1 / 31

Cross Sectional View of FET

Cross Sectional View of FET. FET I-V Characteristic. Saturation Voltage. V pinchoff = V DS,sat = V GS – V TH Separates resistive from saturation region The drain current is given by Solving for V DS,sat :. Early Voltage Function of Length. Early Voltage in MOSFETs.

betty
Download Presentation

Cross Sectional View of FET

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Cross Sectional View of FET Brad Noble Chapter 3 Slides

  2. FET I-V Characteristic Brad Noble Chapter 3 Slides

  3. Saturation Voltage • Vpinchoff = VDS,sat = VGS – VTH • Separates resistive from saturation region • The drain current is given by • Solving for VDS,sat: Brad Noble Chapter 3 Slides

  4. Early Voltage Function of Length Brad Noble Chapter 3 Slides

  5. Early Voltage in MOSFETs • Due to channel length modulation: • Good to solve for quiescent voltage-current. Brad Noble Chapter 3 Slides

  6. Ex: Find VDS,sat for an NFET Brad Noble Chapter 3 Slides

  7. Body Effect Brad Noble Chapter 3 Slides

  8. Cox Gate oxidecapacitance inversionlayer Depletion cap,function of x Cdep Variations in VTHAcross Channel • We assume VTH is constant across channel THIS IS NOT TRUE! • Depletion region is thick at S and thin at D. Brad Noble Chapter 3 Slides

  9. Small Signal Equivalent Ckt Brad Noble Chapter 3 Slides

  10. Parasitic Capacitance Brad Noble Chapter 3 Slides

  11. Capacitance Equivalent Circuit Brad Noble Chapter 3 Slides

  12. Variation in Capacitance Brad Noble Chapter 3 Slides

  13. Notes on PFETs • PFETs typically have a shape factor 3 or 4 times larger than NFETs • Body effect can be eliminated in PFETs by tying the n-well to VDD • Need 6m spacing between n-wells to isolate. • Dr. Engel always does this on input devices, not always elsewhere. Brad Noble Chapter 3 Slides

  14. Subthreshold Conduction Brad Noble Chapter 3 Slides

  15. Weak Inversion • What really happens if VGS < VTN? • In digital design, IDS = 0. • We call it “weak inversion” or W.I. • IDS is primarily due to Idrift in strong inversion and Idiffusion in weak inversion. Brad Noble Chapter 3 Slides

  16. Modes of Inversion • IDS = Idrift + Idiffusion • If VGS > VTN the channel has been inverted. • To be more precise, we can say the channel has been “strongly inverted” (S.I.) due to an abundance of carriers in the channel. • Inversion is independent of whether the FET is in the linear or saturation region. Brad Noble Chapter 3 Slides

  17. Weak Inversion Idiffusion • Drain is more reverse biased than source: • To find Idiff, compute gradient • Because no carriers are lost as they travel from S to D, current is the same for all x and gradient is not a function of x. • Note: This is not really true due to recombination, but its close! Brad Noble Chapter 3 Slides

  18. W.I. Surface Potential Brad Noble Chapter 3 Slides

  19. Deriving Weak Inversion IDS Brad Noble Chapter 3 Slides

  20. W.I. FET As Exp. Law Dev. • S must be big for device to be useful. • If VDS = 100mV, can be neglected. • For W.I. vDS,Sat 100mV • Looks like a BJT Brad Noble Chapter 3 Slides

  21. Inversion Coefficient • Let • Shape factor as a function of  : Lets you chose shape to match inversion mode. Brad Noble Chapter 3 Slides

  22. Ex. Using Inversion Coeff. Brad Noble Chapter 3 Slides

  23. Small Signal Analysis Brad Noble Chapter 3 Slides

  24. Ex: Quiescent Point Question: How many digits are significant? Brad Noble Chapter 3 Slides

  25. Small Signal Model Limits • Suppose the previous circuit is the input device of an amplifier. • Small-signal model holds as long as the deviations are small Brad Noble Chapter 3 Slides

  26. Taylor Series Expansion • Taking a Taylor expansion of one variable: Brad Noble Chapter 3 Slides

  27. Small Signal Model Params Brad Noble Chapter 3 Slides

  28. Example: Small Signal Analysis Brad Noble Chapter 3 Slides

  29. Small Signal Low-Freq Model Brad Noble Chapter 3 Slides

  30. Ex: Find gm and rO Brad Noble Chapter 3 Slides

  31. Transconductance: W.I. & M.I. • What is gm for a weakly inverted FET? • What is gm for a moderately inverted FET? Not in textbooks! Brad Noble Chapter 3 Slides

More Related