Probabilistic modelling of performance parameters of Carbon Nanotube transistors

Department of Electrical and Computer Engineering

ByYaman Sangar

Amitesh NarayanSnehal Mhatre

Overview■ Motivation■ Introduction■ CMOS v/s CNTFETs■ CNT Technology - Challenges■ Probabilistic model of faults■ Modelling performance parameters:◻ION / IOFF tuning ratio

◻Gate delay◻Noise Margin

■ Conclusion

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Overview Motivation Introduction CMOS v/s CNTFETs CNT Technology – Challenges Probabilistic model of faults Modelling performance parameters:

ION / IOFF tuning ratio Gate delay Noise Margin

Conclusion

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MOTIVATION: Why CNTFET?

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■ Dennard Scaling might not last long■ Increased performance by better algorithms?■ More parallelism?■ Alternatives to CMOS - FinFETs, Ge-nanowire FET, Si-

nanowire FET, wrap-around gate MOS, graphene ribbon FET ■ What about an inherently faster and less power consuming

device?■ Yay CNTFET – faster with low power

Overview Motivation Introduction CMOS v/s CNTFETs CNT Technology – Challenges Probabilistic model of faults Modelling performance parameters:

ION / IOFF tuning ratio Gate delay Noise Margin

Conclusion

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5

Carbon Nanotubes

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■ CNT is a tubular form of carbon with diameter as small as 1nm

■ CNT is configurationally equivalent to a 2-D graphene sheet rolled into a tube.

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Types of CNTs

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■ Single Walled CNT (SWNT)■ Double Walled CNT (DWNT)■ Multiple Walled CNT (MWNT)

■ Depending on Chiral angle:• Semiconducting CNT (s-CNT)• Metallic CNT (m-CNT)

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Properties of CNTs

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■ Strong and very flexible molecular material■ Electrical conductivity is 6 times that of copper■ High current carrying capacity■ Thermal conductivity is 15 times more than copper■ Toxicity?

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CNTFET

How CNTs conduct?

■ Gate used to electrostatically induce carriers into tube■ Ballistic Transport

Overview Motivation Introduction CMOS v/s CNTFETs CNT Technology – Challenges Probabilistic model of faults Modelling performance parameters:

ION / IOFF tuning ratio Gate delay Noise Margin

Conclusion

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Circuit FET Delay (In Picoseconds)

Power (In uWatts)

Inverter CMOS 16.58 9.81

CNT 3.78 0.25

2 Input Nand CMOS 24.32 20.67

CNT 5.98 0.69

2 Input Nor CMOS 39.26 22.13

CNT 6.49 0.48

Simulation based Comparison between CMOS and CNT technology

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04/29/2014 11Better delay

Circuit FET Delay (In Picoseconds)

Power (In uWatts)

Inverter CMOS 16.58 9.81

CNT 3.78 0.25

2 Input Nand CMOS 24.32 20.67

CNT 5.98 0.69

2 Input Nor CMOS 39.26 22.13

CNT 6.49 0.48

Simulation based Comparison between CMOS and CNT technology

04/29/2014 12Better delay

At lower power!

Circuit FET Delay (In Picoseconds)

Power (In uWatts)

Inverter CMOS 16.58 9.81

CNT 3.78 0.25

2 Input Nand CMOS 24.32 20.67

CNT 5.98 0.69

2 Input Nor CMOS 39.26 22.13

CNT 6.49 0.48

Simulation based Comparison between CMOS and CNT technology

Overview Motivation Introduction CMOS v/s CNTFETs CNT Technology – Challenges Probabilistic model of faults Modelling performance parameters:

ION / IOFF tuning ratio Gate delay Noise Margin

Conclusion

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■ Major CNT specific variations■ CNT density variation■ Metallic CNT induced count variation■ CNT diameter variation■ CNT misalignment■ CNT doping variation

Challenges with CNT technology

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■ Unavoidable process variations

■ Performance parameters affected

CNT density variation CNT diameter variation

■ Current variation

■ Threshold voltage variation

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CNT Misalignment CNT doping variation

■ Changes effective CNT length■ Short between CNTs■ Incorrect logic functionality■ Reduction in drive current

■ May not lead to unipolar behavior

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Metallic CNT induced count variation

m-CNTs-CNT

■ Excessive leakage current■ Increases power consumption■ Changes gate delay■ Inferior noise performance■ Defective functionality

s-CNT

m-CNT

Vgs

Cur

rent

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Removal of m-CNTFETs

■ VMR Technique : A special layout called VMR structure consisting of inter-digitated electrodes at minimum metal pitch is fabricated. M-CNT electrical breakdown performed by applying high voltage all at once using VMR. M-CNTs are burnt out and unwanted sections of VMR are later removed.

■ Using Thermal and Fluidic Process: Preferential thermal desorption of the alkyls from the semiconducting nanotubes and further dissolution of m-CNTs in chloroform.

