All entries for Han Wang

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Compact Physical Modeling of Graphene Field Effect Transistors

• Authors: H. Wang, A. Hsu, J. Kong, D. A. Antoniadis, T. Palacios
• Sponsorship: SRC/FCRP MSD, ONR GATE MURI Program

Graphene is a two-dimensional (2D) material that has attracted great interest for electronic devices since the demonstration of field effect carrier modulation in 2004 ^[1] . Its high mobility and high saturation velocity make graphene a promising material for next generation of high-frequency devices ^[2] , and its 2D geometry also makes it highly compatible with existing fabrication technology in the semiconductor industry. Furthermore, the possibility of large-scale synthesis of graphene by chemical vapor deposition (CVD) and epitaxial growth ^{[3] [4] [5]} makes graphene integrated circuits a feasible reality in the near future. Hence, it is desirable to develop a compact physical model that can enable the use of computer-aided-design software to simulate future complex circuits. In this work, we develop a compact model for the current-voltage characteristics of graphene field effect transistors (GFETs), which is based on an extension of the “virtual-source” model previously proposed for Si MOSFETs ^{[6] [7]} and is valid for both saturation and non-saturation regions of device operation (Figure 1). This virtual source model provides a simple and intuitive understanding of carrier transport in GFETs, allowing extraction of the virtual source injection velocity v_VS, a physical parameter with great technological significance for short-channel graphene transistors. With only a small set of fitting parameters, the model shows excellent agreement with experimental data (Figure 2). It is also shown that the extracted virtual source carrier injection velocity for graphene devices is much higher than in Si MOSFETs and state-of-the-art III-V HFETs with similar gate length, supporting the great potential of GFETs for high frequency applications. Future work includes extending the model for both small signal and large signal modeling of GFETs RF performance and implementation in Verilog to enable modeling of graphene circuits.



Figure 1: Schematic diagram of the cross-section of a GFET device. The virtual electron source (VES) point and the virtual hole source (VHS) point in the channel are defined for a GFET. The electrostatic potential along the channel and the position of VES and VHS are shown for a GFET operating in the three operating regions: I. Electron conduction region; II. Ambipolar region; III. Hole conduction region.



Figure 2: (a) Top-view of the three-dimensional plot of I_DS vs. V_TGS and V_BGS at V_DS=1.1 V. (b), (c) and (d) are cross-sections in (a) at the corresponding dotted lines. The model results are shown as solid lines; the experiment data is shown as dots. In (b), V_BGS is kept constant at 0 V, and V_TGS is swept from -1 V to 3 V. A family of curves is shown for VDS increasing from 0.35 V to 1.1 V in steps of 0.25 V. In (c), V_TGS is kept constant at 0 V, and V_BGS is swept from -50 V to 100 V. (d) V_TGS is kept constant at 1.41 V, and V_BGS is swept from -30 V to 40 V. A family of curves is shown for V_DS, increasing from 0.35 V to 1.1 V in steps of 0.25 V.




1. K. S. Novoselov, et al., “Electric field effect in atomically thin carbon films,” Science, vol. 306, pp. 666-669, Oct. 2004. [↩]
2. T. Palacios, et al. “Applications of graphene devices in RF communications,” IEEE Comm. Mag., vol. 48,  no. 6, pp. 122-128, June 2010. [↩]
3. A. Reina, et al., “Large area few-layer graphene films on arbitrary substrates by Chemical Vapor Deposition,” Nano Lett., vol. 9, no. 1, pp. 30-35, Jan. 2009. [↩]
4. X. Li, et al. “Large-area synthesis of high-quality and uniform graphene films on copper foils,” Science, vol. 324. no. 5932, June 2009. [↩]
5. C. Berger, et al. “Electronic confinement and coherence in patterned epitaxial graphene,” Science, vol. 312. no. 5777, May 2006. [↩]
6. A. Khakifirooz, O. M. Nayfeh, and D. Antoniadis “A simple semiempirical short-channel MOSFET current-voltage model dontinuous across all regions of operation and employing only physical parameters,” IEEE Trans. Electron Devices, vol. 56, no. 8, Aug. 2009. [↩]
7. D. A. Antoniadis, I. Åberg, C. N. Chleirigh, O. M. Nayfeh, A. Khakifirooz, and J. L. Hoyt, “Continuous MOSFET performance increase with device scaling: The role of strain and channel material innovation,” IBM J. Res. Develop., vol. 50, no. 4/5, pp. 363–376, July 2006. [↩]

