金屬氧化物半導體場效電晶體（Metal-Oxide-Semiconductor Field-Effect Transistor, MOSFET），是一種可以廣泛使用在類比電路與數位電路的場效電晶體，隨著微小化進程的發展，人們希望能得到電流速度更快、等效電阻更小的MOSFET以得到集成度更高、性能更好的電子晶片。然而隨著MOSFET尺寸的減小，我們不得不面對更艱難的物理效應問題，例如：短通道效應（short channel effect）、次臨限傳導（subthreshold limit conduction）等。
面對這一現況，我們選擇另闢蹊徑，用表面聲波構建與傳統的MOSFET不同的場效傳感器。在我們的石墨烯聲電場效傳感器上，我們選擇以石墨烯作為通道（channel），以壓電材料鈮酸鋰（LiNbO3）為基板，用表面聲波得到聲電流訊號，放棄傳統MOSFET源極與汲極的設定，無需加入偏壓VDS。單層石墨烯可視為天然的二維材料，因而我們可以將其作為二維電子氣（Two-dimensional electron gas, 2DEG），用以提供載子，用離子液體作為頂部閘極，對其費米面進行調控，從而得到石墨烯的場效。我們研究是以石墨烯場效為基礎，通過表面聲波對樣品上石墨烯的載子進行操作，從而產生聲電流，再通過對樣品閘極電壓的調控，當費米面調至迪拉克點時，聲電流為零，我們發現在僅改變0.2V閘極電壓的情況下，達到開關比108。
然而上面實驗中我們發現以離子液體作為閘極時，會存在一個較大的延遲時間。因此，我們設計了另一組實驗，即在同一閘極下，用另一組叉指電極對聲電流訊號進行調控，並進行了相關的測試，能得到一個可觀測的時變聲電流訊號，該訊號在10 kHz的脈衝波調制下，上升和下降所需的時間約為10 μs，在這樣品與操作條件下，聲電流訊號的有用成份與雜音的強弱比值約為2.6。因此我們認為石墨烯聲電場效傳感器可以作為邏輯電路的邏輯閘使用。

Metal-oxide-semiconductor field-effect transistor (MOSFET) is widely used in analog and switching device for integrated circuit. To achieve high speed and performance device, in past decades the semiconductor industries indefatigably strive to shrink the chip size; however, this scaling trend has met several seriously technological bottle-neck and fundamental physical limits. To date, the emerging researches on searching novel two-dimensional (2D) semiconductor material are largely motivated to escape this dead end.
In this thesis we attempt to adopt a different approach to implement a logic latch by using a grapheme acoustic-electric transducer. Our device consists of two pairs of dual inter-digital transducer (IDT) to launch surface acoustic wave (SAW) on top of LiNbO3 substrate, graphene prepared by chemical vapor deposition, and ionic-liquid gate used as a gate electrode to tune the Fermi-level of grapheme. We measure the acoustic-electric current Iac of graphene as a function of the gate voltage Vg. We find Iac changes signal and crosses zero as Vg is tuned over a charge neutral point. Accordingly, we can define a current on and off state with ratio (ION/IOFF) over 108. Unlike conventional MOSFET where the conduction channel is formed by the gate voltage, graphene forms an intrinsic 2D channel. Therefore, we can directly modulate the RF source of SAW and get a latch function of the device with the ION/IOFF ~ 104 up to 10 kHz. We also demonstrate a flip-flop function by using one IDT to induce Iac and another crossed IDT to switch it off, which is analogous to the three-terminal operations in MOSFET.
Our device hold several advantages: no use of source-drain voltage so to significantly lower the cost of electric power, potential for high switch speed which is in principle limited by the SAW frequency, and possible integration with graphene-FET device. Our graphene acoustic-electric latch open a route for the future development of various novel logic-gate devices.

[1] Liao L., Lin Y. C., Bao M., et al., High speed graphene transistors with a self-aligned nanowire gate. Nature, 2010. 467(7313): p. 305.
[2] Lin Y. M., Dimitrakopoulos C., Jenkins K. A., et al., 100-GHz transistors from wafer-scale epitaxial graphene. Science, 2010. 327(5966): p. 662-662.
[3] The International Technology Roadmap for Semiconductors http://www.itrs.net/Links/2009ITRS/Home2009.htm (Semiconductor Industry Association, 2009).
[4] Hughes B., Tasker P. J., Bias dependence of the MODFET intrinsic model elements values at microwave frequencies. IEEE Transactions on Electron Devices, 1989. 36(10): p. 2267-2273.
[5] Nguyen L. D., Tasker P. J., Radulescu D. C., et al., Characterization of ultra-high-speed pseudomorphic AlGaAs/InGaAs (on GaAs) MODFETS[J]. IEEE Transactions on Electron Devices, 1989. 36(10): p. 2243-2248.
[6] Schwierz F., Graphene transistors. Nature Nanotechnology, 2010. 5(7): p. 487-496.
[7] Geim A., Novoselov K., The Nobel Prize in Physics 2010. 2010.
[8] Wallace P. R., The band theory of graphite. Physical Review, 1947. 71(9): p. 622.
[9] Liou Y. W., The Fabrication and Characterization of Chemical Vapor Deposited Graphene. NTHU,2013.
[10] Jiang Z., Zhang Y., Stormer H. L., et al., Quantum Hall states near the charge-neutral Dirac point in graphene. Physical Review Letters, 2007. 99(10): p. 106802.
[11] Horng J., Chen C. F., Geng B., et al., Drude conductivity of Dirac fermions in graphene. Physical Review B, 2011. 83(16): p. 165113.
[12] Lazzeri M., Attaccalite C., Wirtz L., et al., Impact of the electron-electron correlation on phonon dispersion: Failure of LDA and GGA DFT functionals in graphene and graphite. Physical Review B, 2008. 78(8): p. 081406.
