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Graphene field-effect transistor for radio-frequency applications : review Jeong-Sun Moon♠

HRL Laboratories LLC, 3011 Malibu Canyon Road, Malibu, CA 90265, USA

Received 4 September 2011Accepted 12 December 2011

*Corresponding AuthorE-mail: jmoon@hrl.com Tel: +1-310-317-5461

Open Access

pISSN: 1976-4251 eISSN: 2233-4998

Carbon Letters Vol. 13, No. 1, 17-22 (2012)Review Articles

Article Info

Copyright © Korean Carbon Society

http://carbonlett.org

AbstractCurrently, graphene is a topic of very active research in fields from science to potential appli-cations. For various radio-frequency (RF) circuit applications including low-noise amplifi-ers, the unique ambipolar nature of graphene field-effect transistors can be utilized for high-performance frequency multipliers, mixers and high-speed radiometers. Potential integration of graphene on Silicon substrates with complementary metal-oxide-semiconductor compat-ibility would also benefit future RF systems. The future success of the RF circuit applications depends on vertical and lateral scaling of graphene metal-oxide-semiconductor field-effect transistors to minimize parasitics and improve gate modulation efficiency in the channel. In this paper, we highlight recent progress in graphene materials, devices, and circuits for RF applications. For passive RF applications, we show its transparent electromagnetic shielding in Ku-band and transparent antenna, where its success depends on quality of materials. We also attempt to discuss future applications and challenges of graphene.

Key words: graphene, field-effect transistor, transistor, ambipolar, low-noise amplifier, radio-frequency, mixer, multiplier, phase noise, radiometer, electromagnetic interference, Antenna, sensors, nanoelectromechanical system, interconnects, complementary metal-oxide- semiconductor

1. Introduction

Graphene is an atomically thin but stable form of hexagonal carbon [1,2] and has attracted a lot of attention in the research community since the seminal papers published in 2004 [3,4]. Along with its unique electronic properties, graphene has shown interesting optical [5], mechanical [6] and thermal [7] properties as summarized in Table 1. Some of these unique properties have been considered for potential applications, such as transparent conductors for touch screens [8,9], electromagnetic interference shielding [10], antenna [11], flexible electronics [12], sensors [13], nanoelectromechanical systems [14], and interconnects [15].

For radio-frequency (RF) electronic applications being considered, graphene exhibits the highest carrier mobility: >100 000 cm2 V-1 s-1 at room temperature [16]. This is not only ~100 times greater than that of Si, but about 10 times greater than state-of-the-art semiconduc-tors lattice-matched to InP, currently regarded the best high-speed materials. The saturation velocity (vsat) of graphene has not been determined clearly yet, but it is estimated to be ~5 times greater than that for Si metal-oxide semiconductor field-effect transistor (MOSFETs) [17]. With expected large on-state current density and transconductance per gate capacitance compared to Si, graphene has the potential to offer excellent switching characteristics (ca-pacitance/on-state current) and short-circuit current gain cut-off frequency [18]. Although it is too early to predict, graphene-on-Si FETs [19] could potentially be further developed and processed in a manner compatible with Si complementary MOS (CMOS) with desirable integration density for system-on-chip applications.

While there are numerous challenges to be overcome for graphene to become a mature

DOI: http://dx.doi.org/ 10.5714/CL.2012.13.1.017

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Synthesis and applications of graphene electrodesDolly Shin, Sukang Bae, Chao Yan, Junmo Kang, Jaechul Ryu, Jong-Hyun Ahn and Byung Hee Hong

Graphene field-effect transistor for radio-frequency applications : review Jeong-Sun Moon

Issues with the electrical characterization of graphene devicesByoung Hun Lee, Young Gon Lee, Uk Jin Jung, Yong Hun Kim, Hyeon Jun Hwang, Jin Ju Kim and Chang Goo Kang

KCS Korean Carbon Society

carbonlett.org

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pISSN: 1976-4251 eISSN: 2233-4998VOL. 13 NO. 1 January 31 2012

Carbon Letters Vol. 13, No. 1, 17-22 (2012)

DOI: http://dx.doi.org/10.5714/CL.2012.13.1.017 18

resulting in an Ion/Ioff ratio of <2. In the case of graphene FETs fabricated on the Si-face of SiC substrates, field-effect mobility has been limited to below 1200 cm2/Vs, but with an improved Ion/Ioff ratio of ~10 [23].

