Notes on Bipolar Junction Transistor (BJT)

Introduction

• Welcome to the channel "All About Electronics".
• Topic: Bipolar Junction Transistor (BJT).
• Importance: Key component in semiconductor devices and integrated circuits, essential for modern electronics.

What is a BJT?

• A three-terminal semiconductor device:
  • Terminals: Emitter, Base, Collector.
  • Acts as a conductor or insulator based on input signal.
• Uses:
  • Digital electronics: Acts as a switch.
  • Analog electronics: Used as an amplifier.

Types of BJTs

1. NPN Transistor:

   • Emitter and Collector: Doped with N-type impurities.
   • Base: Doped with P-type impurities.

2. PNP Transistor:

   • Emitter and Collector: Doped with P-type impurities.
   • Base: Doped with N-type impurities.

• Bipolar: Both electrons and holes contribute to current flow.

Internal Structure of BJT

• Two PN junctions:
  • Between Emitter and Base.
  • Between Base and Collector.
• Emitter: Heavily doped (supplies electrons).
• Base: Lightly doped (narrow width).
• Collector: Moderately doped (wider).

Operating Regions of BJT

1. Active Region:

   • Emitter-Base Junction: Forward biased.
   • Base-Collector Junction: Reverse biased.
   • Conditions for NPN:
     • VE < VB < VC.
   • Conditions for PNP:
     • VE > VB > VC.

2. Cut-Off Region:

   • Both junctions are reverse biased.
   • Conditions for NPN:
     • VE > VB and VC > VB.
   • Conditions for PNP:
     • VE < VB and VC < VB.

3. Saturation Region:

   • Both junctions are forward biased.
   • Conditions for NPN:
     • VB > VE and VB < VC.
   • Conditions for PNP:
     • VB < VE and VB > VC.

4. Reverse Active Region:

   • Base-Collector Junction: Forward biased.
   • Base-Emitter Junction: Reverse biased.
   • Generally avoided due to low gain.

BJT Configurations

• Common Emitter Configuration:
  • Emitter is common between input and output.
• Common Collector Configuration:
  • Collector is common between input and output.
• Common Base Configuration:
  • Base is common between input and output.

Working of BJT in Active Region (NPN Example)

• Notation:
  • VBB: Base supply voltage.
  • VCC: Collector supply voltage.
  • VBE: Voltage difference between Base and Emitter.
  • VCE: Voltage difference between Collector and Emitter.
• Forward bias on Base-Emitter Junction typically has a voltage drop of 0.6 to 0.7 volts.
• Majority carriers from Emitter (electrons) enter the Base:
  • Some recombine with holes, most flow into the Collector due to thin Base width and light doping.
• Collector current (IC) is influenced by the Emitter current (IE):
  • KCL: IE = IB + IC.
  • IC ≈ α * IE (α: common base current gain).
  • IB = (1 - α) * IE.
  • IC = β * IB (β: current gain, typically 50 to 400).

Current Relationships

• Base Current (IB): Input current.
• Collector Current (IC): Output current.
• Controlling IB allows control of IC (BJT: current control device).

Conclusion

• Discussion on BJTs will continue in upcoming videos focusing on configurations, characteristics, and biasing techniques.
• Questions or suggestions can be left in the comments.
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