臺灣博碩士論文加值系統

透明、高電子遷移率薄膜電晶體是先進顯示器發展之關鍵，因其透明則亮度高，可以低功率操作，因電子遷移率高反應速度快，可供高解析度顯示器使用，近年來迅速發展之金屬氧化物如氧化鋅(ZnO)可滿足這些要求。
在本研究中，主要探討下閘極氧化鋅薄膜電晶體，利用射頻濺鍍法製備氧化鋅薄膜，並以使用原子層沉積系統(ALD)在ITO基板上生長之氧化鋁(Al2O3)薄膜做為氧化層。在本研究中濺鍍氧化鋅薄膜經空氣退火後，以X-ray繞射儀分析所沉積之氧化鋅薄膜晶格展現(002)從優取向，以原子力顯微鏡分析氧化鋅薄膜粗糙度，並以霍爾量測分析載子濃度與電子遷移率。
在金屬-氧化層-半導體結構中發現，退火在空氣中有最好的電容特性表現。在氧化鋅薄膜電晶體元件特性研究發現，其中通道寬度300 μm電子遷移率達到1.01 cm2/V-s，電流開關比達到1.68 × 107且次臨界擺幅達到350 mV/decade。
其中，原子力沉積系統成長的氧化鋁可藉由自我清潔的能力使的氧化層與半導體界面得到更佳的改善，且氧化鋁擁有寬能隙的優點可以使漏電流維持在10-12 A。

Transparent and high electron mobility thin film transistor (TFT) is the key technology for modern displays. The transparency can enhance the brightness of display at lower operated power. The high electron mobility can enhance the switching speed and resolution. Metal oxides, for example zinc oxide (ZnO) can meet those requirements.
In our study, the ZnO thin film is deposited by sputtering. The ALD-Al2O3 as gate dielectric is deposited on ITO/glass substrate by atomic layer deposition (ALD). Moreover, annealing of ZnO thin film in air shows the greater carrier concentration and mobility for fabrication of bottom-gate TFT. XRD spectrum of sputtered ZnO shows (002) preferred orientation with a c-axis perpendicular to the glass substrate. AFM morphology of sputtered ZnO shows a good surface roughness. And carrier concentration and mobility is measured by Van der Pauw Hall measurement system.
In metal-oxide-semiconductor structure, annealing in air obtains good characteristics of capacitor-voltage. In characteristics of ZnO bottom-gate thin film transistors, the mobility can reach 1.01 cm2/V-s, on-off ratio is 1.68 × 107, and subthreshold swing is 350 mV/decade. In addition, ALD-Al2O3 has many advantages, such as high bandgap and self-cleaning which could improve interface state between oxide and semiconductor. Thus, the leakage current is kept at 10-12 A.

CONTENTS
摘要 I
Abstract II
CONTENTS III
LIST of FIGURES V
LIST of TABLES VIII
Chapter 1 1
Introduction 1
1-1 Characterization of bulk ZnO 1
1-2 Growth methods for ZnO thin films 2
1-3 Native defects of ZnO thin film 2
1-4 Comparison of thin film transistors 3
1-5 Structures of thin film transistors 4
1-6 Development of displays and thin film transistors 8
1-7 Motivation of ZnO bottom-gate thin film transistor 10
Chapter 2 19
Experiments 19
2-1 Fabrication process for Al contact resistance on ZnO/Al2O3/glass substrate by transmission line method (TLM) 19
2-2 Fabrication process for ZnO MOS capacitors with Al2O3 as dielectric 21
2-3 Fabrication process for ZnO bottom-gate thin film transistor with Al2O3 as gate dielectric 23
2-4 Deposition and measurement systems 26
Chapter 3 44
Results and Discussion 44
3-1 Dependence of carrier concentrations and electron mobilities of annealed ZnO/glass in N2, N2O, O2 and air as a function of RF sputtering power 44
3-2 Electrical characteristics of Al ohmic contacts on sputtered ZnO (40 nm)/glass substrates annealed in air at 300 ℃ for 1 hr 46
3-3 C-V measurement of Al/sputtered ZnO (40 nm)/ALD-Al2O3 (50 nm)/ITO/glass MOS capacitors 48
3-4 Electrical characteristics of sputtered ZnO bottom-gate thin film transistors 49
Chapter 4 74
Conclusions 74
References 75

 
LIST of FIGURES
Figure 1-1 The atomic structure of wurtzite ZnO 12
Figure 1-2 The total pixel area 12
Figure 1-3 Structures of thin film transistors 14
Figure 1-4 Operations of bottom-gate thin film transistor and ID-VG characteristics (a) Accumulation (b) Depletion (c) Inversion 15
Figure 1-5 Operations of bottom-gate thin film transistor and ID-VD characteristics (a) Low drain voltage (b) Pinch-off (c) Beyond saturation 16
