1. ECE2030 Introduction to Computer Engineering Lecture 3: Switches and CMOS Prof. Hsien-Hsin Sean Lee School of Electrical and Computer Engineering Georgia Tech

2. 2 2 Basic Switch pathA path exists when the Switch Control is closed open (OFF) –If (Open) OUTPUT = unknown ; Switch is open (OFF) closed –Else OUTPUT = INPUT ; Switch is closed (ON) INPUT OUTPUT Switch Control

3. 3 3 The Analogy of A Transistor Cross Section An N-Channel Metal-Oxide Semiconductor Field Effect Transistor (MOSFET) INPUT OUTPUT Switch Control (Gate)

4. 4 4 Transistor Characteristics Cut-offCut-off Region –Vgs – Vt  0 –No current (Ids) between drain and source LinearLinear (or Ohmic) Region –0 < Vds < Vgs – Vt –Ids is a function of Vgs and Vds –Ids = β*[(Vgs-Vt)*Vds – Vds*Vds/2] SaturationSaturation Region –0 < Vgs – Vt < Vds –Ids is independent of Vds –Ids = (β/2)*(Vgs-Vt) 2 –β = process factor * (W/L) VtVt : Threshold voltage, a function of materials, doping, insulator thickness, etc. Gate Drain Source Ids Vds Vgs N-type MOS Transistor

5. 5 5 Transistor Characteristics

6. 6 6 Switches in Series INPUT OUTPUT S1 S2 Truth Table S1S2PATH? OFF ON OFF ON

7. 7 7 Switches in Series INPUT OUTPUT S1 S2 Truth Table (OFF/ON=0/1) S1S2PATH? OFF NO OFFONNO ONOFFNO ON YES What Function ??

8. 8 8 Switches in Series INPUT OUTPUT S1 S2 Truth Table (OFF/ON=0/1) S1S2PATH? 000 Function = ??

9. 9 9 Switches in Series INPUT OUTPUT S1 S2 Truth Table (OFF/ON=0/1) S1S2PATH? 000 010 Function = ??

10. 10 Switches in Series INPUT OUTPUT S1 S2 Truth Table (OFF/ON=0/1) S1S2PATH? 000 010 100 Function = ??

11. 11 Switches in Series INPUT OUTPUT S1 S2 Truth Table (OFF/ON=0/1) S1S2PATH? 000 010 100 111 AND Function = Logic AND

12. 12 Switches in Parallel INPUT OUTPUT S1 Truth Table S1S2PATH? OFF NO OFFONYES ONOFFYES ON YES S2

13. 13 Switches in Parallel INPUT OUTPUT S1 Truth Table S1S2PATH? 000 Function =?? S2

14. 14 Switches in Parallel INPUT OUTPUT S1 Truth Table S1S2PATH? 000 011 Function =?? S2

15. 15 Switches in Parallel INPUT OUTPUT S1 Truth Table S1S2PATH? 000 011 101 Function =?? S2

16. 16 Switches in Parallel INPUT OUTPUT S1 Truth Table S1S2PATH? 000 011 101 111 OR Function = Logic OR S2

17. 17 CMOS Transistor Complementary MOS –P-channel MOS (pMOS) –N-channel MOS (nMOS) pMOS –P-type source and drain diffusions –N substrate –Mobility by holes nMOS –N-type source and drain diffusions –P substrate –Mobility by electrons Gate Drain Source Gate Source Drain pMOS nMOS

18. 18 Pass Transistor using NMOS Assume capacitor (C L ) is initially discharged Gate=1, Vin=1 –C L begins to conduct and charges toward 1 (Vdd) and stops at (Vdd-Vt) –Signal is degraded Gate=Vdd Vin=Vdd Vout Ground Load Capacitor Vgs I Gate=Vdd Vin=0 Vout=Vdd Ground Load Capacitor Vgs I Gate=1, Vin=0 –C L begins to discharge toward 0 –

19. 19 Transmission Degradation using Pass Transistor Vdd - Vt Vdd Vdd (1) Vdd - 2Vt Vdd Vout = Vdd- N*Vt Still 1??

20. 20 CMOS Signal Transfer Property GatePath 0Closed 1Open Gate Drain Source Gate Source Drain GatePath 0Open 1Closed pMOS nMOS Transmits 1 well Transmits 0 poorly Transmits 0 well Transmits 1 poorly

21. 21 CMOS Transmission Gate Transmit signal from INPUT to OUTPUT when Gate is closed complementary of Gate Gate (complementary of Gate) SourceDrain Gate INPUT OUTPUT GatepMOSnMOSOUTPUT 0OFF Z 1ON INPUT Z Z : High-Impedance State, consider the terminal is “floating”

22. 22 High Impedance When a path exists –Impedance is low to allow ample flow of current When no path –Impedance is high allowing almost no current flow between two terminals Gate=1 Drain Source << 10K  >> 100M  Closed Gate=0 Drain Source Open

23. 23 Transmission Gates Gate = 1 0 0 Gate = 0 Transmit Logic 0 Gate = 1 1 1 Gate = 0 Transmit Logic 1

24. 24 Transmission Gate Symbol Gate INPUT OUTPUT Gate INPUT OUTPUT 