1. 1ECE 584, Summer 2002Brad Noble Chapter 3 Slides Cross Sectional View of FET

2. 2ECE 584, Summer 2002Brad Noble Chapter 3 Slides FET I-V Characteristic

3. 3ECE 584, Summer 2002Brad Noble Chapter 3 Slides 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 :

4. 4ECE 584, Summer 2002Brad Noble Chapter 3 Slides Early Voltage Function of Length

5. 5ECE 584, Summer 2002Brad Noble Chapter 3 Slides Early Voltage in MOSFETs Due to channel length modulation: Good to solve for quiescent voltage-current.

6. 6ECE 584, Summer 2002Brad Noble Chapter 3 Slides Ex: Find V DS,sat for an NFET

7. 7ECE 584, Summer 2002Brad Noble Chapter 3 Slides Body Effect

8. 8ECE 584, Summer 2002Brad Noble Chapter 3 Slides Variations in V TH Across Channel We assume V TH is constant across channel THIS IS NOT TRUE! Depletion region is thick at S and thin at D. C ox C dep inversion layer Gate oxide capacitance Depletion cap, function of x

9. 9ECE 584, Summer 2002Brad Noble Chapter 3 Slides Small Signal Equivalent Ckt

10. 10ECE 584, Summer 2002Brad Noble Chapter 3 Slides Parasitic Capacitance

11. 11ECE 584, Summer 2002Brad Noble Chapter 3 Slides Capacitance Equivalent Circuit

12. 12ECE 584, Summer 2002Brad Noble Chapter 3 Slides Variation in Capacitance

13. 13ECE 584, Summer 2002Brad Noble Chapter 3 Slides 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 V DD –Need 6  m spacing between n-wells to isolate. –Dr. Engel always does this on input devices, not always elsewhere.

14. 14ECE 584, Summer 2002Brad Noble Chapter 3 Slides Subthreshold Conduction

15. 15ECE 584, Summer 2002Brad Noble Chapter 3 Slides Weak Inversion What really happens if V GS < V TN ? In digital design, I DS = 0. We call it “weak inversion” or W.I. I DS is primarily due to I drift in strong inversion and I diffusion in weak inversion.

16. 16ECE 584, Summer 2002Brad Noble Chapter 3 Slides Modes of Inversion I DS = I drift + I diffusion If V GS > V TN 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.

17. 17ECE 584, Summer 2002Brad Noble Chapter 3 Slides Weak Inversion I diffusion Drain is more reverse biased than source: To find I diff, 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!

18. 18ECE 584, Summer 2002Brad Noble Chapter 3 Slides W.I. Surface Potential

19. 19ECE 584, Summer 2002Brad Noble Chapter 3 Slides Deriving Weak Inversion I DS

20. 20ECE 584, Summer 2002Brad Noble Chapter 3 Slides W.I. FET As Exp. Law Dev. S must be big for device to be useful. If V DS = 100mV, can be neglected. For W.I. v DS,Sat  100mV Looks like a BJT

21. 21ECE 584, Summer 2002Brad Noble Chapter 3 Slides Inversion Coefficient Let Shape factor as a function of  : Lets you chose shape to match inversion mode.  < 0.1 Weakly Inverted (W.I.)  > 10 Strongly Inverted (S.I.) 0.1 <  < 10 Moderately Inverted (M.I.)

22. 22ECE 584, Summer 2002Brad Noble Chapter 3 Slides Ex. Using Inversion Coeff.

23. 23ECE 584, Summer 2002Brad Noble Chapter 3 Slides Small Signal Analysis

24. 24ECE 584, Summer 2002Brad Noble Chapter 3 Slides Ex: Quiescent Point Question: How many digits are significant?

25. 25ECE 584, Summer 2002Brad Noble Chapter 3 Slides 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

26. 26ECE 584, Summer 2002Brad Noble Chapter 3 Slides Taylor Series Expansion Taking a Taylor expansion of one variable:

27. 27ECE 584, Summer 2002Brad Noble Chapter 3 Slides Small Signal Model Params

28. 28ECE 584, Summer 2002Brad Noble Chapter 3 Slides Example: Small Signal Analysis

29. 29ECE 584, Summer 2002Brad Noble Chapter 3 Slides Small Signal Low-Freq Model

30. 30ECE 584, Summer 2002Brad Noble Chapter 3 Slides Ex: Find g m and r O

31. 31ECE 584, Summer 2002Brad Noble Chapter 3 Slides Transconductance: W.I. & M.I. What is g m for a weakly inverted FET? What is g m for a moderately inverted FET? Not in textbooks!