Figure 1-6 Growth mechanism of ALD-Al2O3 17
Figure 1-7 Developments of displays 17
Figure 1-8 Dependence of mobilities and pixels 18
Figure 1-9 Developments of thin film transistors 18
Figure 2-1 Fabrication process for Al contact resistance on ZnO/Al2O3/glass substrate by transmission line method (TLM) 32
Figure 2-2 Fabrication process for ZnO MOS capacitors with Al2O3 as dielectric 35
Figure 2-3 The C-V electrical measurement of ZnO MOS capacitors 35
Figure 2-4 Fabrication process for ZnO bottom-gate thin film transistor 41
Figure 2-5 (a) RF sputtering system (TRUMPF, PFG-300RF), (b) atomic layer deposition (ALD) system (CNT, Fiji-202), and (c) e-beam evaporation system (AST, Peva-600EI) 42
Figure 2-6 (a) show atomic force microscope (AFM) measurement system (Veeco, D-5000), (b) field-emission scanning electron microscope (FE-SEM) measurement system (JEOL, JSM-6500F), and (c) x-ray diffraction (XRD) measurement system (Bruker, D8 ADVANCE ECO) 42
Figure 2-7 (a) Van der Pauw Hall measurement system (Ecopia, HMS-5000), (b) capacitance-voltage measurement system (Agilent, E4980A), and (c) current-Voltage measurement system (Agilent, HP-4145B) 43
Figure 3-1 (a) carrier concentrations and (b) electron mobilities of annealed ZnO/glass in N2, N2O, O2, and air as a function of RF sputtering power 58
Figure 3-2 Van der Pauw Hall mobility, Electron mobilities of ZnO/glass as a function of RF sputtering power and annealed in N2, N2O, O2 and air 58
Figure 3-3 (a) XRD spectrum of sputtered ZnO annealed in air at 300 ℃ for 1hr 59
Figure 3-3 (b) FE-SEM morphology of sputtered ZnOannealed in air at 300 ℃ for 1hr 59
Figure 3-3 (c) AFM morphology of sputtered ZnO annealed in air at 300 ℃ for 1hr 60
Figure 3-4 shows I-V characteristics of Al ohmic electrodes annealed in the range of 250 to 400 °C at a step of 25 °C for 10 min in N2 on sputtered ZnO (40 nm)/glass substrates annealed in air at 300 ℃ for 1 hr 60
Figure 3-5 (a) Contact resistance (Rc) of Al electrode on sputtered ZnO thin film measure by TLM 61
Figure 3-5 (b) Contact pads and distances 61
Figure 3-6 Annealing of sputtered ZnO (40 nm)/ALD-Al2O3 (50 nm)/ITO/glass MOS capacitors in (a) N2, (b) N2O, (c) O2, and (d) air at 300 ℃ for 1 hr 63
Figure 3-7 (a) ID-VG characteristics and (b) log(ID)-VG characteristics with ALD-Al2O3 (20 nm) gate insulator and 300 μm channel width 64
Figure 3-8 ID-VD characteristics with 300 μm channel width 65
Figure 3-9 Current crowding effect 65
Figure 3-10 (a) Activation energy of O2 molecules and (b) absorption of O2 molecules at the back surface of ZnO channel layer 66
Figure 3-11 C-V characteristics 67
Figure 3-12 (a) FE-SEM morphology of ITO gate and (b) C-V measurement of bottom-gate ZnO TFT 65 67
Figure 3-13 leakage current of gate dielectric 68
Figure 3-14 Log(ID)-VG characteristics with ALD-Al2O3 (20 nm) gate insulator and 250 μm channel width 69
Figure 3-15 ID-VD characteristics with 250 μm channel width 69
Figure 3-16 Log(ID)-VG characteristics with ALD-Al2O3 (20 nm) gate insulator and 200 μm channel width 70
Figure 3-17 ID-VD characteristics with 200 μm channel width 70
Figure 3-18 Log(ID)-VG characteristics with ALD-Al2O3 (20 nm) gate insulator and 150 μm channel width 71
Figure 3-19 ID-VD characteristics with 150 μm channel width 71
Figure 3-20 Log(ID)-VG characteristics with ALD-Al2O3 (20 nm) gate insulator and 100 μm channel width 72
Figure 3-21 ID-VD characteristics with 100 μm channel width 72

[1]I. Shalish, H. Temkin, and V. Narayanamurti, “Size-dependent surface luminescence in ZnO nanowires,” Phys. Rev. B, vol. 69, no. 24, pp. 245401-245404, 2004.
