Lecture Notes: How Transistor Works

Introduction

• Recap of previous lecture on transistors
• Focus on how transistors work in this lecture

BJT (Bipolar Junction Transistor)

• B stands for Bipolar:
  • Involves both types of charge carriers (electrons and holes)
• J stands for Junction:
  • Two junctions: Junction j1 and Junction j2

Meaning of Transistor

• Name derived from "transferred resistor"
  • Trans + istor (from resistor) = Transistor
• In active mode:
  • Junction j1: forward biased (low resistance)
  • Junction j2: reverse biased (high resistance)

Amplification

• Input (VI) measured across small resistance (r)
• Output (VO) measured across large resistance (R)
• VI < VO: weak input signal becomes amplified at output

NPN Transistor in Active Mode

• Regions:
  • Emitter (N-type)
  • Base (P-type)
  • Collector (N-type)
• Operation in Active Mode:
  • Junction j1: forward biased (VEB)
    • Emitter connected to negative terminal
    • Base connected to positive terminal
  • Junction j2: reverse biased (VCB)
    • Collector connected to positive terminal
    • Base connected to negative terminal
  • Barrier Potentials:
    • Junction j1: reduced (VB - VEB)
    • Junction j2: increased (VB + VCB)

Movement of Electrons and Holes

• Forward-Biased Junction j1:
  • Electrons cross from emitter to base, some recombine with holes
  • Base is thin and lightly doped – low recombination
  • Most electrons move to collector
• Reverse-Biased Junction j2:
  • Minority charge carriers contribute to reverse saturation current (ICo)

Important Currents

• Reverse Saturation Current (ICo):
  • Measured when emitter terminal is open-circuited
  • Associated with Junction j2
• Collector Current (IC):
  • IC = α * IE + ICo*

Kirchhoff's Current Law (KCL)

• Sum of currents entering = sum of currents leaving
  • IE (emitter current) = IB (base current) + IC (collector current)

Concluding Remarks

• Importance of α (current gain) to be discussed in future lectures
• End of lecture