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Physics of Bipolar Junction Transistor

Physics of Bipolar Junction Transistor. Overview. Structure and Circuit Symbol. (collects charge carriers). (controls the carriers that make the journey from E to C). (emits charge carriers). Two PN junctions (BE and BC). C and E cannot be swapped.

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Physics of Bipolar Junction Transistor

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  1. Physics of Bipolar Junction Transistor

  2. Overview

  3. Structure and Circuit Symbol (collects charge carriers) (controls the carriers that make the journey from E to C) (emits charge carriers) Two PN junctions (BE and BC) C and E cannot be swapped.

  4. A Bipolar Transistor in the Active Region VBE=0.8 V VBC=-0.2 V BE Junction: Forward Bias BC Junction: Reverse Bias Over Simplified View of BJT In the Active Region (not necessarily the right way to understand circuit)

  5. Review: Forward Bias Diode Depletion region shrinks due to charges from the battery. The electric field is weaker. Majority carrier can cross via diffusion; Greater diffusion current. Current flows from P side to N side

  6. c02f28 Review: Forward Bias Diode Equilibrium Forward Biased Diode Majority carriers cross the junction via diffusion. Minority carriers increased on both sides of the junction.

  7. Energy Band Diagram of a Semiconductor Under an Applied Voltage Electrons roll downhill like stones. Holes float up like bubles!

  8. Forward Biased Diode

  9. Graphical Illustration

  10. Review: PN Junction under Reverse Bias Reverse: Connect the + terminal to the n side. Depletion region widens. Therefore, stronger E. Minority carriers cross the PN junction easily through diffusion. Current is composed mostly of drift current contributed by minority carriers. np to the left and pn to the right. Current from n side to p side, the current is negative. E

  11. Energy Band Diagram of a Reverse Biased PN Junction np pn np Stronger E field in the depletion region pn

  12. Injection of Electrons into Depletion Region Into depletion region on the p side. Outcome: The electron is swept to the n side by E.

  13. An Overview Electrons are injected into the BC junction Electrons are injected into the B; holes to the E. Electrons are swept across the reversed biased BC

  14. Thin Base Region The base region is made thin in order to reduce recombination as electrons travel from BE junction to BC junction.

  15. Highly Doped Emitter In order to emphasize the current contribution due to the electrons (which can cross the BC junction), the emitter is heavily doped by N type materials.

  16. Electrons in the Base Electrons are swept Into the collector; low electron density at x2 Electrons injected into the base; high electron density at x1. The electron gradient allows electrons to travel through diffusion.

  17. BJT Current Assumption: BEJ: Forward Biased BCJ: Reverse Biased

  18. Emitter Area

  19. Determine the Output Voltage

  20. Base Current The proportional of hole current and electron current is determined by dopants (ND and NA). Even though the presence of holes are minimized, a small number holes still must enter through the base.

  21. Recombination Recombination Base must supply holes that will enter the emitter and for recombination with the electrons.

  22. KCL

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