1. EE 5340 Semiconductor Device Theory Lecture 22 - Fall 2010
Professor Ronald L. Carter

2. Non-ideal effects in BJTs
Base-width modulation (FA: xB changes with changes in VBC)
Current crowding in 2-dim base
High-level injection (minority carriers g.t. dopant - especially in the base).
Emitter Bandgap narrowing (NE ~ density of states at cond. band. edge)
Junction breakdown at BC junction
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3. Base-width modulation (Early Effect, cont.)
Fig 9.16*
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4. Emitter current crowding in base
Fig 9.21*
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5. Interdigitated base fixes emitter crowding
Fig 9.23*
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6. Base region high- level injection (npn)
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7. Effect of HLI in npn base region
Fig 9.17*
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8. Effect of HLI in npn base region (cont)
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9. Effect of HLI in npn base region (cont)
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10. Emitter region high- level injection (npn)
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11. Effect of HLI in npn emitter region
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12. Effect of HLI in npn base region
Figs 9.18 and 9.19*
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13. Bandgap narrowing effects
Fig 9.20*
Replaces ni2
throughout
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14. Junction breakdown at BC junction
Reach-through or punch-through when WCB and/or WEB become large enough to reduce xB to zero
Avalanche breakdown when Emax at EB junction or CB junction reaches Ecrit.
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15. Hybrid-pi circuit model
Adapted from linking current version of E-M model with parasitic Rs and CSubstr
C-E branch is linking current
B-E branch is the reduced B-E diode with diffusion (for and rev) resistance and capacitance and junction cap.
B-C branch is the reduced B-C diode with diffusion (for and rev) resistance and capacitance and junction cap.
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16. Hybrid-pi Circuit model
Fig 9.33*
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17. Gummel-Poon Static npn Circuit ModelB
RBB
ILC
IBR
ICC - IEC =
IS(exp(vBE/NFVt
- exp(vBC/NRVt)/QB B’
ILE
IBF RE E
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18. BJT Characterization Forward GummeliC RC iB RE RB
vBEx
vBC
vBE + -
vBCx= 0 = vBC + iBRB - iCRC
vBEx = vBE +iBRB +(iB+iC)RE
iB = IBF + ILE =
ISexpf(vBE/NFVt)/BF
+ ISEexpf(vBE/NEVt)
iC = bFIBF/QB =
ISexpf(vBE/NFVt)/QB
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19. Ideal F-G Data iC and iB (A) vs. vBE (V) N = 1  1/slope = 59.5 mV/dec
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20. References
* Semiconductor Physics and Devices, 2nd ed., by Neamen, Irwin, Boston, 1997.
**Device Electronics for Integrated Circuits, 2nd ed., by Muller and Kamins, John Wiley, New York, 1986.
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