[2]J. L. G. Fierro, “Metal Oxides: Chemistry & Applications,” 6000 Broken Sound Parkway NW, Suite 300: Taylor & Francis Group. 2006: 182. ISBN 0-8247-237-6.
[3]T. Minami, H. Sato, H. Nanto, and S. Takata, “Group III Impurity Doped Zinc Oxide Thin Films Prepared by RF Magnetron Sputtering,” Jpn. J. Appl. Phys., vol. 24, no. 10A, pp. L781-L784, 1985.
[4]R. F. Service, “Will UV lasers beat the blues?,” Science, vol. 276, no. 5314, pp. 895-895, 1997.
[5]B. J. Ingram, G. B. Gonzalez, D. R. Kammler, M. I. Bertoni, and T. O. Mason, “Chemical and structural factors governing transparent conductivity in oxides,” J. Electroceram, vol. 13, no. 1, pp. 167-175, 2004.
[6]S. C. Minne, S. R. Manalis, and C. F. Quate, “Parallel atomic force using cantilevers with integrated piezoresistive sensors and integrated piezoelectric actuators,” Appl. Phys. Lett., vol. 67, no. 26, pp. 3918-3920, 1995.
[7]I. Shalish, H. Temkin, and V. Narayanamurti, “Size-dependent surface luminescence in ZnO nanowires,” Phys. Rev. B, vol. 69, no. 24, pp. 245401-245404, 2004.
[8]C. L. Wu, L. Chang, H. G. Chen, C. W. Lin, T. F. Chang, Y. C. Chao, and J. K. Yan, “Growth and characterization of chemical-vapor-deposited zinc oxide nanorods,” Thin Solid Films, vol. 498, no. 1, pp. 137-141, 2006.
[9]E. M. Kaidashev, M. Lorenz, H. von Wenckstern, A. Rahm, H. C. Semmelhack, K. H. Han, G. Benndorf, C. Bundesmann, H. Hochmuth, and M. Grundmann, “High electron mobility of epitaxial ZnO thin films on c-plane sapphire grown by multistep pulsed-laser deposition,” Applied Physics Letters, vol. 82, no. 22, pp. 3901-3903, 2003.
[10]K. Ogata, K. Koike, S. Sasa, M. Inoue, and M. Yano, “Fabrication of ZnO nanorods on O-polar ZnO layers grown by molecular beam epitaxy and electrical characterization using conductive atomic force microscopy,” Semiconductor Science and Technology, vol. 24, no. 015006, pp. 1-4, 2009.
[11]Tynell, Tommi, and M. Karppinen, "Atomic layer deposition of ZnO: a review," Semiconductor Science and Technology, vol. 29, no. 043001, pp. 1-15, 2014.
[12]C. Li, M. Furuta, T. Matsuda, T. Hiramatsu, H. Furuta, and T. Hirao, ”RF Power and Thermal Annealing Effect on the Properties of Zinc Oxide Films Prepared by Radio Frequency Magnetron Sputtering,” Research Letters in Materials Science, vol. 2007, pp. 1-5, 2007.
[13]Q. Li, V. Kumar, Y. Li, H. Zhang, T. Marks, and R. Chang, “Fabrication of ZnO Nanorods and Nanotubes in Aqueous Solutions,” Chemistry of Materials, vol. 17, no. 5, pp. 1001-1006, 2005.
[14]J. Lee, K. Ko and B. Park, “Electrical and optical properties of ZnO transparent conducting films by the sol–gel method,” Journal of Crystal Growth, vol. 247, no. 1-2, pp. 119-125, 2003.
