L.-Y. Chen

Massachusetts Institute of Technology, Cambridge, Massachusetts, United States

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Publications (5)2.06 Total impact

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    ABSTRACT: We report micro-fabricated double-gated vertically aligned carbon nanofiber (CNF) arrays for ionization of gasses in low power portable mass spectrometers. The devices can be operated in one of two modes - electron impact ionization (EII ) or field ionization (FI). When operated as electron impact ionizer, power dissipation was reduced from >1 W typical of thermionic emission based electron impact ionizers to <100 mW. When operated as a field ionizer, the turn-on voltage for field ionization is reduced from 5-10 kV typical of ungated ionizers to 350 V.
    Electron Devices Meeting, 2007. IEDM 2007. IEEE International; 01/2008
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    ABSTRACT: In this work, we designed and fabricated two types of double-gated isolated vertically aligned carbon nanofiber field emission arrays (VACNF FEAs) to study how the tip position relative to the gate affects the device performance. In the first type, the tip is in-plane with the gate, and in the second type, the tip is 0.9 mum below the gate. To quantify the effectiveness of the two gates to affect the total emission current, the gate field factor (beta<sub>G</sub>) and the focus field factor (beta<sub>F</sub>) are examined for both types of VACNF FEAs experimentally and by simulation.
    Vacuum Nanoelectronics Conference, 2007. IVNC. IEEE 20th International; 08/2007
  • L.-Y. Chen, A.I. Akinwande
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    ABSTRACT: Double-gated silicon field emission arrays (FEAs) are fabricated with the tip 400 nm below the extraction gate and 600 nm below the focus. The diameters of the gate and focus apertures are 0.4 μm and 0.7 μm respectively. For this structure, an abrupt drop in anode current is observed when the focus voltage V<sub>f</sub> is less than 12 V. For the range of focus voltages 4 V < V<sub>f</sub> < 12 V, the anode current initially increases with the gate voltage because the net repulsive force is still small. The emitted current increases with V<sub>G</sub> due to repulsion by the focus. Eventually, the anode current peaks and a negative resistance region ensue. Further increase of V<sub>G</sub> shuts off the anode current. For focus voltage > 12 V, the balance between the emission and repulsive forces is such that there is a monotonic increase in anode current with V<sub>G</sub> and shows no negative resistance region.
    Vacuum Nanoelectronics Conference, 2005. IVNC 2005. Technical Digest of the 18th International; 08/2005
  • L.-Y. Chen, A.I. Akinwande
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    ABSTRACT: In this work, the gate field factor and focus field factor were examined in the double-gated field emission array (FEA) and were shown to have strong influence on the initial beam spread. The optical measurement verified that the device with the tip below the gate aperture provides a small beam spread.
    Vacuum Nanoelectronics Conference, 2004. IVNC 2004. Technical Digest of the 17th International; 08/2004
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    ABSTRACT: The Micro Gas Analyzer (MGA) project aims to develop the technology for real-time sensors intended for chemical warfare. The device is composed of four micro-fabricated subsystems: 1) an ionizer; 2) a mass filter based on a quadrupole (1); 3) a species sensor based on a resonator (2); and 4) a pump (3). We are developing a field ionizer array based on gated CNTs. We plan to use arrays of CNTs because of their small radii, high aspect ratio, and gate proximity to ensure high fields at low voltage. State-of-the-art ionizers use electron impact ionization (thermionic cathodes), incurring in excessive power consumption, low current, current density, ionization efficiency, and short lifetime. Each of the proposed ionizer arrays - the impact and field - offer distinct advantages. The electron impact ionizer and field ionizer arrays both are more efficient and consume less power than thermionic cathodes, and variation of gate voltage in each improves specificity. The field ionizer, however, is based on the concept of electron tunneling (electrons tunnel in the outer shell of the molecule, due to the presence of high electric fields). Because of this, the field ionizer is able to soft-ionize species, thus achieving molecule ionization. The reliability and device lifespan of the field- tunneling ionizer is increased by biasing CNTs to the highest potential in the circuit, thus making it unlikely for ionized molecules to back-stream. In the case of the electron impact ionizer, the reliability and lifespan of the ionizer is improved by using a double gate.