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Microelectronics 2

Electrical Engineering 2. Lecture 18. Microelectronics 2. Dr. Peter Ewen. (Room G08, SMC; email - pjse). base. base. emitter collector. emitter collector. n p n. p n p. The Bipolar Junction Transistor (BJT).

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Microelectronics 2

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  1. Electrical Engineering 2 Lecture 18 Microelectronics 2 Dr. Peter Ewen (Room G08, SMC; email - pjse)

  2. base base emitter collector emitter collector n p n p n p The Bipolar Junction Transistor (BJT) npn (Discrete) Transistor Fabrication (e.g. BC107, 108, 109) base emitter base SiO2 epitaxial n-type layer n p 10m n n-type wafer n+ 200m collector

  3. base emitter collector n p n • BJT’s should be connected as labelled, otherwise gains and breakdown voltages will be drastically reduced base emitter base n p 10m n • base is deliberately made thin, ~1 n+ 200m collector

  4. Energy bands for an npn transistor under zero applied bias Fig. 112 depletion regions emitter base collector n p n Conduction Band EF Electron Energy Valence Band

  5. Energy bands for an npn transistor under normal biasing conditions Fig. 113 emitter base collector n p n VCB VBE + + electrons Conduction Band Electron Energy Valence Band

  6. Fig. 114 BJT CARRIER FLOWS  - emitter efficiency α – common-base current gain n-type emitter p-type base n-type collector (lightly doped) α|IE| electrons |IE| IC IE electrons ICBO holes holes (1-)|IE| holes IC = αIE + ICBO base-emitter junction collector-base junction IB VCB VBE + +

  7. For diode: Fig. 116: Input Characteristic – CB Configuration IE / mA VCB = 0V VCB = 25V rd – dynamic resistance 10 8 6 4 2 0 For BJT: Increasing VCB re – dynamic emitter resistance For  = 1, T = 300 K and IE, IC in mA: 0 0.2 0.4 0.6 0.8 VBE / V

  8. IE ≈ IC VCB = 0V VCB = 25V IE / mA INPUT OUTPUT IE IC e c 10 8 6 4 2 0 b VBE VCB IB TRANSFER CHARACTERISTIC IC(VBE) Transconductance, gm, is slope of transfer characteristic, hence: 0 0.2 0.4 0.6 0.8 VBE / V

  9. LECTURE 18 BJT CHARACTERISTICS • Common base  Common emitter • Common collector • The Early effect

  10. IE IC e c Fig. 117: Output Characteristics – CB Configuration VBE b IC(VCB) VCB IB IC / mA Active region IC≈ αIE, α≈ 1 IE = 1.0 mA 1 0.75 0.5 0.25 IE = 0.75 mA Breakdown region IE = 0.50 mA Saturation region IE = 0.25 mA ICBO Cutoff region IE = 0 -1 0 1 2 3 4 5 6 7 8 VCB / V

  11. COMMON EMITTER CONFIGURATION IC Fig. 115 (b) c IB b OUTPUT VCE e INPUT VBE IE INPUT CHARACTERISTICS OUTPUT CHARACTERISTICS IC(VCE) IB(VBE) c n p n b e

  12. Fig. 118: Input Characteristic – CE Configuration IB / A VCE = 20V VCE = 5V 10 8 6 4 2 0 Increasing VCE 0 0.2 0.4 0.6 0.8 VBE / V

  13. IC Fig. 119: Output Characteristics – CE Configuration c IB b VCE IC(VCE) e VBE IE IC / mA Active region IB = 40 A 4 3 2 1 IB = 30 A Breakdown region Saturation region IB = 20 A IB = 10A ICEO Cutoff region IB = 0 0 5 10 15 20 VCE / V

  14. COMMON COLLECTOR CONFIGURATION IE Fig. 115 (c) e IB b OUTPUT VCE c INPUT VCB IC INPUT CHARACTERISTICS OUTPUT CHARACTERISTICS IE(VCE) IB(VCB)

  15. CCCE IE(VCE)  IC(VCE) Fig. 121: Output Characteristics – CC Configuration IE ≈ IC since α ≈ 1 IE / mA Active region IB = 40 A 4 3 2 1 IB = 30 A Breakdown region Saturation region IB = 20 A IB = 10A ICEO Cutoff region IB = 0 0 5 10 15 20 VCE / V

  16. IE Fig. 120: Input Characteristics – CC Configuration 0.7V e IB b VCE c VCB IB vs. VCB for different values of VCE IC IB / A Transistor on  VBE ≈ 0.7V VCE = 15V VCE = 10V VCE = 5V VCB≈ VCE – 0.7V 80 60 40 20 As VCB VCE, VBE  0, transistor turns off 4.3 0 5 10 15 20 VCB / V

