A.I. Akinwande

Massachusetts Institute of Technology, Cambridge, MA, United States

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Publications (122)123.4 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We report propellantless neutralizers resilient to oxygen and low-vacuum environments based on arrays of Pt-coated, self-aligned, and gated Si field emitters. These devices emit currents in excess of 1 mA at bias voltages of less than 120 V, adequate for neutralizing the plume of a small spacecraft's electric propulsion system. The reported devices produce similar currents at fivefold less voltage and emitting area than state-of-the-art CNT neutralizers. Long-term (3 hours) continuous emission in a 1 μTorr oxygen partial pressure environment was demonstrated, confirming the compatibility of these neutralizers with low Earth orbit (LEO) conditions. A robust processing sequence was developed that could be employed for high-yield fabrication of large-area field emission neutralizers with active areas larger than 10 cm2 for current emission higher than 100 mA.
    Journal of Physics Conference Series 12/2013; 476(1):2014-.
  • S.A. Guerrera, A.I. Akinwande
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    ABSTRACT: We report the fabrication of arrays of silicon field emitters with 1-micron pitch with integrated, self-aligned extractor gates and 10-micron tall high-aspect-ratio silicon vertical current limiters.
    Vacuum Nanoelectronics Conference (IVNC), 2013 26th International; 01/2013
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    ABSTRACT: Field ionization of helium is demonstrated at voltages as low as 150 V using large arrays of self-aligned gated tips. The operating voltages reported here are more than 3× lower than the best results reported in literature. These substantially reduced operating biases permit stable operation of the device at pressures as high as 1 Torr by avoiding arcing or plasma formation that can permanently damage the device. The fabricated arrays are composed of 320,000 nano-scale tips with radii below 5 nm. Ion current in nano-ampere-range, sufficient for most analytical instruments, can be produced with tip-to-gate biases below 200V. Long term (> 104 s) ionization of helium was accomplished at 1 Torr pressures.
    Vacuum Nanoelectronics Conference (IVNC), 2013 26th International; 01/2013
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    ABSTRACT: We report the fabrication and characterization of high-aspect-ratio silicon pillar current limiters [vertical ungated field-effect transistors (FETs)] for ballasting individual field emitters within field-emitter arrays (FEAs). Dense (1-$muhbox{m}$ pitch) FEAs that are individually ballasted by 100-nm-diameter and 10-$muhbox{m}$-tall current limiters were fabricated, resulting in an emitter tip radius under 10 nm. When characterized without field emitters, the vertical current limiters (ungated FETs) show current-source-like behavior, with saturation currents up to 15 pA/FET. When the current limiters are incorporated into large arrays of field emitters, the current–voltage characteristics of the FEA show evidence of current limitation at high extraction gate voltages. Emission current densities of over 200 $muhbox{A/cm}^{2}$ were obtained from 1.36 million emitter arrays with 5-$muhbox{m}$ pitch.
    IEEE Transactions on Electron Devices 01/2012; 59(9):2524-2530. · 2.06 Impact Factor
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    ABSTRACT: We report a gas ionizer based on arrays of microfabricated double-gated isolated vertically aligned carbon nanofibers (VA-CNFs) for application in low-power portable mass spectrometers. Field-emitted electrons from VA-CNFs are accelerated to high energy and subsequently collide with neutral gas molecules, leading to ionization/fragmentation of the molecules. Double-gated field-emitter arrays with isolated VA-CNF tips were fabricated using a photoresist planarization technique. Two types of devices were fabricated and characterized. The first type of device has the emitter tip in the same plane as the extraction gate, and the second type of device has the emitter tip 900 nm below the extraction gate. All devices were made using a process that results in gate and focus diameters of 1.7 and 4.2 μm, respectively. When operated as a field-emitted electron impact ionizer (EII), for the same ion current, the ionization efficiency (ratio of ions to emitted electrons) increased from 0.005 to 0.05 as the pressure is increased between 5×10<sup>-6</sup> and 1×10<sup>-3</sup> torr. In comparison with EIIs based on thermionic electron sources, the power dissipation reduced from >;1 W to 100 mW.
    IEEE Transactions on Electron Devices 08/2011; · 2.06 Impact Factor
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    ABSTRACT: We report the demonstration of electron sources that achieve high-current and uniform emission using dense arrays of Si field emitters (FEs) that are individually ballasted by a current source. Each FE is fabricated on top of a vertical ungated field-effect transistor (FET), a two-terminal device based on a very-high-aspect-ratio Si column. The ungated FET takes advantage of the velocity saturation of electrons in silicon, the high aspect ratio of the ungated FET, and the doping concentration to achieve current-source-like behavior to obtain reliable uniform and high-current electron emission. Emitted currents in excess of 0.48 A were demonstrated.
