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Overview of Bipolar Junction Transistors

Dec 24, 2024

Lecture on Bipolar Junction Transistor (BJT) Summary

Introduction to BJT

  • BJT is comprised of two back-to-back PN junctions.
  • Key parts: Emitter, Base, Collector.
  • Types of BJTs: NPN and PNP.
  • BJT controls current between two terminals using a third terminal.

Current Components and Basic Operation

  • Emitter-Base junction is forward biased, injecting electrons from N-type to P-type.
  • Holes are back-injected from P to N.
  • Electrons recombine in the base and most reach the collector.
  • Reverse bias the Base-Collector junction to sweep electrons to the collector.

Current Components in Detail

  • Electron Current (I_EN): Electrons injected from Emitter to Base.
  • Hole Current (I_EP): Holes back-injected from Base to Emitter.
  • Collector Current (I_C): Electrons that reach the collector.
  • Base Current (I_B): Minor component due to recombination.
    • Base-Emitter voltage (V_BE) forward bias.
    • Base-Collector voltage (V_BC) reverse bias.

Design Considerations

  • Emitter Doping should be much higher than Base Doping to improve electron current.
  • Base Width (W_bn) should be narrow to minimize recombination.

Performance Metrics

  • Emitter Injection Efficiency: Fraction of current due to electrons.
  • Base Transport Factor: Fraction of electrons reaching collector.
  • Current Gain (β): Ratio of Collector Current to Base Current.
  • Alpha (α): Ratio of Collector Current to Emitter Current.

Transistor Action

  • Small change in Base current (I_B) can lead to large change in output current (I_C).
  • Amplification: Input current change leads to significant output current change.

Modes of Operation

  1. Common Base (CB)

    • Base is grounded.
    • Emitter current injected and measured at collector.
    • Output characteristics: Plot of I_C vs V_CB.

  2. Common Emitter (CE)

    • Emitter is grounded.
    • Small base current leads to large collector current.
    • Output characteristics: Plot of I_C vs V_CE.

Current Equations

  • Derive expressions for Emitter, Collector, and Base currents.
  • Use minority carrier profiles and boundary conditions to solve continuity equations.

Summary

  • Mathematical expressions and precise calculations are key to understanding BJT functioning.
  • Upcoming discussions will further explore current equations and transistor performance metrics.

Note: Future classes will cover more advanced topics such as Gummel plot, Gummel number, and detailed current equations.

Full transcript