1. ECE 431 Digital Circuit Design Chapter 3 MOS Transistor (MOSFET) (slides 2: key Notes) Lecture given by Qiliang Li 1

2. Introduction Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is the fundamental building block for modern digital integrated circuit and analog circuit. MOS structure I-V Characteristics of MOSFET MOSFET scaling and short-channel effect MOSFET capacitance 2

3. 3.1-3.3 MOS structure 3

4. Energy band diagrams of the components of MOS structure Flat-band voltage qV FB 4

5. Energy band diagram of a combined MOS structure 5

6. Example 1 for MOS structure The electron affinity of silicon is 4.15eV For a p-Si:, gate metal is Al:, what is V FB ? An: 4.1eV 4.15eV 0.55eV 0.2eV AlSi EcEc EiEi E Fp EvEv V FB 6

7. MOS structure: Inversion 7

8. MOSFET structure 8

9. MOSFET with Bias 9

10. Start of Strong Inversion. MOSFET starts to turn on. Threshold Voltage Body-effect coefficient 10

11. Depletion region charge at V SB = 0 for p-Si Negative b/c hole is depleted for n-Si Positive b/c electron is depleted Oxide fixed charge is positive Q ox > 0 11

12. Find V T0 and V T N-channel MOSFET Substrate Na=1E16 cm -3 Poly-Si gate Nd=2E20 cm -3 Tox=50 nm Oxide interface fixed charge density Nox=4E10 cm -3 Example 2 12

13. 13

14. 3.4 MOSFET I-V Characteristics Why it is negative? n channel 14

15. Resistance of tiny part of the channel dy : Voltage drop across dy : 15

16. Saturation? Find the maximum Example 3 16

17. Channel length modulation effect Channel length modulation effect: The effective channel length during saturation is significantly shorter than before saturation. 17

18. 18

19. Example 4 from Eq 3.23 19

20. Full Scaling (Constant-Field Scaling) QuantityBefore ScalingFull Scaling Channel LengthL1/S Channel WidthW1/S Gate oxide thicknesst ox 1/S Junction depthxjxj 1/S Power supply voltageV DD 1/S Threshold voltageV T0 1/S Doping densitiesN A and N D S Oxide capacitanceC ox S Drain currentIDID 1/S Power dissipationP1/S 2 Power densityP/area1 Time delay t delay Capacitance ? C ox W L -> 1/S Power = I DD V DD 3.5 MOSFET Scaling and Small-Geometry Effects 20

21. Short Channel Effects V T0 decreases Why? How much? Mobility degradation Oxide breakdown because of hot carrier effect 21

22. Example 5 22

23. V T decreases significantly Example 5 continue… 23

24. 3.6: MOSFET Capacitance 24

25. 25

26. Oxide Capacitance 26

27. Junction Capacitance 27