Bipolar Junction Transistor (BJT) Summary

BJT Overview

  • A Bipolar Junction Transistor (BJT) uses both electron and hole carriers.
  • Composed of three layers: P-type, N-type, and another P-type forming NPN or PNP configurations.

History of Transistors

  • 1904: John Ambrose Fleming invents the vacuum tube.
  • 1947: Creation of point-contact transistor by John Bardeen and Walter Brattain.
  • 1948: Conceptualization of the junction transistor by William Shockley.
  • 1950: First practical junction transistor built using germanium.
  • 1951: Bell Labs announces junction transistor for commercial production.
  • 1954: Texas Instruments produces the first silicon transistor.
  • Late 1950s: Advancements in planar technology and integrated circuits.
  • 1960s-1970s: BJTs play a key role in miniaturizing electronics.

BJT Structure

  • Terminals:
    • Emitter: Supplies majority charge carriers.
    • Collector: Collects charge carriers from emitter.
    • Base: Controls current flow between emitter and collector.

Modes of Operation

  • Cut-off: Both B-E and B-C junctions are reverse biased.
  • Active: B-E is forward biased, B-C is reverse biased.
  • Saturation: Both B-E and B-C are forward biased.

BJT Configurations

  1. Common Emitter
    • High gain configuration.
  2. Common Base
    • Used for high-frequency applications.
  3. Common Collector
    • Voltage follower configuration.

DC Biasing Techniques

  • Fixed Bias: Simple biasing but sensitive to temperature changes.
  • Collector Feedback Bias: Stabilizes the operating point.
  • Dual Feedback Bias: Combines two feedback parameters for stability.
  • Emitter Feedback Bias: Provides improved stability with temperature variations.
  • Voltage Divider Bias: Uses resistors to maintain stable biasing.

Applications

  • Amplifiers: Used in audio, RF, and sensor applications.
  • Switches: Can function as electronic switches in circuits.
  • Logic Gates: Forms the basis of digital circuits (AND, OR, NOT, NAND, NOR).