Graphene Ambipolar Electronics for RF Applications

• Authors: H. Wang, A. Hsu, J. Kong, K. K. Kim, T. Palacios
• Sponsorship: ONR GATE MURI Program, Institute for Soldier Nanotechnologies

Graphene is a one-atom-thick layer of carbon atoms arranged in a honeycomb lattice through sp² bonding ^[1] . Considered for many years an impossible goal, the isolation of graphene triggered a revolution not only among condensed-matter physicists but also among chemists and engineers, eager to take advantage of its unique properties ^[2] . The symmetry of its honeycomb lattice structure confers to graphene very unique transport properties ^[3] . For example, the carriers in graphene lose their effective mass and can be described by a Dirac-like equation instead of by the Schrödinger equation used in traditional semiconductors. This very low effective mass is responsible for a very high electron and hole mobility in excess of 100,000 cm²/Vs at room temperature ^[4] , the highest ever reported for any semiconductor. In this work, we develop novel graphene electronic devices using graphene grown by chemical vapor deposition. These new applications of graphene rely on its ambipolar transport properties to provide unique device level functionalities, which can significantly simplify the designs of many basic RF circuit blocks, such as frequency multipliers and mixers ^{[5] [6]} . The new graphene ambipolar frequency multipliers, integrated on a sapphire substrate, can operate at 16 GHz with extremely high output spectral purity (> 90%) (Figure 1) and are the fastest circuit level demonstration ever made using graphene. These Ku-band graphene frequency multipliers, made from wafer-scale graphene synthesis and fabrication processes, show the great potential of graphene-based ambipolar devices for RF and mixed-signal applications. Other applications such as graphene ambipolar mixers (Figure 2) can suppress odd order intermodulations more effectively than in traditional unipolar FET mixers [6]. These graphene devices have great potential for applications in future high performance transparent and flexible electronics.



Figure 1: (a) Experimental demonstration of frequency doubling measured by an Agilent DSA90604A oscilloscope. The input is at 3 GHz. The output fundamental frequency is 6 GHz. (b) Power spectrum of the output signal from the graphene frequency doublers. The input is at 8 GHz. Frequency doubling is clearly visible. The signal power at f_out=2f_in=16 GHz is about 10 dB higher than signal power at f_out= f_in=8 GHz without filtering. More than 90% of the total RF power in the output signal is at 16 GHz. All data are obtained using Agilent N9010A spectrum analyzer.



Figure 2: (a) Demonstration circuit of graphene ambipolar mixers. (b) Spectrum analysis: Output spectrum with RF input f_RF=10.5 MHz and local oscillator f_LO=10 MHz at equal power. The presence of strong signal power at f_RF – f_LO=500 kHz and f_RF + f_LO=21.5 MHz clearly demonstrates mixing operations. All data are obtained using Agilent N9010A spectrum analyzer, and the measurements were done at room temperature in air.




1. K. S. Novoselov, et al., “Electric field effect in atomically thin carbon films” Science, vol. 306, pp. 666-669, Oct. 2004. [↩]
2. A. K. Geim, “Graphene: status and prospects,” Science, vol. 324, no. 5934, pp. 1530-1534, June 2009. [↩]
3. N. M. R. Peres, et al., “Electronic properties of disordered two-dimensional carbon,” Phys. Rev. B, vol. 73, pp. 125411, Mar. 2006. [↩]
4. K. Bolotin et al., “Ultrahigh electron mobility in suspended graphene,” Solid State Communications, vol. 146, pp. 351-355, June 2008. [↩]
5. H. Wang, D. Nezich, J. Kong, and T. Palacios “Graphene frequency multipliers,” IEEE Electron Device Lett., vol. ED-30, May 2009. [↩]
6. H. Wang, A. Hsu, J. Wu, J. Kong, and T. Palacios “Graphene-based ambipolar RF mixers,” IEEE Electron Device Lett., vol. ED-31, no. 9, Sept. 2010. [↩]