Bipolar Junction Transistor (BJT)

Introduction

• BJT: Bipolar Junction Transistor
• Importance: Key topic in the course, inventors received Nobel Prize
• Inventors: John Bardeen, Walter Brattain, William Shockley at Bell Labs (1947)
  • Nobel Prize: 1956
  • John Bardeen: Nobel Prize in 1972 for BCS Theory

Historical Context

• Transistor invention: December 1947
• Team: Led by William Shockley; later invented the Junction version (1951)
• Impact: Replaced bulky vacuum tubes
  • Image comparisons: First transistor, vacuum tube, modern transistor

BJT Overview

• Three-terminal doped semiconductor device
  • Amplifies weak signals
  • Used in switching operations
  • Applicable in both analog and digital electronics

Structure of BJT

• Types: NPN and PNP Transistors
• NPN: p-type material between two n-type materials
• PNP: n-type material between two p-type materials

Regions

• Collector region: Largest region
• Base region: Smallest region
• Emitter region: Connects to the emitter terminal

Terminals

• **Emitter terminal (E) **Base terminal (B) **Collector terminal (C)
• Represented by capital letters (E, B, C)**

Junctions

• Junction j1: Between emitter and base (emitter-base junction)
• Junction j2: Between collector and base (collector-base junction)
• Depletion layers across j1 and j2

Region Width and Doping

• Collector: Maximum width, collects electrons, better heat dissipation
• Emitter: Maximum doping
• Base: Minimum width and doping

Cross-sectional View

• Detailed comparison for NPN and PNP transistors

Symbols

• Vertical line represents the base terminal
• Differentiation by current direction
  • NPN: Current from base to emitter
  • PNP: Current from emitter to base

Name Meaning

• Bipolar: Two charge carriers (electrons and holes)
• Junction: Two junctions (j1 and j2)
• Transistor: Combination of Transfer and Resistor
  • Transfers signal from low to high resistance
• BJT: Transferring signal from low to high resistance, part of resistor family

Regions of Operation

• Active Mode: j1 forward biased, j2 reverse biased (Amplifier)
• Saturation Mode: j1 and j2 forward biased (Logical ON/Closed switch)
• Cutoff Mode: j1 and j2 reverse biased (Logical OFF/Open circuit)
• Inverted Mode: j1 reverse biased, j2 forward biased (Roles of emitter and collector switched)
• Active mode is primarily used for amplification; other modes used for switching.

Conclusion

• Understanding BJT operation modes important for applications in electronics
• Next lecture: Working of transistors

Q&A

• Students encouraged to ask questions in the comment section