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MOSFET

MOSFET. Flat-Band Voltage. The flat-band voltage is defined as the applied gate voltage such that there is no band bending in the semiconductor and, as a result, zero net space charge in this region. What does the charge density plot look like for the above band diagram?.

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MOSFET

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  1. MOSFET

  2. Flat-Band Voltage The flat-band voltage is defined as the applied gate voltage such that there is no band bending in the semiconductor and, as a result, zero net space charge in this region. What does the charge density plot look like for the above band diagram?

  3. Ideal Structure – Cross-Section

  4. Ideal Structure – Cross-Section

  5. Ideal Structure – Cross-Section

  6. MOSFET– I-V Where is pinch-off?

  7. MOSFET– I-V VDsat = VG – VT when VD < VDsat, linear or triode operation when VD >VDsat, saturation region of operation

  8. MOSFET– I-V Do these equations make sense?

  9. MOSFET– I-V Accommodate the charge in the depletion region Induce the inverted region

  10. Surface Scattering

  11. Effective Mobility Channel depth Charge/cm2 in the channel at an arbitrary point

  12. Surface Scattering Mobility is not constant in the channel

  13. Square-Law Theory

  14. Square-Law Theory

  15. Square-Law Theory Oxide capacitance per unit area of the gate

  16. Square-Law Theory

  17. Square-Law Theory

  18. Square-Law Theory Linear or Triode mode

  19. Square-Law Theory

  20. Square-Law Theory vs. Bulk-Charge Theory Square-law assumes that the depletion width at all channel points from the source to the drain remains fixed at WT as VD changes. But we know this isn’t true!

  21. Square-Law Theory vs. Bulk-Charge Theory The variation in the depletion layer or “bulk” charge must be included in any charge balance relationship. Changes in the depletion width

  22. Square-Law Theory vs. Bulk-Charge Theory

  23. A.C. Response • In many applications, the MOSFET is used in a linear amplifier circuit. • A small-signal equivalent circuit for the MOSFET is needed to mathematically analyze the electronic circuit. • The equivalent circuit contains capacitances and resistances that introduce frequency effect. • Small-circuit equivalent circuit • Physical factors that limit frequency response

  24. A.C. Response

  25. A.C. Response The transconductance is sometimes referred to as the transistor gain.

  26. A.C. Response

  27. A.C. Response Transconductance is a function of geometry of the device as well as of carrier mobility and threshold voltage. It increases as oxide thickness decreases.

  28. A.C. Response – Cutoff Frequency fmax -> frequency where the MOSFET is no longer amplifying the input signal. Channel length is the key parameter in determining fmax.

  29. Small-signal Characteristics

  30. Nonideal MOS

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