Bipolar Junction Transistor (BJT) Lecture Notes

Introduction

• Welcome to the topic of Bipolar Junction Transistor (BJT).
• The invention of transistors led to the development of modern semiconductor devices and integrated circuits.
• BJTs are crucial in digital electronics (as switches) and analog electronics (as amplifiers).

BJT Basics

• Definition: A three-terminal semiconductor device that can act as a conductor or insulator based on the input signal.
• Terminology:
  • Emitter: Heavily doped region supplying electrons.
  • Base: Lightly doped, thinner region.
  • Collector: Moderately doped, wider region, collects electrons.
• Types of BJTs:
  • NPN: Emitter and Collector are N-type, Base is P-type.
  • PNP: Emitter and Collector are P-type, Base is N-type.
• Bipolar: Indicates both electrons and holes contribute to current flow.

Internal Structure

• Contains two PN junctions:
  • Emitter-Base Junction
  • Base-Collector Junction
• Interaction between these junctions is crucial; they do not operate independently.

Regions of Operation

1. Active Region:

   • Biasing: Emitter-Base Junction is forward biased, Base-Collector Junction is reverse biased.
   • Conditions:
     • VC > VB > VE

2. Cut-off Region:

   • Both junctions are reverse biased.
   • Conditions:
     • VE > VB and VC > VB

3. Saturation Region:

   • Both junctions are forward biased.
   • Conditions:
     • VB > VE and VB > VC

4. Reverse Active Region:

   • Base Collector Junction is forward biased; Base Emitter Junction is reverse biased.
   • Not commonly used due to low gain.

BJT Symbols

• NPN Symbol: Arrow indicates current flow direction (Base to Emitter).
• PNP Symbol: Arrow direction reversed (Emitter to Base).

Configurations

• Common Emitter Configuration: Input between Base and Emitter; output between Collector and Emitter.
• Common Collector Configuration: Input and output share the Collector.
• Common Base Configuration: Input and output share the Base.
• Each configuration has distinct advantages and disadvantages.

Working of NPN Transistor in Active Region

• Supply Voltages:
  • VBB for Base
  • VCC for Collector
  • VBE = VB - VE (Forward bias for Base-Emitter)
  • VCE = VC - VE (Reverse bias for Base-Collector)
• Electron Flow: Electrons move from Emitter towards Base; most enter Collector due to light doping of Base.
• Current Relationships:
  • IE = IB + IC
  • IC = α * IE
  • IB = (1 - α) * IE
  • β (current gain) relates IB and IC: IC = β * IB*

Conclusion

• BJTs are current-controlled devices; by controlling IB, IC can be regulated.
• Different methods exist for signal amplification using BJTs.
• Future videos will cover various BJT configurations and characteristics.
• For questions or feedback, engage in the comments.