Top-gated graphene n-channel FETs (n-FET) and p-channel FETs (p-FETs) from epitaxial graphene layers were demonstrat-ed with excellent I-V saturation behaviors [20] with a record peak extrinsic transconductance of 770 mS/mm. Also, the effec-tive mobility and field-effect mobility versus Eeff were character-ized and compared with Hall mobility. The epitaxial graphene layers were grown on Si-face 6H-SiC substrates on 75-mm wa-fers via Si sublimation [26]. The sheet electron carrier density of the epitaxial graphene layer was typically 8.8 × 1012 cm-2 at room temperature and had electron mobility of ~1192 cm2/Vs, characterized by non-contact Hall Lehighton 1600. The number of epitaxial graphene layers was found to be one layer on the SiC terraces and two layers on the step edges over the entire 75-mm wafer as characterized by Raman analysis and transmission electron microscopy analysis.

Figs. 1a-c shows the graphene FET processed in a layout, where the gate metal is aligned with respect to the ohmic metals in an under lap layout with a gate-to-source/drain separation of <100 nm to minimize the access resistance over a source-drain spacing (Lsd) of 1 µm. The source access resistance was <0.2 Ω·mm via the standard end-point measurements on transmission line method structures. A graphene channel width of 6 μm was defined by O2 plasma etching. The metal gates were processed with Ti/Pt/Au metal deposition and lift-off process on top of a 35-nm-thick SiO2 gate dielectric layer deposited by electron beam evapora-tion. The gate leakage current was in the range of ~nA/μm2 or less, which is negligible in the device characterization presented here.

Figs. 2a and b shows the measured room temperature and the common-source current-voltage characteristics of a two-gate finger 6-μm-wide n-channel graphene FET (denoted as 2f × 6 μm), in which excellent drain current saturation was observed. The source-to-drain voltage (Vds) increased to 3 V, where the gate-to-source (Vgs) voltage was stepped from 3 V (top-curve) in steps of -0.5 V. At Vds = 1 V, on-state current at Vgs = 3 V was 590 mA/mm. The Ion/Ioff ratio was ~13. At Vds = 0.5 V, the Ion/Ioff ratio increased to 20. At Vds = 3 V, the on-state current at Vgs = 3 V was measured as high as 1650 mA/mm. The on-resistance was 1.6 Ω·mm. The peak extrinsic gm of 770 mS/mm was measured at Vds = 3.05 V, which is the highest ever, amongst epitaxial graphene FETs.

technology, this material offers unique device and potential cir-cuit applications ranging from conventional unipolar RF elec-tronics, such as low-noise amplifiers (LNAs), to unique ambi-polar RF electronics such as frequency multipliers, mixers and high-speed radiometers. With observed constant device trans-conductance over the gate voltages [20], graphene FETs could potentially offer LNAs with higher dynamic range per given DC power, beyond the antimony-based low-bandgap devices. With graphene FETs biased near the ambipolar point, graphene FETs behave close to ideal “square-law” devices near the ambipolar point i.e., I(Vg) ∝ Vg

2, where Vg is the gate voltage. It would greatly suppress odd-order harmonics and/or third-order inter-modulation products and improve dynamic range in communi-cations.

2. Status of Epitaxial Graphene-on-SiC Transistors

Graphene FETs are being developed on the wafer-scale uti-lizing graphene-on-Si [19,21], graphene-on-SiC [20,22-24], and graphene transferred to SiO2 substrates [25]. Several promising device parameters have been demonstrated. For example, epi-taxial graphene RF FETs have been demonstrated in a top-gated layout with the highest ever on-state current density of 3 A/mm [24]. On the other hand, the current-voltage characteristics are quasi-linear with weak saturation behaviors, yielding low trans-conductance (gm) per capacitance (i.e., <140 mS/mm at 3.4 fF/μm2) and poor voltage gain (transconductance /output conduc-tance). Also, the ratio of on-state current over off-state current, Ion/Ioff, was ~4 with field-effect mobility below 200 cm2/Vs. While graphene field-effect mobility as high as 5400 cm2/Vs for electron has been demonstrated [22], it was achieved using six to seven layers of epitaxial graphene on C-face SiC substrates,

Table 1. A brief summary of properties of graphene

Vsat (cm/Sec) 4-5 × 107

Carrier mobility (cm2/Vs) >100 000

Current density (A/cm2) ~109

Thermal conductivity (W/m-K) 4800

Optical opacity (%) 2.3% per layer

Young’s modulus (Pa) 0.5-1 tera

Fig. 1. (a) A schematic of the top-gated graphene field-effect transistor (FET). (b) A scanning electron microscopy photograph of 2f x 6 µm graphene FET. (c) A photograph of 75 mm graphene wafer.

