•Large-signal equivalent circuit modelLecture 18 The Bipolar Junction Transistor (II) Regions of Operation Outline • Regions of operation • Large-signal equivalent circuit model • Output characteristics Reading Assignment: Howe and Sodini; Chapter 7, Sections 7.3, 7.4 & 7.5 6.012 Spring 2009 Lecture 18 11. BJT: Regions of Operation VBE C -+ forward� saturation VBC active + B VCE + VBC VBE reverse -cut-off -E • Forward active: device has high voltage gain and high β; • Reverse active: poor β; not useful; • Cut-off: negligible current: nearly an open circuit; • Saturation: device is flooded with minority carriers; – ⇒ takes time to get out of saturation 6.012 Spring 2009 Lecture 18 2Minority Carrier profiles (not to scale):Forward-Active Region: VBE > 0, VBC <0 n-Emitter p-Base n-Collector IE<0 IB>0 IC>0 VBE > 0 VBC< 0 Minority Carrier profiles (not to scale): npB pnE pnC npBo pnEo pnCo 0 WB-XBE WB+XBC -WE-XBE WB+XBC+WC x emitter base collector 6.012 Spring 2009 Lecture 18 3•Emittercurrent:Forward-Active Region: VBE > 0, VBC < 0 • Emitter injects electrons into base, collector extracts (collects) electrons from base: IC = ISe VBE Vth[ ]; IS = qAEnpBo Dn WB • Base injects holes into emitter, holes recombine at emitter contact: IB = IS ββββF e VBE Vth[ ]− 1 ; IS ββββF = qAE pnEoDp WE • Emitter current: IE = −IC − IB = −ISe VBE Vth[ ]− IS ββββF e VBE Vth[ ]−1 • State-of-the-art IC BJT’s today: IS ≈ 0.1 - 1 fA • βF ≈ 50 - 300. • βF hard to control tightly: ⇒ circuit design techniques required to be insensitive to variations in βF. βF = IC IB = npBo • D n WB pnEo • D p WE = NdE D n WE NaB D p WB 6.012 Spring 2009 Lecture 18 4Reverse-Active Region: VBE < 0, VBC > 0 n-Emitter p-Base n-Collector IE>0 IB>0 IC<0 VBE < 0 VBC > 0 Minority Carrier Profiles (not to scale): npB pnE pnC npBo pnEo pnCo 0 WB-XBE WB+XBC -WE-XBE WB+XBC+WC x emitter base collector 6.012 Spring 2009 Lecture 18 5•Collectorcurrent:Reverse-Active Region: VBE < 0, VBC > 0 • Collector injects electrons into base, emitter extracts (collects) electrons from base: IE = ISe VBC Vth[ ]; IS = qACnpBoDn WB • Base injects holes into collector, holes recombine at collector contact and buried layer: IB = IS ββββR e VBC Vth( )−1 ; IS ββββR = qAC pnCoDp WC • Collector current: IC = −IE − IB = −ISe VBC Vth[ ]− IS ββββR e VBC Vth[ ]−1 • Typically, βR ≈ 0.1 - 5 << βF . βR = IE IB = npBo • D n WB pnCo • D p WC = NdC D n WC NaB D p WB 6.012 Spring 2009 Lecture 18 6Cut-Off Region: VBE < 0, VBC < 0 n-Emitter p-Base n-Collector IE>0 IB<0 IC>0 VBE < 0 VBC < 0 Electrons flow only if Generation occurs Minority Carrier Profiles (not to scale): npB pnE pnC npBo pnEo pnCo 0 WB-XBE WB+XBC -WE-XBE WB+XBC+WC x emitter base collector 6.012 Spring 2009 Lecture 18 7•These are tiny leakagecurrents (≈10A).-Cut-Off Region: VBE < 0, VBC < 0 • Base extracts holes from emitter: IB1 = − IS = −IE ββββF • Base extracts holes from collector: IB2 = − IS = −IC ββββR -15 • These are tiny leakage currents (≈10 15 A). 6.012 Spring 2009 Lecture 18 8Saturation Region: VBE > 0, VBC > 0 n-Emitter p-Base n-Collector IE IB<0 IC VBE > 0 VBC > 0 IB>0 Minority Carrier profiles (not to scale): npB pnE pnC npBo pnEo pnCo 0 WB-XBE WB+XBC -WE-XBE WB+XBC+WC x emitter base collector 6.012 Spring 2009 Lecture 18 9ES =ththSthSaturation Region: VBE > 0, VBC > 0 Saturation is superposition of forward active + reverse active: [VBE ][VBC ] IS [VBC ] IC = IS e Vth − e Vth −ββββR e Vth − 1 IS [VBE ] IS [VBC ] IB =e Vth − 1 + e Vth − 1 ββββF ββββR [VBE ][VBC ] IS [VBE ] VthI =−I e Vth − e Vth − e −1IE −IS e[ ]− e[ ] − ββββF e[ ]−1 • IC and IE can have either sign, depending on relative magnitudes of VBE and VBC and βF and βR. 6.012 Spring 2009 Lecture 18 10Saturation -The Flux Picture Both junctions are injecting and collecting. Electrons injected from emitter into base are collected by the collector as in Forward Active case. Electrons injected from collector into the base are collected by the emitter as in Reverse Active case. Holes injected into emitter recombine at ohmic contact as in Forward Active case. Holes injected into collector recombine with electrons in the n+ buried layer 6.012Spring 2009 Lecture 18Equivalent-circuit model representation (non-linear2. Large-signal equivalent circuit model System of equations that describes BJT operation: IC = IS e VBE Vth[ ]− e VBC Vth[ ] − IS ββββR e VBC Vth[ ]− 1 IB = IS ββββF e VBE Vth[ ]− 1 + IS ββββR e VBC Vth[ ]− 1 IE = −IS e VBE Vth[ ]− e VBC Vth[ ] − IS ββββF e VBE Vth[ ]−1 Equivalent-circuit model representation (non-linear hybrid-π model) [particular rendition of Ebers-Moll model in text]: Three parameters in this model: IS, βF, and βR. B C E IB IR IF IS/βR IS/βF IE IC βFIF -βRIR =IS[exp(qVBE/kT) - exp(qVBC/kT)]+ -+ -VBE VBC 6.012 Spring 2009 Lecture 18 12Simplification of equivalent circuit model: • Forward-active region: VBE > 0, VBC < 0 For today’s technology: VBE,on ≈ 0.7 V. IB depends on outside circuit. B C E B C E VBE,on ISe VBE Vth[ ] • Reverse-active region: VBE < 0, VBC > 0 For today’s technology: VBC,on ≈ 0.6 V B C E E B C VBC,on ISe VBC Vth[ ] 6.012 Spring 2009 Lecture 18 13Simplification of equivalent circuit model: • Saturation region: VBE > 0, VBC > 0 For today’s technology: VCE,sat ≈ 0.1 V. IC and IB depend on outside circuit. B C E B C E VBE,on VBE,on VCE,sat VBC,on VBC,on B C E + -6.012 Spring 2009 Lecture 18 14 • Cut-off region: VBE < 0, VBC < 0 on outside circuit. Only negligible leakage currents. B C EI =03. Output Characteristics Common-emitter output characteristics: IC IB IB=0 6.012 Spring 2009 Lecture 18 15 VCE=VCB+VBE VCE,sat B 0 0Common-Emitter Output Characteristics 6.012 Spring 2009 Lecture 18 16What did we
View Full Document
Unlocking...