[15]Ü. Özgür, Y. Alivov, C. Liu, A. Teke, M. Reshchikov, S. Doğan, V. Avrutin, S. Cho, and H. Morkoç, “A comprehensive review of ZnO materials and devices,” Journal of Applied Physics, vol. 98, no. 4, p. 041301, 2005.
[16]A. Janotti and C. Van de Walle, “Fundamentals of zinc oxide as a semiconductor,” Reports on Progress in Physics, vol. 72, no. 12, p. 126501, 2009.
[17]J. Conley, “Instabilities in Amorphous Oxide Semiconductor Thin-Film Transistors,” IEEE Transactions on Device and Materials Reliability, vol. 10, no. 4, pp. 460-475, 2010.
[18]T. Serikawa, S. Shirai, A. Okamoto, and S. Suyama, “Low-temperature fabrication of high-mobility poly-Si TFTs for large-area LCDs,” IEEE Transactions on Electron Devices, vol. 36, no. 9, pp. 1929-1933, 1989.
[19]A. Pecora, L. Maiolo, M. Cuscunà, D. Simeone, A. Minotti, L. Mariucci, and G. Fortunato, “Low-temperature polysilicon thin film transistors on polyimide substrates for electronics on plastic,” Solid-State Electronics, vol. 52, no. 3, pp. 348-352, 2008.
[20]H. Klauk, “Organic thin-film transistors,” Chemical Society Reviews, vol. 39, no.7, pp. 2643-2666, 2010.
[21]J. Park, J. Jeong, Y. Mo, H. Kim, and S. Kim, “Improvements in the device characteristics of amorphous indium gallium zinc oxide thin-film transistors by Ar plasma treatment,” Applied Physics Letters, vol. 90, no. 26, p. 262106, 2007.
[22]H. Hosono, “Ionic amorphous oxide semiconductors: Material design, carrier transport, and device application,” Journal of non-crystalline solid, vol. 352, no. 9, pp. 851-858, 2006.
[23]A. Kumar, A. Nathan, and G. Jabbour, “Does TFT Mobility Impact Pixel Size in AMOLED Backplanes?,” IEEE Transactions on Electron Devices, vol. 52, no. 11, pp. 2386-2394, 2005.
[24]H. Klauk, “Organic thin-film transistors,” Chemical Society Reviews, vol. 39, no.7, pp. 2643-2666, 2010.
[25]J. Park, J. Jeong, Y. Mo, H. Kim, and S. Kim, “Improvements in the device characteristics of amorphous indium gallium zinc oxide thin-film transistors by Ar plasma treatment,” Applied Physics Letters, vol. 90, no. 26, p. 262106, 2007.
[26]S. M. Sze, M. K. Lee, Semiconductor devices: physics and technology, 3rd ed., Wiley, New York, (2013).
[27]A. C. Tickle, Thin-film transistors: a new approach to microelectronics, Wiley, New York, (1969).
[28]S. M. Sze, Semiconductor Devices: physics and technology, Wiley, New York, 1985.
[29]B. G. Streetman and S. K. Banerjee, Solid state electronic devices, 7th ed., Prentice Hall, New Jersey, 2016.
[30]A. Correia, P. C. Barquinha, and J. Goes, A second-order ΣΔ ADC using sputtered IGZO TFTs.
[31]T. Suntola, ”Atomic layer epitaxy,” Materials Science Reports, vol. 4, no. 5, pp. 261-312, 1989.
[32]J. W. Lim, H. S. Park, and S. W. Kang, “Analysis of a transient region during the initial stage of atomic layer deposition,” J. Appl. Phys., vol. 88, no. 11, pp. 6327-6331, 2000.
[33]M. Leskela, M. Ritala, ”Atomic layer deposition (ALD): from precursors to thin film structures,” Thin Solid Films, vol. 409, no. 1, pp. 138-146, 2002.
[34]P. F. Carcia, R. S. McLean, M. D. Groner, A. A. Dameron, and S. M. George,” Gas diffusion ultrabarriers on polymer substrates using Al2O3 atomic layer deposition and SiN plasma-enhanced chemical vapor deposition.” Journal of Applied Physics, vol. 106, no. 2, p. 023533, 2009.
