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Low-Power Design of CMOS Circuits: Capacitance Insights

Explore capacitance optimization techniques for low-power electronic circuit design in CMOS technology. Learn about gate capacitance, bulk nMOSFET, and device scaling to enhance energy efficiency and reduce voltage leakage.

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Low-Power Design of CMOS Circuits: Capacitance Insights

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  1. ELEC 5970-001/6970-001(Fall 2005)Special Topics in Electrical EngineeringLow-Power Design of Electronic CircuitsCapacitance in a CMOS Circuit Vishwani D. Agrawal James J. Danaher Professor Department of Electrical and Computer Engineering Auburn University http://www.eng.auburn.edu/~vagrawal vagrawal@eng.auburn.edu ELEC5970-001/6970-001 Lecture 3

  2. Capacitances VDD C1 In Out C2 CW GND ELEC5970-001/6970-001 Lecture 3

  3. Miller Capacitance VDD C1 In Out CM C2 CW GND ELEC5970-001/6970-001 Lecture 3

  4. Before Transition VDD C1 +VDD 0 In Out CM C2 CW GND ELEC5970-001/6970-001 Lecture 3

  5. After Transition VDD Energy from supply = 2 CM VDD2 Effective capacitance = 2 CM C1 -VDD 0 In Out CM C2 CW GND ELEC5970-001/6970-001 Lecture 3

  6. Capacitances in MOSFET Cgs Cgd Gate Gate oxide Source Drain Cg Cd Cs Bulk ELEC5970-001/6970-001 Lecture 3

  7. Bulk nMOSFET Polysilicon Gate Drain W Source n+ n+ L p-type body (bulk) SiO2 Thickness = tox ELEC5970-001/6970-001 Lecture 3

  8. Gate Capacitance Cg = Cox WL = C0, intrinsic cap. Cg = Cpermicron W εox Cpermicron = Cox L = ── L tox where εox=3.9ε0 for Silicon dioxide = 3.9×8.85×10-14 F/cm ELEC5970-001/6970-001 Lecture 3

  9. Intrinsic Capacitances Weste and Harris, CMOS VLSI Design, Addison-Wesley, 2005, p. 78. ELEC5970-001/6970-001 Lecture 3

  10. Low-Power Transistors • Device scaling to reduce capacitance and voltage. • Body bias to reduce threshold voltage and leakage. • Multiple threshold CMOS (MTCMOS). • Silicon on insulator (SOI) ELEC5970-001/6970-001 Lecture 3

  11. Device Scaling • Reduced dimensions • Reduce supply voltage • Reduce capacitances • Reduce delay • Increase leakage due to reduced VDD/ Vt ELEC5970-001/6970-001 Lecture 3

  12. Optimum Threshold Voltage Vt = 0.7V Vt = 0.3V Delay Delay or Energy-delay product Energy-delay product 0 1 2 3 4 5 6 • VDD / Vt ELEC5970-001/6970-001 Lecture 3

  13. Bulk CMOS Inverter Polysilicon (input) SiO2 Output GND VDD metal 1 p+ n+ p+ p+ n+ n+ n-well p-substrate (bulk) ELEC5970-001/6970-001 Lecture 3

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