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Lecture #26

OUTLINE Modern BJT Structures Poly-Si emitter Heterojunction bipolar transistor (HBT) Charge control model Base transit time Reading: Finish Chapter 11, 12.2. Lecture #26. Modern BJT Structure. Narrow base n+ poly-Si emitter Self-aligned p+ poly-Si base contacts

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Lecture #26

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  1. OUTLINE Modern BJT Structures Poly-Si emitter Heterojunction bipolar transistor (HBT) Charge control model Base transit time Reading: Finish Chapter 11, 12.2 Lecture #26 EE130 Lecture 26, Slide 1

  2. Modern BJT Structure • Narrow base • n+ poly-Si emitter • Self-aligned p+ poly-Si base contacts • Lightly-doped collector • Heavily-doped epitaxial subcollector • Shallow trenches and deep trenches filled with SiO2 • for electrical isolation EE130 Lecture 26, Slide 2

  3. Polycrystalline-Silicon (Poly-Si) Emitter • bdcis larger for a poly-Si emitter BJT as compared with an all-crystalline emitter BJT, due to reduced dpE(x)/dx at the edge of the emitter depletion region Continuity of hole current in emitter EE130 Lecture 26, Slide 3

  4. Emitter Gummel Number w/ Poly-Si Emitter where SpDEpoly/WEpoly is the surface recombination velocity For a uniformly doped emitter, EE130 Lecture 26, Slide 4

  5. Emitter Band Gap Narrowing To achieve large bdc, NE is typically very large, so that band gap narrowing (Lecture 8, Slide 5) is significant. DEGE is negligible for NE < 1E18/cm3 N = 1018 cm-3: DEG = 35 meV N = 1019 cm-3: DEG = 75 meV EE130 Lecture 26, Slide 5

  6. Narrow Band Gap (SiGe) Base • To improve bdc, we can increase niB by using a base material (Si1-xGex) that has a smaller band gap • for x = 0.2, DEGB is 0.1eV • Note that this allows a large bdc to be achieved with large NB (even >NE), which is advantageous for • reducing base resistance • increasing Early voltage (VA) EE130 Lecture 26, Slide 6

  7. EXAMPLE: Emitter Band Gap Narrowing If DB = 3DE , WE = 3WB , NB = 1018 cm-3, and niB2 = ni2, find bdc for (a) NE = 1019 cm-3, (b) NE = 1020 cm-3, and (c) NE = 1019 cm-3 and a Si1-xGex base with DEgB = 60 meV (a) At NE = 1019 cm-3, DEgE35 meV (b) At NE = 1020cm-3, DEgE 160 meV: (c) EE130 Lecture 26, Slide 7

  8. Charge Control Model A PNP BJT biased in the forward-active mode has excess minority-carrier charge QB stored in the quasi-neutral base: In steady state, EE130 Lecture 26, Slide 8

  9. Base Transit Time, tt • time required for minority carriers to diffuse across the base • sets the switching speed limit of the transistor EE130 Lecture 26, Slide 9

  10. Relationship between tB and tt • The time required for one minority carrier to recombine in the base is much longer than the time it takes for a minority carrier to cross the quasi-neutral base region. EE130 Lecture 26, Slide 10

  11. Drift Transistor: Built-in Base Field The base transit time can be reduced by building into the base an electric field that aids the flow of minority carriers. • Fixed EgB , NB decreases from emitter end to collector end. B - E C Ec Ef Ev • Fixed NB , EgB decreases from emitter end to collector end. - E B C 1 dE = C E Ec Ef q dx Ev EE130 Lecture 26, Slide 11

  12. EXAMPLE: Drift Transistor • Given an npn BJT with W=0.1mm and NB=1017cm-3 (mn=800cm2/Vs), find tt and estimate the base electric field required to reduce tt EE130 Lecture 26, Slide 12

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