[35]M. Leskela, M. Ritala, ”Atomic layer deposition (ALD): from precursors to thin film structures,” Thin Solid Films, vol. 409, no. 1, pp. 138-146, 2002.
[36]D. Buchanan, “Scaling the gate dielectric: materials, integration, and reliability.,” IBM J. Res. Dev., vol. 43, no. 3, pp. 245-264, 1999.
[37]L. Feldman, E. P. Gusev, and E. Garfunkel, Fundamental Aspects of Ultrathin Dielectrics on Si-based Devices, Springer Science & Business Media, vol. 47, 2012.
[38]L. Manchanda, W. H. Lee, J. E. Bower, F. H. Baumann, W. L. Brown, C. J. Case, R. C. Keller, Y. O. Kim, E. J. Laskowski, M. D. Morris, R. L. Opila, P. J. Silverman, T. W. Sorsch, and G. R. Weber, “Gate quality doped high K films for CMOS beyond 100 nm: 3-10 nm Al/sub 2/O/sub 3/with low leakage and low interface states.,” Tech. Dig. Int. Electron Devices Meet. 1998, pp. 605-608, 1998.
[39]S. A. Campbell, D. C. Gilmer, X. C. Wang, M. T. Hsieh, H. S. Kim, W. Gladfelter, and J. Yan, “MOSFET transistors fabricated with high permitivity TiO/sub 2/dielectrics.,” IEEE Trans. Electron Devices, vol. 44, no. 1, pp. 104-109, 1997.
[40]J. A. Aboaf, “Deposition and properties of aluminum oxide obtained by pyrolytic decomposition of an aluminum alkoxide,” J. Electrochem. Soc., vol. 114, no. 9, pp. 948-952, 1967.
[41]M. T. Duffy and A. G. Revesz, “Interface Properties of Si‐(SiO2)‐Al2O3 Structures,” J. Electrochem. Soc., vol. 117, no. 3, pp. 372-377, 1970.
[42]M. T. Duffy and W. Kern, “Chemical vapor deposition of aluminum oxide films from organo-aluminum compounds”, RCA Rev., vol. 31, no. 4, pp. 754-770, 1970.
[43]J. Pearton, D. P. Nortona, K. Ip, Y. W. Heo, and T. Steiner, "Recent progress in processing and properties of ZnO," Progress in Materials Science, vol. 50, no. 3, pp. 293-340, 2005.
[44]Y. Kuo, "Thin film transistor technology—Past, present, and future," The Electrochemical Society Interface, vol. 22, no. 1, pp. 55-61, 2013.
[45]S. Masuda, K. Kitamura, Y. Okumura, S. Miyatake, H. Tabata, and T. Kawai, “Transparent thin film transistors using ZnO as an active channel layer and their electrical properties,” J. Appl. Phys., vol. 93, no. 3, pp. 1624–1630, 2003.
[46]R. B. M. Cross, M. M. D. Souza, S. C. Deane, and N. D. Young, “A Comparison of the Performance and Stability of ZnO-TFTs With Silicon Dioxide and Nitride as Gate Insulators,” IEEE Transactions on Electron Devices, vol. 55, no. 5, 2008.
[47]J. M. Myoung, W. H. Yoon, D. H. Lee, I. Yun, S. H. Bae, and S. Y. Lee, “Effects of Thickness Variation on Properties of ZnO Thin Films Grown by Pulsed Laser Deposition”, Jpn. J. Appl. Phys., vol. 41, no. 1, pp. 28-31, 2002.
[48]C. Li, M. Furuta, T. Matsuda, T. Hiramatsu, H. Furuta, and T. Hirao, “RF Power and Thermal Annealing Effect on the Properties of Zinc Oxide Films Prepared by Radio Frequency Magnetron Sputtering”, Advances in Materials Science and Engineering, vol. 2007, no. 26459, pp. 1-5, 2007.
[49]A. Janotti and C. Van de Walle, “Fundamentals of zinc oxide as a semiconductor,” Reports on Progress in Physics, vol. 72, no. 12, p. 126501, 2009.
[50]K. Ip, G.T. Thaler, H. Yang, S. Y. Han, Y. Li, D.P. Norton, S.J. Pearton, S. Jang, and F. Ren, “Contacts to ZnO”, Journal of Crystal Growth, vol. 287, no. 1, pp. 149-56, 2006.
