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Lecture 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 1 1. BJT: Regions of Operation VBE - C + VBC + B forward� active saturation VCE + VBC VBE - - cut-off reverse E • Forward active: device has high voltage gain and high β; • Reverse active: poor β; not useful; • Cut­off: 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 2 Forward­Active Region: VBE > 0, VBC <0 n-Emitter p-Base n-Collector IC>0 IE<0 IB>0 VBE > 0 VBC < 0 Minority Carrier profiles (not to scale): emitter base pnE npB collector pnC pnCo npBo pnEo x -WE-XBE 6.012 Spring 2009 -XBE 0 WB WB+XBC Lecture 18 WB+XBC+WC 3 Forward­Active Region: VBE > 0, VBC < 0 • Emitter injects electrons into base, collector extracts (collects) electrons from base: [ ]; I C = I Se VBE Vth IS = qAE npBo Dn WB • Base injects holes into emitter, holes recombine at emitter contact: [ ]− 1; I S IB = e βF VBE Vth IS βF = qAE pnEo D p WE • Emitter current: IS [ ] [ ] I E = −IC − I B = −I Se − e −1 VBE Vth βF VBE Vth • 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. Dn I WB N dE DnWE βF = C = = D IB N aB D pWB pnEo • p WE n pBo • 6.012 Spring 2009 Lecture 18 4 Reverse­Active Region: VBE < 0, VBC > 0 n-Emitter p-Base n-Collector IE>0 IC<0 IB>0 VBE < 0 VBC > 0 Minority Carrier Profiles (not to scale): emitter base pnE npB collector pnC pnCo npBo pnEo x -WE-XBE 6.012 Spring 2009 -XBE 0 WB WB+XBC WB+XBC+WC Lecture 18 5 Reverse­Active Region: VBE < 0, VBC > 0 • Collector injects electrons into base, emitter extracts (collects) electrons from base: VBC qAC n pBo Dn Vth IE = IS e ; IS = WB [ ] • Base injects holes into collector, holes recombine at collector contact and buried layer: I S IB = e βR ( )−1; VBC Vth IS βR = qAC pnCo D p WC • Collector current: IS [ ] [ ] I C = −I E − I B = −I Se − e −1 VBC Vth βR VBC Vth • Typically, βR ≈ 0.1 - 5 << βF . Dn IE WB N dC DnWC βR = = = D p N aB D pWB IB pnCo • WC n pBo • 6.012 Spring 2009 Lecture 18 6 Cut­Off Region: VBE < 0, VBC < 0 n-Emitter p-Base n-Collector IE>0 IC>0 Electrons flow only if Generation occurs IB<0 VBE < 0 VBC < 0 Minority Carrier Profiles (not to scale): emitter base pnE npB collector pnC pnCo npBo pnEo x -WE-XBE 6.012 Spring 2009 -XBE 0 WB WB+XBC WB+XBC+WC Lecture 18 7 Cut­Off Region: VBE < 0, VBC < 0 • Base extracts holes from emitter: I B1 = − IS βF = −I E • Base extracts holes from collector: I B2 = − IS βR = −I C -15 • These are tiny leakage currents (≈10-15 A). 6.012 Spring 2009 Lecture 18 8 Saturation Region: VBE > 0, VBC > 0 n-Emitter p-Base n-Collector IC IE <0 IBIB>0 VBE > 0 VBC > 0 Minority Carrier profiles (not to scale): emitter base pnE npB collector pnC pnCo npBo pnEo x -WE-XBE 6.012 Spring 2009 -XBE 0 WB WB+XBC WB+XBC+WC Lecture 18 9 Saturation Region: VBE > 0, VBC > 0 Saturation is superposition of forward active + reverse active: [ ]− e[ ] IC = IS e VBE Vth VBC Vth [ ] I S IB = e βF VBE Vth I − S e βR [ ]− 1 V BC Vth [ ] I − 1 + S e βR VBC Vth − 1 [ ]− e[ ] − IS e[ ]−1 I E = −I S e VBE Vth VBC Vth βF VBE Vth • 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 10 Saturation - 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 18 2. Large­signal equivalent circuit model System of equations that describes BJT operation: [ ] [ ] [ ] VBC VBE V BC I V V V I C = I S e th − e th − S e th − 1 βR V I VBC I S VBE IB = e th − 1 + S e Vth − 1 βR βF VBC VBE VBE I V V V I E = −I S e th − e th − S e th −1 βF [ ] [ ] [ ] [ ] [ ] Equivalent-circuit model representation (non­linear hybrid­π model) [particular rendition of Ebers-Moll C model in text]: IC VBC IS/βR B + IR + IB IF βFIF - βRIR =IS[exp(qVBE/kT) - exp(qVBC/kT)] IS/βF VBE IE E Three parameters in this model: IS, βF, and βR. 6.012 Spring 2009 Lecture 18 12 Simplification of equivalent circuit model: • Forward­active region: VBE > 0, VBC < 0 C C [ ] Ie VBE Vth B B S VBE,on E E For today’s technology: VBE,on ≈ 0.7 V. IB depends on outside circuit. • Reverse­active region: VBE < 0, VBC > 0 C C VBC,on B [ ] Ie VBC Vth B S E E For today’s technology: VBC,on ≈ 0.6 V 6.012 Spring 2009 Lecture 18 13 Simplification of equivalent circuit model: • Saturation region: VBE > 0, VBC > 0 C C C + VBC,on B VBC,on B B VBE,on VCE,sat VBE,on E E E For today’s technology: VCE,sat ≈ 0.1 V. IC and IB depend on outside circuit. • Cut­off region: VBE < 0, VBC < 0 C B E Only negligible leakage currents. 6.012 Spring 2009 Lecture 18 14 3. Output Characteristics Common­emitter output characteristics: IC IB IB=0 0 0 VCE=VCB+VBE VCE,sat 6.012 Spring 2009 Lecture 18 15 Common­Emitter Output Characteristics 6.012 Spring 2009 Lecture 18 16 What did we learn today? Summary of Key Concepts • Forward­active region: For bias calculations: C B VBE,on [ ] I Se VBE Vth E • Saturation Region: For bias calculations: C + VBC,on B VCE,sat VBE,on E • Cut­off Region: For bias calculations: C B E 6.012 Spring 2009 Lecture 18 17 MIT OpenCourseWare http://ocw.mit.edu 6.012 Microelectronic Devices and Circuits Spring 2009 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.
