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Digital Integrated Circuits A Design Perspective

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1 Digital Integrated Circuits A Design Perspective
EE141 Digital Integrated Circuits A Design Perspective Jan M. Rabaey Anantha Chandrakasan Borivoje Nikolic The Devices July 30, 2002

2 Goal of this chapter Present intuitive understanding of device operation Introduction of basic device equations Introduction of models for manual analysis Introduction of models for SPICE simulation Analysis of secondary and deep-sub-micron effects Future trends

3 The Diode Mostly occurring as parasitic element in Digital ICs n p B A
SiO 2 Al Cross-section of pn -junction in an IC process One-dimensional representation diode symbol Mostly occurring as parasitic element in Digital ICs

4 Depletion Region

5 Diode Current

6 Forward Bias Typically avoided in Digital ICs

7 Reverse Bias The Dominant Operation Mode

8 Models for Manual Analysis

9 Junction Capacitance

10 Diffusion Capacitance

11 Secondary Effects Avalanche Breakdown 0.1 ) A ( I –0.1 –25.0 –15.0
I D –0.1 –25.0 –15.0 –5.0 5.0 V (V) D Avalanche Breakdown

12 Diode Model

13 SPICE Parameters

14 What is a Transistor? A Switch! |V GS | An MOS Transistor

15 The MOS Transistor Polysilicon Aluminum

16 MOS Transistors - Types and Symbols
G G S S NMOS Enhancement NMOS Depletion D D G G B S S NMOS with PMOS Enhancement Bulk Contact

17 Threshold Voltage: Concept

18 The Threshold Voltage

19 The Body Effect

20 Current-Voltage Relations A good ol’ transistor
0.5 1 1.5 2 2.5 3 4 5 6 x 10 -4 V DS (V) I D (A) VGS= 2.5 V VGS= 2.0 V VGS= 1.5 V VGS= 1.0 V Resistive Saturation VDS = VGS - VT Quadratic Relationship

21 Transistor in Linear

22 Transistor in Saturation
Pinch-off

23 Current-Voltage Relations Long-Channel Device

24 A model for manual analysis

25 Current-Voltage Relations The Deep-Submicron Era
-4 V DS (V) 0.5 1 1.5 2 2.5 x 10 I D (A) VGS= 2.5 V VGS= 2.0 V VGS= 1.5 V VGS= 1.0 V Early Saturation Linear Relationship

26 Velocity Saturation u ( m / s ) u = 10 x = 1.5 x (V/µm) 5 sat n c
Constant velocity Constant mobility (slope = µ) x c = 1.5 x (V/µm)

27 Perspective I V Long-channel device V = V Short-channel device V V - V
GS DD Short-channel device V V - V V DSAT GS T DS

28 ID versus VGS linear quadratic quadratic Long Channel Short Channel
0.5 1 1.5 2 2.5 3 4 5 6 x 10 -4 V GS (V) I D (A) 0.5 1 1.5 2 2.5 x 10 -4 V GS (V) I D (A) linear quadratic quadratic Long Channel Short Channel

29 ID versus VDS Resistive Saturation VDS = VGS - VT Long Channel
0.5 1 1.5 2 2.5 3 4 5 6 x 10 -4 V DS (V) I D (A) VGS= 2.5 V VGS= 2.0 V VGS= 1.5 V VGS= 1.0 V Resistive Saturation VDS = VGS - VT -4 V DS (V) 0.5 1 1.5 2 2.5 x 10 I D (A) VGS= 2.5 V VGS= 2.0 V VGS= 1.5 V VGS= 1.0 V Long Channel Short Channel

30 A unified model for manual analysis
G B

31 Simple Model versus SPICE
0.5 1 1.5 2 2.5 x 10 -4 Velocity Saturated Linear Saturated VDSAT=VGT VDS=VDSAT VDS=VGT (A) I D V (V) DS

32 A PMOS Transistor Assume all variables negative! -2.5 -2 -1.5 -1 -0.5
-0.8 -0.6 -0.4 -0.2 x 10 -4 V DS (V) I D (A) VGS = -1.0V VGS = -1.5V VGS = -2.0V Assume all variables negative! VGS = -2.5V

33 Transistor Model for Manual Analysis

34 The Transistor as a Switch

35 The Transistor as a Switch

36 The Transistor as a Switch

37 MOS Capacitances Dynamic Behavior

38 Dynamic Behavior of MOS Transistor

39 The Gate Capacitance x L Polysilicon gate Top view Gate-bulk overlap
d L Polysilicon gate Top view Gate-bulk overlap Source n + Drain W t ox n + Cross section L Gate oxide

40 Gate Capacitance Cut-off Resistive Saturation Most important regions in digital design: saturation and cut-off

41 Gate Capacitance Capacitance as a function of VGS
(with VDS = 0) Capacitance as a function of the degree of saturation

42 Measuring the Gate Cap

43 Diffusion Capacitance
Channel-stop implant N 1 A Side wall Source W N D Bottom x Side wall j Channel L S Substrate N A

44 Junction Capacitance

45 Linearizing the Junction Capacitance
Replace non-linear capacitance by large-signal equivalent linear capacitance which displaces equal charge over voltage swing of interest

46 Capacitances in 0.25 mm CMOS process

47 The Sub-Micron MOS Transistor
Threshold Variations Subthreshold Conduction Parasitic Resistances

48 Threshold Variations V V Low V threshold Long-channel threshold VDS L
Threshold as a function of Drain-induced barrier lowering the length (for low V ) (for low L ) DS

49 Sub-Threshold Conduction
0.5 1 1.5 2 2.5 10 -12 -10 -8 -6 -4 -2 V GS (V) I D (A) VT Linear Exponential Quadratic The Slope Factor S is DVGS for ID2/ID1 =10 Typical values for S: mV/decade

50 Sub-Threshold ID vs VGS
VDS from 0 to 0.5V

51 Sub-Threshold ID vs VDS
VGS from 0 to 0.3V

52 Summary of MOSFET Operating Regions
Strong Inversion VGS > VT Linear (Resistive) VDS < VDSAT Saturated (Constant Current) VDS  VDSAT Weak Inversion (Sub-Threshold) VGS  VT Exponential in VGS with linear VDS dependence

53 Parasitic Resistances

54 Latch-up

55 Future Perspectives 25 nm FINFET MOS transistor

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