Graphene field-effect transistor for radio-frequency applications

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Fig. 3 shows extracted effective mobility (μeff) and field-effect mobility (μFE) of graphene n-FETs versus the effective electric field, Eeff. In comparison, the universal and field-effect mobility of Si n-MOSFETs [27] and strained Si n-MOSFETs on SiGe-on-oxide [28] are shown. While both the μeff and μFE of the graphene n-FETs depend on Eeff, both values were higher than 1000 cm2/Vs over a wide range of the effective electric field up to 1.6 MV/cm. The peak field-effect mobility values ranged from 3200 cm2/Vs to 6000 cm2/Vs. A record field-effect mobility of 6000 cm2/Vs was obtained at an effective electric field of 0.27 MV/cm. The measured field-effect mobility of graphene n-FETs was at least seven times higher than that of Si n-MOSFETs in international technology roadmap for semiconductor and ~80 times higher than ultra-thin-body SOI n-MOSFETs. The peak field-effect mobility of graphene p-FETs was also determined to be 3200 cm2/Vs at an effective electric field of 0.2 MV/cm.

3. Graphene FET RF Performance

Regarding the graphene FET speed performance, in 2009 an extrinsic current-gain cutoff frequency (ft) and maximum os-cillation frequency (fmax) of 4.2 GHz and 14 GHz was reported with a 2 μm gate length [24]. The speed performance was not optimized due to the overlap capacitance between the gate and ohmic metal pads. In 2010, graphene FETs with a 240 μm gate length reported de-embedded ft/fmax of 100 GHz/10 GHz, while there is no extrinsic ft/fmax reported [29]. With graphene FETs processed with self-aligned nanowire gate with a gate length of 140 nm, de-embedded ft of 300 GHz was claimed while the ex-trinsic ft of 2.4 GHz was measured [30].

As an example of graphene FET RF performance, Fig. 4a shows the measured magnitude of H21 (|H21|) and Mason’s uni-lateral gain (U) of the 2 × 12 μm graphene FETs with source-drain spacing (Lds) of 1 μm. The S-parameters were measured at Vds = 5 V and Vgs = -2.5 V. An extrinsic ft of 4.1 GHz was extracted, yielding an extrinsic ft·Lg of 8.2 GHz·μm. The extrin-sic gm was 195 mS/mm. A fmax of 11.5 GHz was extracted from the unilateral gain with a slope of -20 dB/decade. Fig. 4b shows a plot of extrinsic ft and fmax measured from the graphene FETs. For graphene FET with Lds = 3 μm, the extrinsic gm improved to 148 mS/mm at Vds = 9 V, yielding the extrinsic ft and fmax of 4.4 GHz and 6 GHz, respectively. For graphene FET with Lds = 1 μm, the ft and fmax were 4.2 GHz and 14 GHz, respectively, at Vds = 7 V. With a source access resistance of 1.9 Ω·mm, the intrinsic gm becomes 205 mS/mm. This yields an intrinsic ft of 5 GHz with an intrinsic ft·Lg of 10 GHz·μm, which is slightly better than 8.9 GHz·μm from the bulk Si NMOS.

At present, the RF performance of graphene FETs is not close to what had been predicted by the intrinsic saturation velocity of the graphene channel, 4-5 × 107 cm/sec. The theoretical carrier transport velocity provides ft·Lg product >50 GHz·μm, which is far from the currently observed values of ft·Lg products. The ft·Lg product of graphene FETs is expected to improve as the quality of the epitaxial graphene film, gate dielectric layer, transcon-ductance, and transistor fabrication processing improves with reduced parasitic charging delay [31].

In most RF applications, fmax is very important to provide am-plifiers with gain. The weak I-V saturation leads to a smaller

Fig. 2. (a) Measured common-source current-voltage characteristics of 1f x 6 µm graphene FET. (b) Both p-channel and n-channel graphene MOSFET operations. MOSFET: metal-oxide semiconductor field-effect transistor.

