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CMOS VLSI Design
Introduction to
CMOS VLSI
Design
Lecture 2B: MOSFET Characteristics
Peter Kogge Joseph Nahas University of Notre Dame Fall 2009 Slightly modified and rearranged from original 2008 slides by Jay Brockman Based on lecture slides by David Harris, Harvey Mudd College http://www.cmosvlsi.com/coursematerials.html 2B: MOSFET Characteristics CMOS VLSI Design Slide 3
Outline
!! Introduction !! MOS Capacitor !! nMOS I-V Characteristics !! pMOS I-V Characteristics !! Gate and Diffusion Capacitance !! Pass Transistors !! RC Delay Models 2B: MOSFET Characteristics CMOS VLSI Design Slide 4
Outline for Today
!! nMOS I-V Characteristics !! pMOS I-V Characteristics !! Gate and Diffusion Capacitance !! Less Ideal Characteristics !! Pass Transistors CMOS VLSI Design
Review of MOS Characteristics
!! Use figures from Book !! Use more approximations !! Move quickly 2B: MOSFET Characteristics Slide 5
2B: MOSFET Characteristics CMOS VLSI Design Slide 6
Channel Charge
!! MOS structure looks like parallel plate capacitor while operating in inversion
-! Gate – oxide – channel !! Qchannel = 2B: MOSFET Characteristics CMOS VLSI Design Slide 7
Channel Charge
!! MOS structure looks like parallel plate capacitor while operating in inversion
-! Gate – oxide – channel !! Qchannel = CV !! C = 2B: MOSFET Characteristics CMOS VLSI Design Slide 8
Channel Charge
!! MOS structure looks like parallel plate capacitor while operating in inversion
-! Gate – oxide – channel !! Qchannel = CV !! C = Cg = !oxWL/tox = CoxWL !! V = Cox^ =^ !ox^ / tox 2B: MOSFET Characteristics CMOS VLSI Design Slide 9
Channel Charge
!! MOS structure looks like parallel plate capacitor while operating in inversion
-! Gate – oxide – channel !! Qchannel = CV !! C = Cg = !oxWL/tox = CoxWL !! V = Vgc – Vt = (Vgs – Vds/2) – Vt^ Cox^ =^ !ox^ / tox
2B: MOSFET Characteristics CMOS VLSI Design Slide 14
nMOS Linear I-V
!! Now we know
-! How much charge Qchannel is in the channel -! How much time t each carrier takes to cross 2B: MOSFET Characteristics CMOS VLSI Design Slide 15
nMOS Linear I-V
!! Now we know
-! How much charge Qchannel is in the channel -! How much time t each carrier takes to cross 2B: MOSFET Characteristics CMOS VLSI Design Slide 16
nMOS Linear I-V
!! Now we know
-! How much charge Qchannel is in the channel -! How much time t each carrier takes to cross 2B: MOSFET Characteristics CMOS VLSI Design Slide 17
nMOS Saturation I-V
!! If Vgd < Vt, channel pinches off near drain
-! When Vds > Vdsat = Vgs – Vt !! Now drain voltage no longer increases current
2B: MOSFET Characteristics CMOS VLSI Design Slide 18
nMOS Saturation I-V
!! If Vgd < Vt, channel pinches off near drain
-! When Vds > Vdsat = Vgs – Vt !! Now drain voltage no longer increases current 2B: MOSFET Characteristics CMOS VLSI Design Slide 19
nMOS Saturation I-V
!! If Vgd < Vt, channel pinches off near drain
-! When Vds > Vdsat = Vgs – Vt !! Now drain voltage no longer increases current 2B: MOSFET Characteristics CMOS VLSI Design Slide 20
nMOS I-V Summary
!! William Shockley 1 st^ order transistor models
-! 1952 A Unipolar Field Effect Transistor 2B: MOSFET Characteristics CMOS VLSI Design Slide 21
Example
(Same as last lecture.) !! We will be using a 0.6 μm process for your project
-! From AMI Semiconductor -! tox = 100 Å = 10 nm -! μ = 350 cm^2 /V*s -! Vt = 0.7 V -! !r = 3. -!! 0 = 8.85! 10 -14^ F/cm !! Plot Ids vs. Vds -! Vgs = 0, 1, 2, 3, 4, 5 -! Use W/L = 4/2 "# ! " =^ μ# W tox L = (^) ( (^350) ) 3.9^ •^ 8.85^ $^10 % 14 10 $ 10 %^7 & ' ( )
W L & '^ (^ ) *^ +^ = 120 W L & '^ (^ ) *^ + μ A V^2 & '^ (^ ) *^ +
2B: MOSFET Characteristics CMOS VLSI Design Slide 26
Simulated nMOS I-V Plot
!! 180 nm TSMC process !! BSIM 3v3 SPICE models !! What differs?
-! Less ON current -! No square law -! Current increases in saturation CMOS VLSI Design
Modifications to Model
!! Velocity Saturation !! Channel Length Modulation !! The Body Effect 2B: MOSFET Characteristics Slide 27 2B: MOSFET Characteristics CMOS VLSI Design Slide 28
Velocity Saturation
!! We assumed carrier velocity is proportional to E-field
-! v = μElat = μVds/L !! At high fields, this ceases to be true -! Carriers scatter off atoms -! Velocity reaches v sat -! Electrons: 6-10 x 10^6 cm/s -! Holes: 4-8 x 10^6 cm/s -! Better model 2B: MOSFET Characteristics CMOS VLSI Design Slide 29
Vel Sat I-V Effects
!! Ideal transistor ON current increases with VDD^2 !! Velocity-saturated ON current increases with VDD !! Real transistors are partially velocity saturated
-! Approximate with %-power law model -! Ids & VDD% -! 1 < % < 2 determined empirically
2B: MOSFET Characteristics CMOS VLSI Design Slide 30
%-Power Model
2B: MOSFET Characteristics CMOS VLSI Design Slide 31
Channel Length Modulation
!! Reverse-biased p-n junctions form a depletion region
-! Region between n and p with no carriers -! Width of depletion Ld region grows with reverse bias -! Leff = L – Ld !! Shorter Leff gives more current -! Ids increases with Vds -! Even in saturation 2B: MOSFET Characteristics CMOS VLSI Design Slide 32
Chan Length Mod I-V
!! " = channel length modulation coefficient
-! not feature size -! Empirically fit to I-V characteristics 2B: MOSFET Characteristics CMOS VLSI Design Slide 33
Body Effect
!! Vt: gate voltage necessary to invert channel !! Increases if source voltage increases because source is connected to the channel !! Increase in Vt with Vs is called the body effect
2B: MOSFET Characteristics CMOS VLSI Design Slide 38
Pass Transistors
!! We have assumed nmos source is grounded !! What if source > 0?
-! e.g. pass transistor passing VDD !! Vg = VDD -! If Vs > VDD-Vt, Vgs < Vt -! Hence transistor would turn itself off !! nMOS pass transistors pull no higher than VDD-Vtn -! Called a degraded “1” -! Approach degraded value slowly (low Ids) !! pMOS pass transistors pull no lower than Vtp 2B: MOSFET Characteristics CMOS VLSI Design Slide 39
Pass Transistor Ckts
2B: MOSFET Characteristics CMOS VLSI Design Slide 40
Pass Transistor Ckts
