semiconductor Devices for

37、 Integrated Circuits (C. Hu)Slide 8-36Visualization of QF(t) FFFBFQtIdtdQ)( QF(t)QF/FFIB(t)(tIBFFFQModern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-37EXAMPLE : Find IC(t) for a Step IB(t) The solution of isFFFBFQtIdtdQ)()1 (/ )()()1 (/0/0FFFFtBFFFCtBFFFeItQtIeIQWhat is ?)( )?0( ?)

38、(FFBQQIIB(t) IC(t)IC(t) t tIB IB0EBCntQFModern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-388.11 Model for Large-Signal Circuit Simulation Compact (SPICE) model contains dozens of parameters, mostly determined from measured BJT data. Circuits containing tens of thousands of transis

39、tors can be simulated. Compact model is a “contract” between device/manufacturing engineers and circuit designers.) 1(1)(/kTqVFSACBkTqVkTqVSCBCBCBEeIVVeeII C BEQF CCS rBrCrECBEQRCBC ICModern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-39A commonly used BJT compact model is the Gumme

40、l-Poon model, consisting ofEbers-Moll model Current-dependent beta Early effectTransit timesKirk effect Voltage-dependent capacitances Parasitic resistances Other effects8.11 Model for Large-Signal Circuit SimulationModern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-408.12 Chapter S

41、ummary The base-emitter junction is usually forward-biased while the base-collector is reverse-biased. VBE determines the collector current, IC .BBEWBiBiBkTqVBiECdxDpnnGeGqnAI022/2) 1( GB is the base Gummel number, which represents all the subtleties of BJT design that affect IC. Modern Semiconducto

42、r Devices for Integrated Circuits (C. Hu)Slide 8-418.12 Chapter Summary The base (input) current, IB , is related to IC by the common-emitter current gain, F . This can be related to the common-base current gain, F .BEBCFGGII The Gummel plot shows that F falls off in the high IC region due to high-l

43、evel injection in the base. It also falls off in the low IC region due to excess base current.FFECFII1 Base-width modulation by VCB results in a significant slope of the IC vs. VCE curve in the active region (known as the Early effect). Modern Semiconductor Devices for Integrated Circuits (C. Hu)Sli

44、de 8-428.12 Chapter Summary Due to the forward bias VBE , a BJT stores a certain amount of excess carrier charge QF which is proportional to IC. FCFIQF is the forward transit time. If no excess carriers are stored outside the base, then The charge-control model first calculates QF(t) from IB(t) and

45、then calculates IC(t).BBFBFDW22FFFBFQtIdtdQ)(FFCtQtI/ )()(, the base transit time.Modern Semiconductor Devices for Integrated Circuits (C. Hu)Slide 8-438.12 Chapter SummaryThe small-signal models employ parameters such as transconductance, qkTIdVdIgCBECm/input capacitance,and input resistance.mFBEFgdVdQCmFBBEgdIdVr/Modern Semiconductor Devices for Integrated Circuits (C. Hu)