Bipolar Junction Transistor (BJT) Overview

Introduction

• The invention of the transistor led to many other semiconductor devices, including integrated circuits.
• BJTs are three-terminal semiconductor devices that can act as conductors or insulators based on the applied input signal.
• They can function as switches in digital electronics or amplifiers in analog electronics.

BJT Structure

• Three Doped Regions: Emitter, Base, and Collector.
• Types:
  • NPN Transistor: Emitter and Collector doped with N-type impurity, Base with P-type impurity.
  • PNP Transistor: Base doped with N-type impurity, Emitter and Collector with P-type impurity.

Key Terms

• Bipolar: Indicates that both electrons and holes contribute to current flow.
• PN Junctions: Two junctions formed between Emitter-Base and Base-Collector.
• Doping Concentration: Emitter is heavily doped, Base is lightly doped, and Collector is moderately doped.
• Region Widths: Base is the narrowest, Collector is the widest.

Modes of Operation

• Active Region:
  • Emitter-Base Junction: Forward biased.
  • Base-Collector Junction: Reversed biased.
  • Voltage relationships: V_C > V_B > V_E.
• Cut-Off Region:
  • Both junctions are reversed biased.
  • Voltage relationships: V_E > V_B and V_C > V_B.
• Saturation Region:
  • Both junctions are forward biased.
  • Voltage relationships: V_B > V_E and V_B > V_C.
• Reverse Active Region: Base-Collector Junction is forward biased, Base-Emitter Junction is reversed biased (generally avoided).

BJT Symbols

• NPN Transistor: Arrow points from Base to Emitter.
• PNP Transistor: Arrow points from Emitter to Base.

Configurations

• Common Emitter Configuration:
  • Input: Base-Emitter.
  • Output: Collector-Emitter.
• Common Collector Configuration:
  • Input: Base-Collector.
  • Output: Emitter.
• Common Base Configuration:
  • Input: Emitter-Base.
  • Output: Collector.

Working of BJT in Active Region (NPN Example)

• Voltages:
  • V_BB: Base supply voltage.
  • V_CC: Collector supply voltage.
  • V_BE: Voltage between Base and Emitter (V_B - V_E).
  • V_CE: Voltage between Collector and Emitter (V_C - V_E).
• Operation:
  • Electrons from the Emitter move to the Base (heavily doped Emitter supplies electrons).
  • Electrons enter the Collector region due to the applied electric field.
• Current Relationships:
  • I_E = I_B + I_C.
  • I_C ≈ α * I_E.
  • I_B = (1 - α)/α * I_C.
  • I_C = β * I_B (β is the current gain, typically 50-400).
  • I_E = (β + 1) * I_B.
• Current Flow:
  • Electrons flow from Emitter to Collector; holes flow in the opposite direction.

Summary

• BJTs are current-controlled devices; input Base current controls the output Collector current.
• Can be used for amplification (active region) or switching (saturation and cut-off regions).
• Upcoming topics: BJT configurations, input/output characteristics, and biasing techniques.

Conclusion

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