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Principles of VLSI Design: MOS Threshold Voltage & Transistor Characteristics

Learn about energy band diagrams, MOS structures in various bias regions, depletion and inversion regions, and measurement of key parameters related to MOS transistors in VLSI design.

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Principles of VLSI Design: MOS Threshold Voltage & Transistor Characteristics

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  1. Threshold Voltage COE 360 Principles of VLSI Design Dr. Aiman El-Maleh Computer Engineering Department King Fahd University of Petroleum and Minerals

  2. Outline • Energy Band Diagrams for MOS Structure • MOS in Equilibrium State • MOS Biased in Depletion Region • Depletion Region Charge • MOS Biased in Inversion Region • MOS & Depletion Capacitance • Threshold Voltage for MOS Transistors • Measurement of Parameters Based on Slides of Kenneth R. Laker, University of Pennsylvania

  3. TWO TERMINAL MOS STRUCTURE

  4. Energy Band Diagram for P-Type Substrate • The Fermi potential F denotes the difference between the intrinsic Fermi level Ei and the Fermi level EF • The electron affinity of silicon, q, is the potential difference between the conduction level and vacuum (free space) • The energy required for an electron to move from Fermi level into free space is called the work function qS

  5. Energy Band Diagram for P-Type Substrate

  6. Energy Band Diagrams for Components of MOS Structure

  7. MOS in Equilibrium State Under Thermal Equilibrium: Fermi level of all materials Must line up.

  8. Flat Band Voltage • Consider a MOS structure of p-type silicon substrate, a silicon dioxide layer and aluminum gate • Equilibrium Fermi Potential of silicon is qFp = 0.2 eV • The work function qS = 4.15 eV + 1.1/2 eV + 0.2 eV = 4.15 eV + 0.75 eV = 4.9 eV • The built-in potential difference across this MOS systems is qM - qS = 4.1 eV – 4.9 eV = -0.8 eV • If a voltage corresponding to this potential difference is applied between gate and substrate the energy bands become flat.

  9. MOS Biased in Depletion Region

  10. Depletion Region Charge

  11. MOS Biased in Inversion Region

  12. MOS Biased in Inversion Region

  13. MOS & Depletion Capacitance

  14. Threshold Voltage for MOS Transistor • Components of Threshold Voltage: • Work function difference between gate and channel • Gate voltage component to change surface potential • Gate voltage component to offset depletion region charge • Voltage component to offset fixed charges in gate oxide and silicon oxide interface • Threshold voltage is:

  15. Threshold Voltage for MOS Transistors

  16. Threshold Voltage for MOS Transistors

  17. Threshold Voltage for MOS Transistors

  18. Threshold Voltage Example

  19. Threshold Voltage Example

  20. Threshold Voltage Body Bias Example

  21. Threshold Voltage Body Bias Example

  22. Measurement of Parameters (VT0, , , kn, kp)

  23. Measurement of Parameters (VT0, , , kn, kp)

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