Understanding MOSFET Second-Order Effects

Mar 12, 2025

Electronic Circuits Lecture 32 - MOSFET Second-Order Effects

Introduction

  • Lecture by Basil Razavi
  • Focus on the MOSFET device, specifically second-order effects

Review of Last Lecture

  • Discussed VDS (Drain-Source Voltage) exceeding VGS (Gate-Source Voltage) minus VTH (Threshold Voltage).
  • Observed channel pinching near the drain when VDS becomes larger.
  • Identified two regions of operation:
    • Triode Region: VDS < VGS - VTH
    • Saturation Region: VDS >= VGS - VTH
  • In saturation, the drain current (ID) remains constant with changes in VDS, operating as an ideal current source.

Current Source and Voltage Relation

  • In saturation, ID is a function of VGS, not VDS, and follows a quadratic relationship.
  • VGS values affect electron density and current capability.

Channel Length Modulation

  • Similar to the early effect in bipolar transistors.
  • Real ID vs. VDS measurements show a slight increase, not constant.
  • Channel length decreases as VDS increases, affecting ID.
  • Introduced correction factor to model this:
    • ID = (1/2) * 碌n * Cox * (W/L) * (VGS - VTH)^2 * (1 + 位 * VDS)
    • 位 (Lambda) is the channel length modulation coefficient.

Biasing in Circuits

  • Critical for MOSFET amplification.
  • Without proper biasing, MOSFET cannot amplify signals effectively.
  • Biasing sets operating point (VGS, ID) to ensure MOSFET is active.
  • Aim to achieve a reasonable swing in output voltage with a reasonable resistance value (RL).

Transconductance (gm)

  • Indicates how effectively a MOSFET converts voltage changes to current changes.
  • Defined as the derivative of ID with respect to VGS.
  • Higher gm indicates a stronger device.
  • Calculated as:
    • gm = 碌n * Cox * (W/L) * (VGS - VTH)
    • gm = 2 * ID / (VGS - VTH)
    • gm = 2 * ID * 碌n * Cox * (W/L)

Amplifier Design Attempt

  • First attempt failed due to lack of biasing.
  • Second attempt added a threshold voltage, but resulted in high resistance requirements.
  • Third attempt improved biasing, selecting V0 > VTH.
    • Resulted in more reasonable resistance value (25 k惟).

Conclusion

  • Proper biasing is essential for MOSFET operation in amplifier circuits.
  • Understanding second-order effects like channel length modulation is crucial for accurate circuit behavior modeling.
  • Transconductance is a key parameter in evaluating MOSFET performance.

Key Takeaway: Properly biasing a MOSFET ensures its effective operation in amplifiers, and understanding its behaviors and model corrections, like channel length modulation, is vital for designing efficient circuits.