    IEEE Transactions on Electron Devices 07/2011; · 2.06 Impact Factor
  • J. Sarik, A.I. Akinwande, I. Kymissis
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    ABSTRACT: A laboratory-based class in flat-panel display technology is presented. The course introduces fundamental concepts of display systems and reinforces these concepts through the fabrication of three display devices-an inorganic electroluminescent seven-segment display, a dot-matrix organic light-emitting diode (OLED) display, and a dot-matrix liquid crystal display (LCD). Instead of fabricating a device over multiple laboratory sessions, students fabricate a functional device in a single session. This approach to teaching fabrication provides students immediate results, can accommodate students with disparate backgrounds, and can be easily adapted and expanded. This paper discusses the laboratory design, its impact on student experiences, and possible improvements and extensions.
    IEEE Transactions on Education 06/2011; · 0.95 Impact Factor
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    ABSTRACT: Analog & digital circuits implemented in a dual threshold voltage (V<sub>T</sub>) p-channel organic technology are presented. The dual V<sub>T</sub> organic technology is compatible with large-area and mechanically flexible substrates due to its low processing temperature (≤ 95°C) and scalable patterning techniques. We demonstrate the first analog & digital organic integrated circuits produced by a dual-gate metal process. The analog circuits are powered by a 5-V supply and include a differential amplifier and a two-stage uncompensated operational amplifier (op-amp). A dynamic comparator is measured to have an input offset voltage of 200 mV and latching time of 119 ms. Both the comparator and the op-amp dissipate 5 nW or less. Area-minimized digital logic is presented. Inverters powered by a 3-V supply were measured to have positive noise margins and consumed picowatts of power. An 11-stage ring oscillator, also powered by a 3-V supply, swings near rail to rail at 1.7 Hz. These results demonstrate dual threshold voltage process feasibility for large-area flexible mixed-signal organic integrated circuits.
    IEEE Transactions on Electron Devices 04/2011; · 2.06 Impact Factor
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    ABSTRACT: In this paper, we report the design and simulation of electron sources composed of arrays of Si field emitters (FEs) that are individually ballasted by a current source. Each FE is fabricated on top of a vertical ungated field-effect transistor (FET), a two-terminal device based on a very-high-aspect-ratio Si column. The ungated FET takes advantage of the velocity saturation of electrons in silicon, the high aspect ratio of the ungated FET, and the doping concentration of the semiconductor to achieve current-source-like behavior. The proposed technology can be used to implement cathodes capable of reliable uniform and high current emission.
    IEEE Transactions on Electron Devices 01/2011; 58:1775-1782. · 2.06 Impact Factor
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    ABSTRACT: A fully photolithographic dual threshold voltage (V<sub>T</sub>) organic thin-film transistor (OTFT) process suitable for flexible large-area integrated circuits is presented. The nearroom-temperature (<; 95 °C) process produces integrated dual V<sub>T</sub> pentacene-based p-channel transistors. The two V<sub>T</sub> 's are enabled by using two gate metals of low (aluminum) and high (platinum) work function. The Al and Pt gate OTFTs exhibit nominally identical current-voltage transfer curves shifted by an amount ΔV<sub>T</sub>. The availability of a high-V<sub>T</sub> device enables area-efficient zero-Vos high-output-resistance current sources, enabling high-gain inverters. We present positive noise margin inverters and rail-to-rail ring oscillators powered by a 3-V supply-one of the lowest supply voltages reported for OTFT circuits. These results show that integrating nand p-channel organic devices is not mandatory to achieve functional area-efficient low-power organic integrated circuits.
