K.A. Jenkins

IBM, Armonk, New York, United States

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Publications (180)331.48 Total impact

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    ABSTRACT: Graphene has attracted much interest as a future channel material in radio frequency electronics because of its superior electrical properties. Fabrication of a graphene integrated circuit without significantly degrading transistor performance has proven to be challenging, posing one of the major bottlenecks to compete with existing technologies. Here we present a fabrication method fully preserving graphene transistor quality, demonstrated with the implementation of a high-performance three-stage graphene integrated circuit. The circuit operates as a radio frequency receiver performing signal amplification, filtering and downconversion mixing. All circuit components are integrated into 0.6 mm(2) area and fabricated on 200 mm silicon wafers, showing the unprecedented graphene circuit complexity and silicon complementary metal-oxide-semiconductor process compatibility. The demonstrated circuit performance allow us to use graphene integrated circuit to perform practical wireless communication functions, receiving and restoring digital text transmitted on a 4.3-GHz carrier signal.
    Nature Communications 01/2014; 5:3086. · 10.74 Impact Factor
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    ABSTRACT: With the rapid advances in graphene field-effect transistor (GFET) performances, graphene has attracted much interest as a future channel material in RF electronics. However, the pace of the development of graphene circuits seems significantly slower. Several graphene circuits demonstrated today showing promising GHz functions still relied on ideal discrete passive components connected at the equipment level, and these circuits are limited to single transistor designs [1-3]. To compete with existing technologies requires that all active and passive components be monolithically integrated for not only the small circuit footprint and low cost but also high circuit complexity and advanced system functionality.
    2013 IEEE International Electron Devices Meeting (IEDM); 12/2013
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    ABSTRACT: The linearity of the radio frequency response of graphene field-effect transistors has been measured as a function of gate bias using the two-tone method. Two kinds of transistors, which differ in both the graphene source material and the device structure, have been compared. Both devices show high linearity compared to contemporary silicon transistors. The physical origins of this behavior are analyzed and discussed.
    Applied Physics Letters 10/2013; 103:173115. · 3.52 Impact Factor
  • Shu-Jen Han, Satoshi Oida, Keith A. Jenkins, Darsen Lu, Yu Zhu
    IEEE Electron Device Letters 10/2013; 34(10):1340-1342. · 2.79 Impact Factor
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    ABSTRACT: The excellent electrical properties of graphene, such as its high carrier mobility, gate tunability, and mechanical flexibility makes it a very promising material for radio frequency (RF) electronics. Here we study the impact of top and bottom gate control on the essential performance metrics of graphene RF transistors. We find that the maximum cut-off frequency improves as the bottom gate voltage is tuned towards the same polarity as the top gate bias voltage. These results can be explained by the bottom-gate tunable doping of the graphene underneath the metal contacts and in the under-lap region. These effects become more dramatic with device down-scaling. We also find that the minimum output conductance occurs, when the drain voltage roughly equals an effective gate voltage (Veff ≈ VTG+VBG⋅CBG/CTG, where VTG and VBG are top and bottom gate voltage, CTG and CBG are the respective gate capacitance). The minimum output conductance is reduced as the bottom gate bias increases, due to the stronger control of the channel from the bottom gate, lessening the influence of the drain voltage on the drain current. As a result of these two influences, when the bottom gate voltage is tuned towards the same polarity as the top gate voltage, both the maximum oscillation frequency (fmax) and the intrinsic gain significantly improve. The intrinsic gain can increase as high as 3–4 times as the gain without the bottom gate bias. Tuning the bottom gate to enhance fmax and gain will be very important elements in the effort to enable graphene RF devices for practical use.
    Journal of Applied Physics 07/2013; 114(4). · 2.21 Impact Factor
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    ABSTRACT: Graphene is a very promising candidate for applications in flexible electronics due to its high carrier mobility and mechanical flexibility. In this paper, we present results on graphene RF devices fabricated on polyimide substrates with cutoff frequencies as high as 10 GHz. Excellent channel mobility and current saturation are observed in graphene long channel devices on polyimide. Graphene devices on polyimide also show very good temperature stability from 4.4 K to 400 K and excellent mechanical flexibility up to a bending radius of 1 mm. These demonstrated properties make graphene an excellent candidate for flexible wireless applications.
    Applied Physics Letters 06/2013; 102(23). · 3.52 Impact Factor
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    ABSTRACT: Graphene is very promising for RF devices due to its high carrier mobility. High cut-off frequency graphene RF devices using CVD grown graphene and epitaxially grown graphene have been reported. Here we report the effect of the back-gate bias on the FET cut-off frequency and current saturation. We found that there are two peak cut-off frequencies corresponding to electron peak trans-conductance and hole peak trans-conductance maxima respectively, as we sweep the top-gate bias. The electron peak cut-off frequency can be significantly increased by applying a positive back-gate bias. The higher the voltage, the larger the maximum cut-off frequency. This can be explained by the additional electron doping introduced by the back-gate bias in the under-lap region, which forms an n-n+-n configuration. Similarly, the hole peak cut-off frequency can be significantly enhanced by applying negative back-gate bias to form the p-p+-p configuration. The shorter the channel, the more pronounced this effect. We also found that the current saturation is also improved by introducing the same type of carrier as the channel in the under-lap region.
