The Physical Structure (NMOS)

1. The Physical Structure (NMOS) Gate oxide Polysilicon Gate Al Al SiO2 SiO2 SiO2 S D Field Oxide Field Oxide n+ channel n+ L P Substrate contact Metal (G) L (S) (D) n+ n+ W Poly

2. RS Rch RD Transistor Resistance Two Components: Drain/ Sources Resistance: RD(S) = Rsh x no. of squares+ contact resistance. Channel Resistance: Depends on the region of operation: (G) : L (S) (D) n+ n+ W Linear Saturation

3. Transistor Geometry

4. Transistor Geometry- Detailed

5. NMOS Operation-Linear KN=K’. W/L Process Transconductance uA/V2 for 0.35u, K’ (Kp)=196uA/ V2 Gate oxide capacitance per unit area eox = 3.9 x eo = 3.45 x 10-11 F/m tox Oxide thickness for 0.35 m , tox=100Ao Quick calculation of Cox: Cox= 0.345/tox (Ao) pf/um2 m = mobility of electrons 550 cm2/V-sec for 0.35 m process

6. NMOS Operation-Linear Effect of W/L Effect of temperature W W Rds W/L temp Rds m

7. Variations in Width and Length polysilicon 1. Width Oxide encroachment Weff= Wdrawn-2WD 2. Length Lateral diffusion LD= 0.7Xj Leff= Ldrawn-2LD Weff WD WD Wdrawn polysilicon Ldrawn LD Leff LD

8. Large Transistors • Rchannel decrease with increase of W/L of the transistor

9. Semiconductor Resistors Resistance R= p(l /A) = (p/t). (l /w) = Rsh. (l /w) Rsh = Sheet resistance / For 0.5u process: N+ diffusion : 70 / M1: 0.06 P+ diffusion : 140  / M2: 0.06 Polysilicon : 12  / M3: 0.03 Polycide:2-3  / P-well: 2.5K N-well: 1K current t l w (A)

10. Semiconductor Resistors polysilicon Diffusion n+ Al Al SiO2 Field oxide n+ Polysilicon Resistor Diffusion Resistor

11. Semiconductor Capacitors 1. Poly Capacitor: a. Poly to substrate b. Poly1 to Poly2 2. Diffusion Capacitor sidewall capacitances depletion region n+ (ND) bottomwall capacitance substrate (NA)

12. Dynamic Behavior of MOS Transistor Prentice Hall/Rabaey

13. SPICE Parameters for Parasitics Prentice Hall/Rabaey

14. SPICE Transistors Parameters Prentice Hall/Rabaey

15. Modelling: Resistance 1. Resistance: Rint= Rsh [l/w] Rsh values for 0.35u CMOS Process: Polysilicon 10 / Polycide 2 / Metal1 0.07 / Metal II 0.07 / Metal III 0.05 / Contact resistance: PolyI to MetalI 50  Via resistance: Metal I to Metal II 1.5  Via resistance: Metal II to metal III 1.

16. CMOS Inverter: Steady State Response V V DD DD R on V = V OH DD V V = 0 out V OL out R on V = V V = 0 in DD in

17. Switching Characteristics of Inverters Transient Response

18. Computing the Capacitances V V DD DD M 2 M 4 C C g 4 db 2 C gd 12 V V V out out 2 in C C C db 1 w g 3 M 3 M 1 Interconnect Fanout V V in out Simplified C Model L

19. Computing the Capacitances

20. Delay Definitions

21. VDD=5V S G VDD MP D Vin Vo D CL G MN S GND Step Response Fall Delay Time: TPHL MN OFF Saturation Linear IDN V = 5 in Vin V = 4 in V = 3 in Vo VDD-VT VDD (VDSAT)

22. Step Response- Fall time, tf vo 1 0.9 1-n vin tf=~ k is a constant 0.1 td1 td2 0.1 tr=~ k is a constant

23. Step Response-tPHL Assume normalized voltages vin= Vin/ VDD vo= Vo/ VDD n = VTN/ VDD p = VTP/ VDD tPHL=td1+td2 Vo VDD VDD-VTN Vx VDD/ 2 Vin td1 td2 vo 1 1-n 0.5 vin td1 td2