    IEEE Transactions on Electron Devices 12/2010; · 2.06 Impact Factor
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    ABSTRACT: Size reduction in quadrupole mass spectrometers (QMSs) is an ongoing requirement driven by the needs of space exploration, portable, and covert monitoring applications. Microelectromechanical systems (MEMS) technology provides a method of achieving this size reduction. A quadrupole mass filter (QMF) is one component of a QMS and is suitable for microfabrication. MEMS manufacturing techniques are more suitable to the production of rectilinear electrodes, instead of the more widely used circular electrodes. Present understanding of the performance characteristics of rectilinear electrodes and the dependence of these characteristics on electrode geometry are not well documented. In this paper, we report on the performance characteristics of a square-electrode QMF. Both the predicted performances obtained by computer simulation and experimental data are presented for operation in stability zone 1 (0.236, 0.706) and zone 3 (3.16, 3.23). A comparison between these results and the simulated data for equivalent devices constructed using hyperbolic and circular electrodes for operation in zone 1 is also made. This comparison demonstrates that, although the field produced by square electrodes is far from the “ideal,” it is still possible to achieve useful filtering action. Our results also show that, for operation in zone 3, performance comparable with that of hyperbolic and circular electrodes operating in zone 1 is achievable.
    IEEE Transactions on Instrumentation and Measurement 10/2010; · 1.36 Impact Factor
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    ABSTRACT: This paper reports the design and fabrication of high-aspect-ratio needlelike silicon structures that can have complex geometry. The structures are hundreds of micrometers tall with submicrometer-sharp protrusions, and they are fabricated using a series of passivated and unpassivated deep reactive-ion etching (DRIE) steps. A simple model is presented to predict the geometry of the structure based on the etch mask and the etch sequence. Model predictions are in good qualitative agreement with fabrication results, making it a useful design tool. The model is compared with literature reports on tapered DRIE.
    Journal of Microelectromechanical Systems 07/2010; · 2.13 Impact Factor
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    K. Cheung, L.F. Velasquez-Garcia, A.I. Akinwande
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    ABSTRACT: We report the design, fabrication, and characterization of a new class of chip-scale quadrupole mass filter (QMF). The devices are completely batch fabricated using a wafer-scale process that integrates the quadrupole electrodes, ion optics, and housing into a single monolithic block. This process eliminates the electrode-to-housing misalignments inherent in other QMFs and enables the implementation of complex device architectures. To achieve the reported integration, 1 mm ?? 1 mm square electrodes of heavily doped silicon were utilized, resulting in quadrupoles with an effective aperture radius of 0.707 mm and a length of 30 mm. Mass filtering was demonstrated with this unconventional device showing a mass range of 650 amu and a resolution of ~30 at a drive frequency of 1.8 MHz. When operated in the second stability region at 2.0 MHz and a mass range of 50 amu, a peak width of 0.3 amu was achieved at mass 28, showing a resolution of ~90. This paper introduces operation in the second stability region as a reliable method for turning QMFs with less than ideal electrode geometries into high-performance devices.
    Journal of Microelectromechanical Systems 07/2010; · 2.13 Impact Factor
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    ABSTRACT: We report the fabrication and experimental characterization of a carbon nanotube (CNT)-based MEMS/NEMS electron impact gas ionizer with an integrated extractor gate for portable mass spectrometry. The ionizer achieves low-voltage ionization using sparse forests of plasma-enhanced chemical-vapor-deposited CNTs as field emitters and a proximal extractor grid with apertures aligned to the CNT forests to facilitate electron transmission. The extractor gate is integrated to the ionizer using a high-voltage MEMS packaging technology based on Si springs defined by deep reactive ion etching. The ionizer also includes a high-aspect-ratio silicon structure (??foam) that facilitates sparse CNT growth and also enables uniform current emission. The devices were tested as field emitters in high vacuum (10<sup>-8</sup> torr) and as electron impact ionizers using argon at pressures of up to 21 mtorr. The experimental data show that the MEMS extractor gate transmits up to 66% of the emitted current and that the ionizers are able to produce up to 0.139 mA of ion current with up to 19% ionization efficiency while consuming 0.39 W.
    Journal of Microelectromechanical Systems 07/2010; · 2.13 Impact Factor
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    ABSTRACT: The effects of bias stress in integrated pentacene organic transistors are studied and modeled for different stress conditions. It is found that the effects of bias stress can be expressed in terms of the shift in applied gate voltage ¿V for a given current. An empirical equation describing ¿V in terms of different gate and drain bias stress measurements and stress times is presented and verified. In the measured devices, ¿ V saturates at 14 V, independent of the gate bias-stress condition. A model based on carrier trapping rate equation that accounts for this ¿V saturation is developed. The model suggests that the ¿V saturation is due to the small density of traps compared to the channel carrier density.