    03/2013;
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    ABSTRACT: This work presents recent advances in the development of graphene technology for various applications from RF to THz frequencies. First, large-scale graphene synthesis methods are reviewed. Graphene FETs suitable for RF applications are then presented along with their DC and RF characteristics. The challenges and current progress toward wafer-scale integration of graphene-based RFICs are also discussed including RFIC measurement results up to 200C. Finally, the absorption properties of graphene at THz frequencies are discussed.
    Microwave Symposium Digest (IMS), 2013 IEEE MTT-S International; 01/2013
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    ABSTRACT: We report the radio-frequency performance of carbon nanotube array transistors that have been realized through the aligned assembly of highly separated, semiconducting carbon nanotubes on a fully scalable device platform. At a gate length of 100 nm, we observe output current saturation and obtain as-measured, extrinsic current gain and power gain cut-off frequencies, respectively, of 7 GHz and 15 GHz. While the extrinsic current gain is comparable to the state-of-the-art the extrinsic power gain is improved. The de-embedded, intrinsic current gain and power gain cut-off frequencies of 153 GHz and 30 GHz are the highest values experimentally achieved to date. We analyze the consistency of DC and AC performance parameters and discuss the requirements for future applications of carbon nanotube array transistors in high-frequency electronics.
    Applied Physics Letters 08/2012; 101(5). · 3.52 Impact Factor
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    ABSTRACT: Recently, graphene field-effect transistors (FET) with cutoff frequencies (f(T)) between 100 and 300 GHz have been reported; however, the devices showed very weak drain current saturation, leading to an undesirably high output conductance (g(ds)= dI(ds)/dV(ds)). A crucial figure-of-merit for analog/RF transistors is the intrinsic voltage gain (g(m)/g(ds)) which requires both high g(m) (primary component of f(T)) and low g(ds). Obtaining current saturation has become one of the key challenges in graphene device design. In this work, we study theoretically the influence of the dielectric thickness on the output characteristics of graphene FETs by using a surface-potential-based device model. We also experimentally demonstrate that by employing a very thin gate dielectric (equivalent oxide thickness less than 2 nm), full drain current saturation can be obtained for large-scale chemical vapor deposition graphene FETs with short channels. In addition to showing intrinsic voltage gain (as high as 34) that is comparable to commercial semiconductor FETs with bandgaps, we also demonstrate high frequency AC voltage gain and S21 power gain from s-parameter measurements.
    ACS Nano 05/2012; 6(6):5220-6. · 12.03 Impact Factor
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    ABSTRACT: High-performance graphene transistors for radio frequency applications have received much attention and significant progress has been achieved. However, devices based on large-area synthetic graphene, which have direct technological relevance, are still typically outperformed by those based on mechanically exfoliated graphene. Here, we report devices with intrinsic cutoff frequency above 300 GHz, based on both wafer-scale CVD grown graphene and epitaxial graphene on SiC, thus surpassing previous records on any graphene material. We also demonstrate devices with optimized architecture exhibiting voltage and power gains reaching 20 dB and a wafer-scale integrated graphene amplifier circuit with voltage amplification.
    Nano Letters 05/2012; 12(6):3062-7. · 13.03 Impact Factor
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    ABSTRACT: High-frequency performance of graphene field-effect transistors (GFETs) with boron-nitride gate dielectrics is investigated. Devices show saturating IV characteristics and fmax values as high as 34 GHz at 600-nm channel length. Bias dependence of fT and fmax and the effect of the ambipolar channel on transconductance and output resistance are also examined.
    Electron Devices Meeting, 1988. IEDM '88. Technical Digest., International 12/2011;
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    ABSTRACT: This letter reports the impact of surface morphology on the carrier transport and radio-frequency performance of graphene FETs formed on epitaxial graphene synthesized on SiC substrates. Such graphene exhibits long terrace structures with widths between 3-5 μm and steps of 10 ± 2 nm in height. While a carrier mobility value above 3000 cm<sup>2</sup>/V·s at a carrier density of 10<sup>12</sup> cmx<sup>2</sup> is obtained in a single graphene terrace, the step edges can result in a step resistance of ~21 kΩ·μm. By orienting the transistor layout so that the entire channel lies within a single graphene terrace and by reducing the access resistance associated with the ungated part of the channel, a cutoff frequency above 200 GHz is achieved for graphene FETs with channel lengths of 210 nm, i.e., the highest value reported on epitaxial graphene thus far.