Fig. 3. Measured effective carrier mobility and field-effect mobility of graphene n-FETs compared with those of Si n-MOSFET and strained Si on SGOI MOSFET. MOSFET: metal-oxide semiconductor field-effect transistor.

Carbon Letters Vol. 13, No. 1, 17-22 (2012)

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If graphene FETs behave as ideal “square-law” devices near the ambipolar point ie., I(Vg) ∝ Vg

2, it would greatly suppress odd-order harmonics and/or third-order intermodulation prod-ucts and improve dynamic range in communications. A suppres-sion of odd-order signals (f, 3f, 5f…) compared to the second harmonic (2f) was shown in the frequency multiplication ex-periments up to 200 kHz with ~10 dB suppression [32,33], and above GHz [34].

Fig. 5a shows a schematic of the frequency multiplication process in the case of ambipolar transfer curve. Fig. 5b shows an example of measured RF output spectra consisting of strong even-order harmonic signals with odd-order harmonics sup-pressed by >25 dB, supporting “square-law” device operation at the ambipolar point.

The phase noise measurement was reported with a carrier-to-noise degradation of 6 dB, which implies noiseless frequency multiplication without additional 1/f noise up-conversion during the nonlinear process [34]. Capitalizing on the ambipolar nature

output resistance (Rds), transconductance, and higher Cgd com-pared to other semiconductor FETs with a finite bandgap. In or-der to maintain high fmax in short channel graphene FETs, chan-nel engineering to reduce band-to-band tunneling will be desired as a mitigation method.

4. Graphene FET Ambipolar RF Applications

Unlike Si, SiGe, and III-IV semiconductor materials, gra-phene has a unique ambipolar characteristic, where excellent field-effect mobility for both electron and holes can be obtained. For instance, field-effect mobility of 6000 cm2/Vs for electrons and 3200 cm2/Vs for holes was obtained [20]. Several potential applications for these unique ambipolar graphene FETs have been under evaluation [32-34]. These include frequency mul-tipliers, mixers, and oscillators. Frequency multiplication or mixing is based on the nonlinearity of graphene FETs near the ambipolar point.

Fig. 4. (a) Measured |H21| and unilateral gain (U) are shown as a function of frequencies at Vds = 5 V and Vgs = -2.5 V of 2 x 12 µm graphene MOS-FETs with Lds = 1 µm. (b) A plot of measured extrinsic fT and fmax of the gra-phene FETs is shown. The gate length is 2 m. The highest ft and fmax are 4.2 GHz and 14 GHz, respectively. MOSFET: metal-oxide semiconductor field-effect transistor.

Fig. 5. (a) A schematic of frequency multiplication process is shown with RF output spectra consisting of even-order harmonic signals only. (b) Measured transfer curves of two 2f x25 µm graphene FETs at Vds = 1 V is shown with the ambipolar point at Vgs = -1.5 V. Output spectra of the gra-phene FETs driven under the 1f = 1 MHz, 0 dBm RF input signal are shown at the ambipolar point.

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[19] Moon JS, Curtis D, Bui S, Marshall T, Wheeler D, Valles I, Kim S, Wang E, Weng X, Fanton M. Top-gated graphene field-effect tran-sistors using graphene on si (111) wafers. IEEE Electron Device Lett, 31, 1193 (2010). http://dx.doi.org/10.1109/led.2010.2065792.

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of graphene FETs, ambipolar mixers were demonstrated in the GHz range [35,36]. The conversion loss is typically in the order of 20-30 dB, which has to be solved in order to make ambipolar frequency doublers and mixers competitive technologies.

5. Conclusions

While it is early in the stage of development to clearly forecast its impact, graphene FETs could potentially offer performance benefits in RF systems including LNAs, frequency multipliers, mixers and high-speed radiometers. The future success of the RF circuit applications depends on high-quality material growth on large-wafer scale, vertical and lateral scaling of graphene MOSFETs to minimize parasitics and improved gate modulation efficiency in the channel, a bandgap engineering of graphene channels in the MOSFETs, and innovative circuit concepts.

Acknowledgements

This work was partially supported by the Defense Advanced Research Projects Agency (DARPA) and monitored by Dr. John Albrecht at DARPA under SPAWAR contract #N66001-08-C-2048. The views, opinions, and/or findings contained in this article/presentation are those of the author/presenter and should not be interpreted as representing the official views or policies, either expressed or implied, of the Defense Advanced Research Projects Agency or the Department of Defense.

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