    IEEE Transactions on Electron Devices 06/2010; · 2.06 Impact Factor
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    ABSTRACT: We report the first remote flight control of an insect using microfabricated flexible neuroprosthetic probes (FNPs) that directly interface with the animal's central nervous system. The FNPs have a novel split-ring design that incorporates the anatomical bi-cylinder structure of the nerve cord and allows for an efficient surgical process for implantation (Figure 1a). Additionally, we have integrated carbon nanotube (CNT)-Au nanocomposites into the FNPs to enhance the charge injection capability of the probe. The FNPs integrated with a wireless system are able to evoke multi-directional, graded abdominal motions in the moths thus altering their flight path.
    Micro Electro Mechanical Systems (MEMS), 2010 IEEE 23rd International Conference on; 02/2010
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    K. Cheung, L.F. Velasquez-Garcia, A.I. Akinwande
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    ABSTRACT: We report exciting experimental data from a low-cost, high-performance square electrode quadrupole mass filter with integrated ion optics intended for chips-cale mass spectrometry. The device showed a mass range of 650 amu and a maximum resolution of ~80, metrics that are comparable to the state-of-the-art. We demonstrated that operating our device in the second stability region leads to higher performance despite the unconventional electrode geometry. The data also suggests ion behavior that deviates from the standard circular rod case, motivating new studies.
    Micro Electro Mechanical Systems (MEMS), 2010 IEEE 23rd International Conference on; 02/2010
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    ABSTRACT: We report a low temperature ( ~ 100°C) lithographic method for fabricating hybrid metal oxide/organic field-effect transistors (FETs) that combine a zinc-indium-oxide (ZIO) semiconductor channel and organic, parylene, dielectric layer. The transistors show a field-effect mobility of (12±0.8) cm<sup>2</sup> V<sup>-1</sup> s<sup>-1</sup>, on/off ratio of 10<sup>8</sup> and turn-off voltage of V<sub>off</sub> = -1 V. This work demonstrates that organic and inorganic layers can be deposited and patterned using a low temperature budget, integrated lithographic process to make FETs suitable for large area electronic applications.
    Journal of Display Technology 02/2010; · 1.66 Impact Factor
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    ABSTRACT: We describe a flexible multisite microelectrode for insect flight biasing using neural stimulation. The electrode is made of two layers of polyimide (PI) with gold sandwiched in between in a split-ring geometry. The split-ring design in conjunction with the flexibility of the PI allows for a simple insertion process and provides good attachment between the electrode and ventral nerve cord of the insect. Stimulation sites are located at the ends of protruding tips that are circularly distributed inside the split-ring structure. These protruding tips penetrate into the connective tissue surrounding the nerve cord. We have been able to insert the electrode into pupae of the giant sphinx moth Manduca sexta as early as seven days before the adult moth emerges, and we are able to use the multisite electrode to deliver electrical stimuli that evoke multidirectional, graded abdominal motions in both pupae and adult moths. Finally, in loosely tethered flight, we have used stimulation through the flexible microelectrodes to alter the abdominal angle, thus causing the flying moth to deviate to the left or right of its intended path.
    IEEE transactions on bio-medical engineering 02/2010; 57(7):1757-64. · 2.15 Impact Factor
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    ABSTRACT: For the first time, we demonstrate control of organic thinfilm transistor's (OTFT) threshold voltage (V<sub>T</sub>) by modifying the gate work function. We present a near-room-temperature, fully lithographic process to fabricate integrated pentacene dual V<sub>T</sub> OTFTs suitable for large-area and flexible mixed signal circuits. Platinum and aluminum are used as the gate metals for the high V<sub>T</sub> (more depletion-like) and low V<sub>T</sub> (more enhancement-like) p-channel devices, respectively. The availability of a high V<sub>T</sub> device enables area-efficient zero-VGS current source loads. We demonstrate positive noise margin inverters which use pico Watts of power and a 3 V supply. Compared to a single V<sub>T</sub> implementation, the dual V<sub>T</sub> inverter occupies an area that is 30× smaller, and is 17× faster. These results show that p-channel only organic technologies can produce functional and low-power circuits without integrating a complementary device.
    Electron Devices Meeting (IEDM), 2009 IEEE International; 01/2010

Publication Stats

567 Citations
123.40 Total Impact Points

Institutions

  • 1995–2011
    • Massachusetts Institute of Technology
      • • Department of Electrical Engineering and Computer Science
      • • Department of Materials Science and Engineering
      Cambridge, MA, United States
  • 1989–1997
    • Honeywell
      Morristown, New Jersey, United States
  • 1990
    • Signal Processing Inc.
      Maryland, United States
    • University of Minnesota Twin Cities
      • School of Physics and Astronomy
      Minneapolis, MN, United States