Lecture: Bipolar Junction Transistor (BJT)

Introduction

• Bipolar Junction Transistor (BJT) is a three-terminal semiconductor device.
• Acts as a conductor or insulator based on input signal.
• Can be used as a switch (digital electronics) or amplifier (analog electronics).
• Despite the prevalence of field effect transistors, BJTs are still widely used.

Structure and Types

• Three doped regions: Emitter, Base, and Collector.
• NPN Transistor:
  • Emitter and Collector: N-type doping.
  • Base: P-type doping.
• PNP Transistor:
  • Emitter and Collector: P-type doping.
  • Base: N-type doping.
• Bipolar indicates both electrons and holes contribute to current flow.

Internal Construction

• Emitter: Heavily doped, supplies electrons.
• Base: Lightly doped.
• Collector: Moderately doped, wider than emitter and base.
• Base region is much narrower than Emitter and Collector.

Regions of Operation

1. Active Region:

   • Emitter-Base Junction: Forward biased.
   • Base-Collector Junction: Reverse biased.
   • Voltage Condition: VC > VB > VE.

2. Cut-Off Region:

   • Both junctions: Reverse biased.
   • Voltage Condition: VE > VB and VC > VB.

3. Saturation Region:

   • Both junctions: Forward biased.
   • Voltage Condition: VB > VE and VB > VC.

4. Reverse Active Region:

   • Collector-Base Junction: Forward biased.
   • Base-Emitter Junction: Reverse biased.
   • Provides low gain, usually avoided.

Symbol and Current Flow

• NPN Transistor:
  • Current flows from Base to Emitter.
• PNP Transistor:
  • Current flows from Emitter to Base.

Configurations

• Common Emitter:
  • Emitter is common between input and output.
• Common Collector:
  • Collector is common.
• Common Base:
  • Base is common.
• Each has its own advantages and applications.

Working Principle (Active Region)

• Example: NPN Transistor.
• Supply Voltages:
  • VBB: Base supply voltage.
  • VCC: Collector supply voltage.
  • VBE: Voltage between Base and Emitter.
  • VCE: Voltage between Collector and Emitter.

Current Relations

• KCL: IE = IB + IC
• Current Gain:
  • Collector Current, IC = β × IB.
  • β (Beta) is typically 50 to 400.
  • Emitter current, IE = (β + 1) × IB.

Current Control Device

• Controls output current by controlling base input current.
• Field Effect Transistor (FET): Voltage control device.

Amplification and Biasing

• Output collector current amplified by β.
• Possible to amplify input signals by biasing BJT.

Minority Carrier Contribution

• Collector current, ICT = IC + ICO (minority charge contribution).
• ICO: current due to minority carriers (microamperes).

Conclusion

• Upcoming topics: BJT configurations, input/output characteristics, biasing techniques.
• Overview of BJT provided in this lecture.