  17. THE EARLY EFFECT, OR BASE-WIDTH MODULATION depletion regions base emitter collector n p n e-b junction c-b junction IB VCB VBE + +

  18. Effect of bias on width of the depletion region 0 Fig. 55 -2 -14 Reverse bias (p-type -ve w.r.t. n-type) Volts -4 -12 -6 -10 -8 V VB - + - + - + - + - + - + - + - + + – n p Depletion region widens Potential VB+V VB Distance

  19. Effect of bias on width of the depletion region 0 Fig. 55 0.1 0.7 Forward bias (p-type +ve w.r.t. n-type) Volts 0.2 0.6 0.3 0.5 0.4 V VB + - + - + - + - + – n p - + - + Depletion region narrows Potential VB VB-V Distance

  20. THE EARLY EFFECT, OR BASE-WIDTH MODULATION depletion regions base emitter collector effective width of base n p n e-b junction c-b junction VCB=1V VCB=2V VCB=3V VCB=4V VCB=6V VCB=5V IB VCB VBE + +

  21. If the effective width of the base decreases: – 1. There will be less recombination in the base, so α (and hence β) will increase. β = α / (1- α)

  22. If the effective width of the base decreases: – • 2. The minority carrier concentration gradient (Δn/Δx) will increase: • so |IE| will increase. |IE|  Δn/Δx (Δx is the basewidth) n p n emitter base collector |IE| Electron concentration Δn Δx

  23. If the effective width of the base decreases: – 3. The c-b depletion region may extend all the way over to the e-b junction – PUNCH-THROUGH base emitter collector n p n e-b junction c-b junction IB VCB VBE + +

  24. Energy bands for an npn transistor under normal biasing conditions Fig. 113 emitter base collector n p n VCB VBE + + electrons Conduction Band Electron Energy Valence Band

  25. Energy bands for an npn transistor under normal biasing conditions Fig. 113 emitter base collector n p n VCB VBE + + electrons Conduction Band Electron Energy Valence Band

  26. Energy bands for an npn transistor under normal biasing conditions Fig. 113 emitter base collector n p n VCB VBE + + electrons Conduction Band Electron Energy Valence Band

  27. Energy bands for an npn transistor under normal biasing conditions Fig. 113 emitter base collector n p n VCB VBE + + electrons Conduction Band Electron Energy Valence Band

  28. IE IC e c Early effect implies α and |IE| increases as VCB increases, hence IC (≈ αIE )increases VBE b IC(VCB) VCB IB IC / mA Active region IC≈ αIE IE = 1.0 mA 1 0.75 0.5 0.25 IE = 0.75 mA Breakdown region IE = 0.50 mA Saturation region IE = 0.25 mA ICBO Cutoff region IE = 0 -1 0 1 2 3 4 5 6 7 8 VCB / V Common-base output characteristics

  29. As VCE increases, VCB increases. Early effect implies α, and hence β, increases as VCB increases. IC ≈ βIB, hence IC increases IC c IB b VCE e VBE IE IC / mA Active region IB = 40 A IC ≈ βIB 4 3 2 1 IB = 30 A Breakdown region Saturation region IB = 20 A IB = 10A ICEO Cutoff region IB = 0 0 5 10 15 20 VCE / V Common-emitter output characteristics

  30. IE / mA VCB = 0V VCB = 25V 10 8 6 4 2 0 Early effect implies |IE| increases as VCB increases. Increasing VCB 0 0.2 0.4 0.6 0.8 VBE / V Common-base input characteristics

  31. IB / A VCE = 20V VCE = 5V 10 8 6 4 2 0 Increasing VCE Early effect implies less recombination in the base as VCB/VCE increases hence IB decreases 0 0.2 0.4 0.6 0.8 VBE / V Common-emitter input characteristics

  32. Summary • CB output characteristics are plots of IC vs. VCB: • IC = ICBO for IE = 0 • IC≈ IE in flat regions • IC 0 as VCB goes negative • IC increases at large values of VCB due to breakdown at the reverse-biased c-b junction

  33. Common-emitter configuration • Input characteristics (IB vs. VBE) resemble that for a forward-biased diode but depend on output voltage VCE due to Early effect

  34. Output characteristics are plots of IC vs. VCE for different values of the input current IB: • IC = ICEO for IB = 0 • IC>> IB in flat regions due to current amplification • IC increases at large values of VCE due to breakdown at the reverse-biased c-b junction

  35. Common-collector configuration • Input characteristics (IB vs. VCB) • VCB = VCE – VBE hence: • If transistor is “on” VCB is fixed at VCE – 0.7V • As VCB VCE device turns off, IB  0

  36. Output characteristics are plots of IE vs. VCE for different values of IB • Since IE≈ IC, CC output characteristics are essentially the same as those for CE • THE EARLY EFFECT, OR BASE-WIDTH MODULATION

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