Graphene Transistors: Status,Prospects, and Problems

Brajesh RawatResearch Scholar

Email-id: r.brajesh@iitg.ernet.in

Department of Electronic & Electrical Engineering

Indian Institute of Technology

Guwahati,Assam

6th November, 2014

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OUTLINE

Why is graphene popular these days?

What is really matter for FET?

Graphene properties relevant to transistors

Current status of graphene transistors

Graphene fabrication techniques

Conclusion

Reference

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Why Graphene is popular these days?

Arrangement of carbon atom on the graphene

Figure 1: Graphene sheet

1 Transparent and bendable

2 Thinnest and lightest object

3 Its conduct electricity muchbetter than copper.

4 300 times stronger than steeland harder than a diamond.

Current technology trends may get replace by graphene 1.

1Source:http://graphene-flagship.eu3 / 18

Graphene Contd...

Graphene is the mother of all carbon-based systems.

Figure 2: Graphene: mother (top left), Graphite finds in our pencils is simply a stackof graphene layers (top right), Carbon nanotubes are made of rolled-up sheets ofgraphene (bottom left) and Buckminsterfullerene are spheres of wrapped-up graphene(bottom right)1.

1Source: Neto A. et al. Phys World (2006).4 / 18

What is really matter for FET?

Figure 3: Conventional MOSFETs [1].

S.No Essential fea-tures

Main FOM Essential Re-quirement

Additional Re-quirements

1 LogicTransistor(switch)[1, 2]

Ion/Ioff in be-tween 104 −106

Bandgap of0.4eV.

Shorter gateand fast carri-ers.

2 AnalogTransistor(amplify)[1, 2]

fT should bein GHz (MostApplications)and AV =30

Current satura-tion in outputcharacteristics

Shorter gates,faster carriersand lower seriesresistance.

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Graphene properties relevant to transistors

1 Carrier mobility is much higher then III-V and Silicon at roomtemperature [3].

2 High saturation velocity [4].

3 Electrons and holes behavior is same for electric field effect [3].

4 Ballistic Transport over a length of 0.4µm [5].

5 High intrinsic carrier concentration and very high current den-sity [3].

6 It is a self-cooling material [4].

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Challenges with Graphene for Transistor ApplicationsEnergy

k

Conduction band

Valance band

Fermi level

Energy

k

Conduction band

Valance band

Ban

dgap

Silicon Bandstructure

Graphene Bandstructure

Figure 4

1 Zero bandgap limits minimumchannel conductivity in FET andleads to a low Ion/Ioff ratio.

2 Weak saturation lower the intrinsicgain.

3 Bulk and source/drain contacteffect the transports [4].

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Bandgap Openning in Graphene

The bandstructure of graphene can be modified following ways:

Graphene on substrate

Graphene nanoribbon (GNR)

Bilayer Graphene

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Graphene Bandgap on Substrate

Graphene Type Size Bandgap Remark

SL graphene on SiO2 LA No Experimental and Theory [1]

SL graphene on SiO2 GNR Yes Experimental and Theory: Gapdue to lateral confinement [1]

BL graphene on SiO2 LA Yes Experimental and Theory:gap dueto symmetry breaking by perpen-dicular interlayer field [2]

Epitaxial SL LA Unknown Controversial discussion* [1]

Epitaxial BL LA Yes Experimental and Theory [1]

Epitaxial SL,BL GNR Yes Theory [1]

Stained SL LA Yes Theory: gap due to level crossing[1]

Graphene on h-BZ LA Yes Experimental [6]

SL: Single layer,BL: Bilayer GNR: Graphene nanoribbon,LA:Large Area 9 / 18

Performance comparison

S No. GrapheneType

Bandgap I on/I off f T AV Drawback

1 NanoribbonFET [1, 2]

0.3-0.4eV 104 − 106 708GHzfor LG =50nm

20 Edge disorder andedge geometeryaffect bandgap.

2 BilayerFET [7, 8]

0.2eV 100 .... 35 Requires a largerperpendicularelectric field

3 Larger areaFET [5]

0 2-5 427GHzfor LG =67nm [2]

6 ...

4 NominalSi-MOSFETs

1.12eV 104 − 107 200GHzfor LG =65nm [2]

5 Short channel ef-fects.

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Electronic transport in graphene

Figure 5: Electron mobility versus bandgap for different materials (left) and electrondrift velocity versus electric field for common semiconductors (right) [1].

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Output characteristics for Si-MOSFETs and grapheneMOSFETs

Figure 6: Typical Output characteristics for conventional Si MOSFETs (left) andlarger area graphene MOSFETs (right) [1].

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Graphene transistor configurations

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Graphene Preparation

Most popular approaches to prepare graphene are [9]

1 Mechanical exfoliation of natural graphite using an adhesivetape and transfer to oxidized Si wafers.

2 Chemical vapour deposition (CVD) by catalytic decompositionof a gaseous precursor on transition metal substrates such asnickel and copper.

3 Epitaxial growth on SiC substrate by desorption of Si.

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Conclusion

1 The primary challenge for the graphene devices is to create abandgap in a controlled and practical fashion.

2 During the past 4 years, it made a huge progress in the de-velopment of graphene transistors. Most impressive were thedemonstration of a GFET with 1.4 THz cut-off frequency andGNR MOSFETs with excellent switch-off.

3 ITRS roadmap strongly recommends intensified research intographene.

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References I

[1] S. Frank, “Graphene transistor,” Nature Nanotechnology, vol. 5, no. 7, pp.487–496, 2010.

[2] F. Schwierz, “Graphene transistors: Status, prospects, and problems,”Proceedings of the IEEE, vol. 101, no. 7, pp. 1567–1584, July 2013.

[3] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos,I. V. Grigorieva, and A. A. Firsov, “Electric field effect in atomically thin carbonfilms,” Science, vol. 306, no. 5696, pp. 666–669, 2004.

[4] Y. Wu, D. Farmer, F. Xia, and P. Avouris, “Graphene electronics: Materials,devices, and circuits,” Proceedings of the IEEE, vol. 101, no. 7, pp. 1620–1637,July 2013.

[5] J. Chauhan and J. Guo, “Assessment of high-frequency performance limits ofgraphene field-effect transistors,” Nano Research, vol. 4, no. 6, pp. 571–579,2011. [Online]. Available: http://dx.doi.org/10.1007/s12274-011-0113-1

[6] L. W. S. W. T. K. S. L. H. YoungA. F., MericI., “Boron nitride substrates forhigh-quality graphene electronics,” Nature Nanotechnology, vol. 5, no. 10, pp.722–726, 2010.

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References II

[7] B. N. Szafranek, G. Fiori, D. Schall, D. Neumaier, and H. Kurz, “Currentsaturation and voltage gain in bilayer graphene field effect transistors,” NanoLetters, vol. 12, no. 3, pp. 1324–1328, 2012.

[8] G. Fiori, D. Neumaier, B. Szafranek, and G. Iannaccone, “Bilayer graphenetransistors for analog electronics,” Electron Devices, IEEE Transactions on,vol. 61, no. 3, pp. 729–733, March 2014.

[9] L. Colombo, R. Wallace, and R. Ruoff, “Graphene growth and device integration,”Proceedings of the IEEE, vol. 101, no. 7, pp. 1536–1556, July 2013.

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Thank You

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