Lecture Notes on BJT (Bipolar Junction Transistor)

Overview

• Previous discussions on BJT covered:
  • Current transport
  • Gain
  • Base transport factor
  • Current gain
  • Gummel number and Early voltage
  • Components of base current, emitter-collector current
  • BJT as a current input device and signal amplifier

Today's Focus: Delay in BJT

• Delay: Speed at which a BJT can operate
• Importance in RF transistors
• Factors affecting delay:
  • Emitter and base doping
  • Band gap shrinkage from high doping

Key Concepts

Early Voltage and Doping

• Higher emitter doping than base doping increases gain
• Problem with high doping:
  • Band Gap Shrinkage: High doping reduces band gap
    • Silicon BJT example: Shrinks from 1.1 eV to 0.9 eV
    • Increases intrinsic carrier concentration ni
  • Reduces gain (beta) due to e^(−ΔEG/kT) factor
  • Gain starts decreasing at very high doping levels (~10^18 per cm^3)

BJT Delay

• Definition: Total time tau for electrons to move from emitter to collector
• Cut-off Frequency (fT): Frequency where current gain is 1

Factors in Delay

• Emitter Delay: Charging/discharging of base-emitter capacitance
  • RC delay
  • Calculated as: kT/q * I_C * (ε/ε_0) / W_BE
• Base Delay:
  • Transit time across base: W_Bn / V_sat
  • Charge clearing time: W_Bn^2 / (2 * Dn)
• Collector Delay:
  • Base-collector capacitance delay: ε/ε_0 * (R_E + R_C) / W_BC
  • Transit time: W_BC / (2 * V_sat)*

Implications of Delay

• Optimization:
  • Trade-off between reducing W_BC for faster transit and increasing W_BC for less capacitance
  • Thinner base (W_Bn) reduces delay

Advanced BJT Concepts

• Advanced BJT, e.g., Hetero Junction Bipolar Transistor (HBT)
• HBT uses different materials for emitter and base to overcome BJT limitations
• Cut-off frequency: Greater than 700 GHz achievable in systems like Indium Arsenide

Conclusion

• Completed coverage of BJT fundamentals
• Next sessions will touch on compound semiconductors and silicon MOSFETs
• Advanced topics like Kirk effect and Ebers-Moll model were not covered but available for further inquiry

Note: If any confusion arises from the current topics, particularly with current equations, further clarification can be sought. Always welcome to reach out for discussions or questions.

This lecture concludes the discussion on BJTs in the course. Further exploration will include hetero junctions and compound semiconductors.