24. Step Response Rise Delay tPLH and Rise Time tr VDD S G VDD MP D Vin Vo D 0.1 CL G MN S  (P= - 0.2) GND

25. Factors Influence Delay • Inverter Delay,td = (tPHL+tPLH)/2 • The following factors influence the delay of the inverter: • Load Capacitance • Supply Voltage • Transistor Sizes • Junction Temperature • Input Transition Time

26. Delay as a function of VDD

27. Delay as a function of Transistor Size • tPHL and tf decrease with the increase of W/L of the NMOS • tPLH and tr decrease with the increase of W/L of the PMOS

28. Temperature Effect • Temperature ranges: commercial : 0 to700C industrial: -40 to 850C military: -55 to 1250C • Calculation of the junction temperature tj= ta + ja X Pd • Effect of temperature on mobility • Delay and speed implications

29. Effect of Input Transition Times The delay of the inverter increases with the increase of the input transition times r and f tPHL = (tPHL) step + (r /6).(1-2p) tPLH = (tPLH) step + (f/6).(1+2n) Vo r Vin

30. Transistor Sizing • Define  = (W/L)p/(W/L)n • For Equal Fall and Rise Delay • KN=KP •  = n/ p • For Minimum Delay • dtD/d  = 0 • opt = Sqrt (n/ p)

31. Two Components contribute to the power dissipation: Static Power Dissipation Leakage current Sub-threshold current Dynamic Power Dissipation Short circuit power dissipation Charging and discharging power dissipation Power Dissipation in CMOS

32. Static Power Dissipation VDD • Leakage Current: • P-N junction reverse biased current: • io= is(eqV/kT-1) • Typical value 0.1nA to 0.5nA @room temp. • Total Power dissipation: • Psl= i0.VDD • Sub-threshold Current • Relatively high in low threshold devices S G B MP D Vin Vo D G B MN S GND

33. Analysis of CMOS circuit power dissipation • The power dissipation in a CMOS logic gate can be • expressed as • P = Pstatic + Pdynamic • = (VDD · Ileakage) + (p · f · Edynamic) • Where p is the switching probability or activity factor • at the output node (i.e. the average number of output • switching events per clock cycle). • The dynamic energy consumed per output switching event is defined as Edynamic =

34. Analysis of CMOS circuit power dissipation The first term —— the energy dissipation due to the Charging/discharging of the effective load capacitance CL. The second term —— the energy dissipation due to the input-to-output coupling capacitance. A rising input results in a VDD-VDD transition of the voltage across CM and so doubles the charge of CM. CL = Cload + Cdbp +Cdbn CM = Cgdn + Cgdp

35. The MOSFET parasitic capacitances • • distributed, • • voltage-dependent, and • • nonlinear. • So their exact modeling is quite complex. Even ESC can be modeled, it is also difficult to calculate the Edynamic. On the other hand, if the short-circuit current iSC can be Modeled, the power-supply current iDD may be modeled with the same method. So there is a possibility to directly model iDD instead of iSC.

36. Schematic of the Inverter

38. Analysis of short-circuit current The short-circuit energy dissipation ESC is due to the rail-to-rail current when both the PMOS and NMOS devices are simultaneously on. ESC = ESC_C + ESC_n Where and

39. Charging and discharging currents • Discharging Inverter Charging Inverter

40. Factors that affect the short-circuit current For a long-channel device, assuming that the inverter is symmetrical (n = p =  and VTn = -VTp = VT) and with zero load capacitance, and input signal has equal rise and fall times (r = f = ), the average short-circuit current [Veendrick, 1994] is From the above equation, some fundamental factors that affect short-circuit current are:  , VDD, VT,  and T.

41. Parameters affecting short cct current For a short-channel device,  and VT are no longer constants, but affected by a large number of parameters (i.e. circuit conditions, hspice parameters and process parameters). CL also affects short-circuit current. Imean is a function of the following parameters (tox is process-dependent): CL, , T (or /T), VDD, Wn,p, Ln,p (or Wn,p/Ln,p ), tox, … The above argument is validated by the means of simulation in the case of discharging inverter,

42. The effect of CL on Short CCt Current

43. Effect of tr on short cct Current

44. Effect of Wp on Short cct Current

45. Effect of timestep setting on simulation results

46. Thank you !