    IEEE Electron Device Letters 11/2011; · 2.79 Impact Factor
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    ABSTRACT: While graphene transistors have proven capable of delivering gigahertz-range cutoff frequencies, applying the devices to RF circuits has been largely hindered by the lack of current saturation in the zero band gap graphene. Herein, the first high-frequency voltage amplifier is demonstrated using large-area chemical vapor deposition grown graphene. The graphene field-effect transistor (GFET) has a 6-finger gate design with gate length of 500 nm. The graphene common-source amplifier exhibits ∼5 dB low frequency gain with the 3 dB bandwidth greater than 6 GHz. This first AC voltage gain demonstration of a GFET is attributed to the clear current saturation in the device, which is enabled by an ultrathin gate dielectric (4 nm HfO(2)) of the embedded gate structures. The device also shows extrinsic transconductance of 1.2 mS/μm at 1 V drain bias, the highest for graphene FETs using large-scale graphene reported to date.
    Nano Letters 08/2011; 11(9):3690-3. · 13.03 Impact Factor
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    ABSTRACT: High-performance graphene field-effect transistors are fabricated on two-inch graphene-on-SiC wafers. Epitaxial graphene was synthesized on SiC wafers by thermal annealing to form one to two layers of graphene. The graphene transistors possess high current density of >; 1mA/μm, and a cutoff frequency of 170 GHz is achieved for graphene FETs with a gate length of 90 nm. These results unravel the great potential of graphene for future RF applications.
    Microwave Symposium Digest (MTT), 2011 IEEE MTT-S International; 07/2011
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    ABSTRACT: A wafer-scale graphene circuit was demonstrated in which all circuit components, including graphene field-effect transistor and inductors, were monolithically integrated on a single silicon carbide wafer. The integrated circuit operates as a broadband radio-frequency mixer at frequencies up to 10 gigahertz. These graphene circuits exhibit outstanding thermal stability with little reduction in performance (less than 1 decibel) between 300 and 400 kelvin. These results open up possibilities of achieving practical graphene technology with more complex functionality and performance.
    Science 06/2011; 332(6035):1294-7. · 31.20 Impact Factor
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    ABSTRACT: Owing to its high carrier mobility and saturation velocity, graphene has attracted enormous attention in recent years. In particular, high-performance graphene transistors for radio-frequency (r.f.) applications are of great interest. Synthesis of large-scale graphene sheets of high quality and at low cost has been demonstrated using chemical vapour deposition (CVD) methods. However, very few studies have been performed on the scaling behaviour of transistors made from CVD graphene for r.f. applications, which hold great potential for commercialization. Here we report the systematic study of top-gated CVD-graphene r.f. transistors with gate lengths scaled down to 40 nm, the shortest gate length demonstrated on graphene r.f. devices. The CVD graphene was grown on copper film and transferred to a wafer of diamond-like carbon. Cut-off frequencies as high as 155 GHz have been obtained for the 40-nm transistors, and the cut-off frequency was found to scale as 1/(gate length). Furthermore, we studied graphene r.f. transistors at cryogenic temperatures. Unlike conventional semiconductor devices where low-temperature performance is hampered by carrier freeze-out effects, the r.f. performance of our graphene devices exhibits little temperature dependence down to 4.3 K, providing a much larger operation window than is available for conventional devices.
    Nature 04/2011; 472(7341):74-8. · 38.60 Impact Factor
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    ABSTRACT: In this paper, the authors present experimental studies on transport characteristics of graphene FETs with channel lengths down to 70 nm. The factors limiting the performance of short channel graphene devices are discussed. RF performance of a sub-100 nm graphene transistor fabricated on epitaxial graphene grown on a SiC substrate is also presented. A cut-off frequency as high as 170 GHz is achieved in a 90 nm graphene FET using a scalable top-down fabrication processes. Our results indicate that further improvement of RF performance of graphene FETs can be enabled by channel length scaling with structure optimization and contact resistance reduction.
    Electron Devices Meeting (IEDM), 2010 IEEE International; 01/2011
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    ABSTRACT: By optimizing of the gate dielectrics and device dimensions, we achieve a record high output current and transconductance of 5 mA/µm and 2 mS/µm in epitaxial graphene FETs. A cut-off frequency of 280 GHz is achieved for a 40-nm graphene FET, the highest so far on any synthesized graphene. Also, highest voltage gain of 10 dB has been achieved, with an fmax/fT ratio larger than 1 demonstrated consistently on different devices. For the first time, forward power gain |S21|>1 delivered into a 50-Ω load is demonstrated.
    Electron Devices Meeting, 1988. IEDM '88. Technical Digest., International 01/2011;
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    ABSTRACT: Wafer-scale graphene devices processed entirely in a standard 200 mm silicon fab are demonstrated for the first time. New embedded gate structures enable full saturation of the drain current in graphene FETs with sub-μm channels, resulting in high intrinsic voltage gain. In addition, passive devices were monolithically integrated with graphene transistors to form the first GHz-range graphene IC using large-scale CVD graphene. The demonstration of high performance graphene FETs and IC fabricated using a 200 mm platform is a major step in transitioning this promising material from a scientific curiosity into a real technology.
    Electron Devices Meeting, 1988. IEDM '88. Technical Digest., International 01/2011;