■ Chemical Etching: Diameter dependent etching technique which removes all m-CNTs below a cutoff diameter.

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Overview Motivation Introduction CMOS v/s CNTFETs CNT Technology – Challenges Probabilistic model of faults Modelling performance parameters:

ION / IOFF tuning ratio Gate delay Noise Margin

Conclusion

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Probabilistic model of CNT count variation due to m-CNTs

■ ps = probability of s-CNT■ pm = probability of m-CNT■ ps = 1 - pm

■ Ngs = number of grown s-CNTs■ Ngm = number of grown m-CNTs■ N = total number of CNTs

Probability of grown CNT count

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Conditional probability after removal techniques

■ Ns = number of surviving s-CNTs■ Nm = number of surving m-CNTs■ prs = conditional probability that a CNT is removed given that it is s-CNT ■ prm = conditional probability that a CNT is removed given that it is m-CNT■ qrs = 1 - prs

■ qrm = 1 -prm

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P൛Ns = ns|Ngs = ngsൟ= ngsCnsqrsnsprs

(ngs-ns) P൛Nm = nm|Ngm = ngmൟ= ngmCnmqrm

(ngm-nm)prmnm

Overview Motivation Introduction CMOS v/s CNTFETs CNT Technology – Challenges Probabilistic model of faults Modelling performance parameters:

ION / IOFF tuning ratio Gate delay Noise Margin

Conclusion

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Effect of CNT count variation on ION / IOFF tuning ratio

■ ION / IOFF is indicator of transistor leakage

■ Improper ION / IOFF → slow output transition or low output swing

■ Target value of ION / IOFF = 104

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Current of a single CNT

ICNT = ps Is + pmIm

µ(ICNT) = psµ( Is )+ pmµ(Im )

■ ICNT = drive current of single CNT (type unknown)■ Is = drive current of single s-CNT■ Im = drive current of single m-CNT

■ ps = probability of s-CNT■ pm = probability of m-CNT

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■ Ns = count of s-CNT■ Nm = count of m-CNT

■ Is,on = s-CNT current, Vgs = Vds = Vdd

■ Is,off = s-CNT current, Vgs = 0 and Vds = Vdd

■ Im = m-CNT current, Vds = Vdd

ION / IOFF ratio of CNTFET

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µ (Ns) = ps (1 - prs) N

µ (Nm) = pm (1 - prm) N

ION / IOFF ratio of CNTFET

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Effect of various processing parameters on the ratio µ(ION) / µ(IOFF)

■ µ(ION) / µ(IOFF) is more sensitive to prm

■ µ(ION) / µ(IOFF) = 104 for prm > 1 – 10 -4 = 99.99 % for pm = 33.33%

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1- prm

𝜇 (𝐼 𝑜𝑛)𝜇(𝐼 𝑜𝑓𝑓 )

Overview Motivation Introduction CMOS v/s CNTFETs CNT Technology – Challenges Probabilistic model of faults Modelling performance parameters:

ION / IOFF tuning ratio Gate delay Noise Margin

Conclusion

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Effect of CNT count variation on Gate delay

delay=C load ∆V

I drive

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= =

σ (delay )≈ μ (delay)σ ( I drive)

¿¿σ (delay )≈

Cload V dd

μ2¿¿

μ(delay)≈C load V dd

¿¿

¿𝑝𝑠 σ 2 ( 𝐼 𝑠 )+𝑝𝑚 σ 2 ( 𝐼𝑚 )+𝑝𝑚𝑝𝑠 [ μ ( 𝐼 𝑠 )− μ ( 𝐼𝑚 ) ] 2

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Plot of v/s

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= 0.3σ (delay )μ(delay )

𝜎 𝑠

𝜇𝑠

N = 10

N = 20

N = 30N = 40

N = 50

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Plot of v/s N

σ (delay )μ(delay )

N

0.2 0.4 0.60.8

0.9

Overview Motivation Introduction CMOS v/s CNTFETs CNT Technology – Challenges Probabilistic model of faults Modelling performance parameters:

ION / IOFF tuning ratio Gate delay Noise Margin

Conclusion

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Noise Margin of CNTFET

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VIL and VIH

■ Substituting = Vin, , and

■ =

■ Differentiating with respect to Vin and substituting -1

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nFET

pFET

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For CNTFET, For CMOS,

VIL and VIH

NML = VIL - 0

NMH = VDD – VIH

Overview Motivation Introduction CMOS v/s CNTFETs CNT Technology – Challenges Probabilistic model of faults Modelling performance parameters:

ION / IOFF tuning ratio Gate delay Noise Margin

Conclusion

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CONCLUSION

■ Modeled count variations and hence device current as a probabilistic function

■ Studied the affect of these faults on tuning ratio and gate delay■ Inferred some design guidelines that could be used to judge the correctness

of a process■ Mathematically derived noise margin based on current equations – better

noise margin than a CMOS

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