[51]J. Chang, K. L. Chang, C. Chi, J. Zhang, and J. Wu, “Water induced zinc oxide thin film formation and its transistor performance,” Journal of Materials Chemistry, vol. 2, no. 27, pp. 5397~5403, 2014.
[52]T. Hirao, M. Furuta, T. Hiramatsu, T. Matsuda, C. Li, H. Furuta, H. Hokari, M. Yoshida, H. Ishii, and M. Kakegawa,“Bottom-Gate Zinc Oxide Thin-Film Transistors(ZnO TFTs) for AM-LCDs,” IEEE TRANSACTIONS ON ELECTRON DEVICES, vol. 55, no. 11, pp. 3136-3142, 2008.
[53]H. Mi, J. Seo, C. Ku, J. Shi, X. Wang, Y. Lu, and Z. Ma, “Microwave TFTs Made of MOCVD ZnO With ALD Al2O3Gate Dielectric,” IEEE Journal of the Electron Devices Society, vol. 4, no. 2, pp. 55-59, 2016.
[54]P. Zhang, Y. Lau, and R. Gilgenbach, “Analysis of current crowding in thin film contacts from exact field solution,” Journal of Physics D: Applied Physics, vol. 48, no. 47, p. 475501, 2015.
[55]T. Richards and H. Sirringhaus, “Analysis of the contact resistance in staggered, top-gate organic field-effect transistors,” Journal of Applied Physics, vol. 102, no. 9, p. 094510, 2007.
[56]M. Mativenga, J. Um, D. Kang, R. Mruthyunjaya, J. Chang, G. Heiler, T. Tredwell, and J. Jang, “Edge Effects in Bottom-Gate Inverted Staggered Thin-Film Transistors,” IEEE Transactions on Electron Devices, vol. 59, no. 9, pp. 2501-2506, 2012.
[57]C. Shim, F. Maruoka and R. Hattori, “Structural Analysis on Organic Thin-Film Transistor With Device Simulation,” IEEE Transactions on Electron Devices, vol. 57, no. 1, pp. 195-200, 2010.
[58]J. Y. Wu, H. H. Wang, Y. H. Wang, and M. P. Houng, “Interface induced uphill diffusion of boron: An effective approach for ultrashallow junction,” IEEE Electron Device Lett., vol. 22, no. 2, pp. 64-67, 2001.
[59]X. Xu, L. Feng, S. He, Yizheng Jin, and X. Guo, “Solution-Processed Zinc Oxide Thin-Film Transistors With a Low-Temperature Polymer Passivation Layer,” IEEE Electron Device Letters, vol. 33, no. 10, pp. 1420-1422, 2012.
[60]Y. C. Chen, T. C. Chang, H. W. Li, S. C. Chen, and J. Lu, “Biasinduced oxygen adsorption in zinc tin oxide thin film transistors under dynamic stress,” Appl. Phys. Lett., vol. 96, no. 26, pp. 262104-1–262104-3, 2010.
[61]H. Lee, J.-S. Yoo, C.-D. Kim, I.-J. Chung, and Jerzy Kanicki, “Asymmetric Electrical Properties of Corbino a-Si:H TFT and Concepts of Its Application to Flat Panel Displays”, IEEE Electron Device Letters, vol. 54, no. 4, pp. 654-658, 2007.
[62]M. Zhang, W. Zhou, R. Chen, M. Wong, “A Simple Method to Grow Thermal SiO2 Interlayer for High-Performance SPC Poly-Si TFTs Using Al2O3 Gate Dielectric”, IEEE Electron Device Letters, vol. 35, nO. 5, pp. 548-551, 2014.
[63]R. Chapman, R. Rodriguez-Davila, I. Mejia and M. Quevedo-Lopez, "Nanocrystalline ZnO TFTs Using 15-nm Thick Al2O3Gate Insulator: Experiment and Simulation," IEEE Transactions on Electron Devices, vol. 63, no. 10, pp. 3936-3943, 2016.
[64]T. Hirao, M. Furuta, T. Hiramatsu, T. Matsuda, C. Li, H. Furuta, H. Hokari, M. Yoshida, H. Ishii, and M. Kakegawa, “Bottom-Gate Zinc Oxide Thin-Film Transistors (ZnO TFTs) for AM-LCDs,” IEEE Electron Device Letters, vol. 55, no. 11, pp. 3136-3142, 2008.