[13] Malard L. M., Pimenta M. A. A., Dresselhaus G., et al., Raman spectroscopy in graphene. Physics Reports, 2009. 473(5): p. 51-87.
[14] Ferrari A. C., Meyer J. C., Scardaci V., et al., Raman spectrum of graphene and graphene layers. Physical Review Letters, 2006. 97(18): p. 187401.
[15] Bullis K., Graphene Transistors. MIT Technology Review, 2008.
[16] Wang X., Zhi L., Müllen K., Transparent, conductive graphene electrodes for dye-sensitized solar cells. Nano Letters, 2008. 8(1): p. 323-327.
[17] Stoller M. D., Park S., Zhu Y., et al., Graphene-based ultracapacitors. Nano Letters, 2008, 8(10): p. 3498-3502.
[18] Novoselov K. S., Geim A. K., Morozov S. V., et al., Electric field effect in atomically thin carbon films. Science, 2004. 306(5696): p. 666-669.
[19] Cheol Shin W., Yoon T., Hun Mun J., et al., Doping suppression and mobility enhancement of graphene transistors fabricated using an adhesion promoting dry transfer process. Applied Physics Letters, 2013. 103(24): p. 243504.
[20] Neto A. H. C., Guinea F., Peres N. M. R., et al., The electronic properties of graphene. Reviews of Modern Physics, 2009. 81(1): p. 109.
[21] Bonaccorso F., Sun Z., Hasan T., et al., Graphene photonics and optoelectronics. Nature Photonics, 2010. 4(9): p. 611-622.
[22] Lemme M. C., Echtermeyer T. J., Baus M., et al., A graphene field-effect device. IEEE Electron Device Letters, 2007. 28(4): p. 282-284.
[23] Reddy D., Register L. F., Carpenter G. D., et al., Graphene field-effect transistors. Journal of Physics D: Applied Physics, 2011. 4(31): p. 313001.
[24] Kedzierski J., Hsu P. L., Reina A., et al., Graphene-on-insulator transistors made using C on Ni chemical-vapor deposition. IEEE Electron Device Letters, 2009. 30(7): p. 745-747.
[25] Oshima C., Nagashima A., Ultra-thin epitaxial films of graphite and hexagonal boron nitride on solid surfaces. Journal of Physics: Condensed Matter, 1997. 9(1): p. 1.
[26] Zhang W., Lin C. T., Liu K. K., et al., Opening an electrical band gap of bilayer graphene with molecular doping. ACS Nano, 2011. 5(9): p. 7517-7524.
[27] Yang L., Park C. H., Son Y. W., et al., Quasiparticle energies and band gaps in graphene nanoribbons. Physical Review Letters, 2007. 99(18): p. 186801.
[28] Han M. Y., Özyilmaz B., Zhang Y., et al., Energy band-gap engineering of graphene nanoribbons. Physical Review Letters, 2007. 98(20): p. 206805.
[29] Szafranek B. N., Schall D., Otto M., et al., Electrical observation of a tunable band gap in bilayer graphene nanoribbons at room temperature. Applied Physics Letters, 2010. 96(11): p. 112103.
[30] Balog R., Jørgensen B., Nilsson L., et al., Bandgap opening in graphene induced by patterned hydrogen adsorption. Nature Materials, 2010. 9(4): p. 315-319.
[31] Varykhalov A., Scholz M. R., Kim T. K., et al., Effect of noble-metal contacts on doping and band gap of graphene. Physical Review B, 2010. 82(12): p. 121101.
[32] Radisavljevic B., Radenovic A., Brivio J., et al., Single-layer MoS2 transistors. Nature Nanotechnology, 2011. 6(3): p. 147-150
[33] Liu W., Kang J., Sarkar D., et al., Role of metal contacts in designing high-performance monolayer n-type WSe2 field effect transistors. Nano Letters, 2013. 13(5): p. 1983-1990.
[34] Rayleigh L., On waves propagated along the plane surface of an elastic solid. Proceedings of the London Mathematical Society, 1885. 1(1): p. 4-11.
[35] Campbell C., Surface acoustic wave devices for mobile and wireless communications. Academic press, 1998.
[36] Wixforth A., Scriba J., Wassermeier M., et al., Surface acoustic waves on GaAs/Alx Ga1− x As heterostructures. Physical Review B, 1989. 40(11): p. 7874.
[37] Zhang S. H., Xu W., Badalyan S. M., et al., Piezoelectric surface acoustical phonon limited mobility of electrons in graphene on a GaAs substrate. Physical Review B, 2013. 87(7): p. 075443.
[38] Chien C. C. The design of surface acoustic wave devices and its application in broad oscillator. NTCU, 2000.
[39] Parmenter R. H., The acousto-electric effect. Physical Review, 1953. 89(5): p. 990.
[40] Miseikis V., Cunningham J. E., Saeed K., et al., Acoustically induced current flow in graphene. Applied Physics Letters, 2012. 100(13): p. 133105.
[41] Tang C. C., Chen Y. F., Ling D. C., et al., Ultra-low acoustoelectric attenuation in graphene. Journal of Applied Physics, 2017. 121(12): p. 124505.
[42] Chen F., Qing Q., Xia J., et al., Electrochemical gate-controlled charge transport in graphene in ionic liquid and aqueous solution. Journal of the American Chemical Society, 2009. 31(29): p. 9908-9909.
[43] Misra R., McCarthy M., Hebard A F. Electric field gating with ionic liquids. Applied Physics Letters, 2007. 90(5): p. 052905.
[44] Shen M. T., Modulation of surface acoustic waves in grapheme via two cross interdigitated transducers. NTHU, 2016.