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Understanding BJT Operation: Minority Carrier Diffusion and Ebers-Moll Model Insights

This discussion delves into the fundamental concepts of Bipolar Junction Transistors (BJTs), focusing on the derivation of minority carrier profiles in quasi-neutral regions and the analysis of terminal currents using the Ebers-Moll model. It covers aspects such as the minority carrier diffusion equation, boundary conditions, and the significance of narrow base design for high current gain. Additionally, it explores the relationships between base and collector voltages and the implications for transistor performance, providing a comprehensive overview for students in EE130/230A.

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Understanding BJT Operation: Minority Carrier Diffusion and Ebers-Moll Model Insights

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  1. EE130/230A Discussion 14 PengZheng

  2. “Game Plan” for I-V Derivation • Solve the minority-carrier diffusion equation in each quasi-neutral region to obtain excess minority-carrier profiles • different set of boundary conditions for each region • Find minority-carrier diffusion currents at depletion region edges • Add hole & electron components together  terminal currents EE130/230A Fall 2013 Lecture 26, Slide 2

  3. BJT Terminal Currents • We know: • Therefore: EE130/230A Fall 2013 Lecture 26, Slide 3

  4. BJT with Narrow Base • In practice, we make W << LB to achieve high current gain. Then, since we have: EE130/230A Fall 2013 Lecture 26, Slide 4 R. F. Pierret, Semiconductor Device Fundamentals, Fig. 11.2

  5. Ebers-Moll Model increasing (npn) or VEC (pnp) C. C. Hu, Modern Semiconductor Devices for Integrated Circuits, Figure 8-2 • The Ebers-Moll model is a large-signal equivalent circuit which describes both the active and saturation regions of BJT operation. • Use this model to calculate IB and IC given VBE and VBC EE130/230A Fall 2013 Lecture 26, Slide 5

  6. V V EB CB I B E B C I C If only VEB is applied (VCB = 0): aR : reverse common base gain aF : forward common base gain If only VCBis applied (VEB = 0): : Reciprocity relationship: EE130/230A Fall 2013 Lecture 26, Slide 6

  7. In the general case, both VEB and VCB are non-zero: IC: C-B diode current + fraction of E-B diode current that makes it to the C-B junction IE: E-B diode current + fraction of C-B diode current that makes it to the E-B junction Large-signal equivalent circuit for a pnp BJT R. F. Pierret, Semiconductor Device Fundamentals, Fig. 11.3 EE130/230A Fall 2013 Lecture 26, Slide 7

  8. Summary: BJT Performance Requirements • High gain (bdc >> 1) • One-sided emitter junction, so emitter efficiency g  1 • Emitter doped much more heavily than base (NE >> NB) • Narrow base, so base transport factor aT 1 • Quasi-neutral base width << minority-carrier diffusion length (W << LB) • IC determined only by IB (IC function of VCE,VCB) • One-sided collector junction, so quasi-neutral base width W does not change drastically with changes in VCE (VCB) • Based doped more heavily than collector (NB > NC) (W = WB – xnEB – xnCB for PNP BJT) EE130/230A Fall 2013 Lecture 26, Slide 8

  9. Questionsregarding the MOSFET design project? Good luck to Quiz#6!

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