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L17 March 221 EE5342 – Semiconductor Device Modeling and Characterization Lecture 17 - Spring 2005 Professor Ronald L. Carter ronc@uta.edu http://www.uta.edu/ronc/
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L17 March 222 npn BJT topology Charge Neutral Region Depletion Region x x’ p-Base -CollectorN-Emitter z 0 WBWB W B +W C -W E 0 x” c x” 0 xBxB 0 x’ E IEIE ICIC IBIB
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L17 March 223 Defining currents in FA mode npn BJT (Fig 9.13*)
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L17 March 224 npn BJT currents (F A region, ©RLC ) I C = J C A C I B =-(I E +I C ) J nE J nC I E = -J E A E J RB =J nE -J nC J pE J GC J RE J pC
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L17 March 225 E current equations in npn BJT (w/o gen/rec)
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L17 March 226 C current equations in npn BJT (w/o gen/rec)
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L17 March 227 FA npn figure of merit emitter eff
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L17 March 228 FA npn figure of merit base transp
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L17 March 229 FA npn figure of merit recomb fact
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L17 March 2210 Common base current gain, TT
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L17 March 2211 Common base current gain, (cont.)
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L17 March 2212 Common emitter current gain,
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L17 March 2213 Ebers-Moll Model (Neglecting G-R curr) (Fig. 9.30* Semiconductor Physics & Devices, by Neamen, Irwin, Chicago, 1997, * throughout) -J E A E =I E J C A C =I C
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L17 March 2214 Source of Ebers- Moll Equations (E)
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L17 March 2215 Source of Ebers- Moll Equations (C)
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L17 March 2216 Recombination/Gen Currents (FA)
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L17 March 2217 Non-ideal effects in BJTs Base-width modulation (FA: x B changes with changes in V BC ) Current crowding in 2-dim base High-level injection (minority carriers g.t. dopant - especially in the base). Emitter Bandgap narrowing (N E ~ density of states at cond. band. edge) Junction breakdown at BC junction
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L17 March 2218 npn Base-width mod. (Early Effect) Fig 9.15*
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L17 March 2219 Base-width modulation (Early Effect, cont.) Fig 9.16*
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L17 March 2220 Emitter current crowding in base Fig 9.21*
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L17 March 2221 Interdigitated base fixes emitter crowding Fig 9.23*
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L17 March 2222 Non-ideal effects in BJTs º Base-width modulation (FA: x B changes with changes in V BC ) º Current crowding in 2-dim base High-level injection (minority carriers g.t. dopant - especially in the base). Emitter Bandgap narrowing for N E --> density of states at cond. band. edge Junction breakdown at BC junction
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L17 March 2223 Base region high- level injection (npn)
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L17 March 2224 Effect of HLI in npn base region Fig 9.17*
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L17 March 2225 Effect of HLI in npn base region (cont)
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L17 March 2226 Effect of HLI in npn base region (cont)
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L17 March 2227 Emitter region high- level injection (npn)
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L17 March 2228 Effect of HLI in npn emitter region
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L17 March 2229 Effect of HLI in npn base region Figs 9.18 and 9.19*
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L17 March 2230 Bandgap narrowing effects Fig 9.20* Replaces n i 2 throughout
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L17 March 2231 Junction breakdown at BC junction Reach-through or punch-through when W CB and/or W EB become large enough to reduce x B to zero Avalanche breakdown when E max at EB junction or CB junction reaches E crit.
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L17 March 2232 Static diode from the npn Gummel-Poon Static Model C E B B’ I LC I LE I BF I BR I CC - I EC = IS(exp(v BE /NFV t - exp(v BC /NRV t )/Q B RCRC RERE R BB
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L17 March 2233 Gummel Poon npn Model Equations I BF = IS expf(v BE /NFV t )/BF I LE = ISE expf(v BE /NEV t ) I BR = IS expf(v BC /NRV t )/BR I LC = ISC expf(v BC /NCV t ) Q B = (1 + v BC /VAF + v BE /VAR ) {½ + ¼ + (BF IBF/IKF + BR IBR/IKR) }
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L17 March 2234 Making a complete diode with G-P BJT RB = RC = 0 Set RE to the desired RS value Set ILE and NE to ISR and NR so this is the rec. current Set BR=BF>>1, ~1e8 so IBR, IBF are neglibigle Set ISC = 0 so ILC is = 0 Set IS to IS for diode so ICC-IEC is the injection curr. Set VAR = VAF = 0 IKF gives the desired high level injection, set IKR = 0
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L17 March 2235 References 1 OrCAD PSpice A/D Manual, Version 9.1, November, 1999, OrCAD, Inc. 2 Semiconductor Device Modeling with SPICE, 2nd ed., by Massobrio and Antognetti, McGraw Hill, NY, 1993. * Semiconductor Physics & Devices, by Donald A. Neamen, Irwin, Chicago, 1997.
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