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  • Modeling electrical characteristics of thin-film field-effect transistors III. Normally-on devices
    Publication . Stallinga, Peter; Gomes, Henrique L.
    The thin-film field-effect-transistor model recently developed is applied to devices based on materials that already show current even without a bias present at the gate resulting in so-called normally-on transistors. These fall in three categories: (i) narrow-band-gap semiconductors, where the thermal energy is sufficient to excite carriers across the band-gap, here analyzed for unipolar and ambipolar materials, (ii) doped semiconductors, and (iii) metals. It is shown what the impact is on the IV and transfer curves. (C) 2008 Elsevier B.V. All rights reserved.
  • Light-emitting thin-film field-effect transistors
    Publication . Stallinga, Peter; Gomes, Henrique L.
    A model for thin-film field-effect transistors in which the active layer is treated as purely two-dimensional is applied to ambipolar devices that have shown to be light emitting. This results in an adequate description of the electrical characteristics.
  • A microelectrode impedance method to measure interaction of cells
    Publication . Gomes, Henrique L.; Leite, Ricardo; Afonso, R.; Stallinga, Peter; Cancela, M. Leonor
    An impedance method was developed to determine how immune system cells (hemocyte) interact with intruder cells (parasites). When the hemocyte cells interact with the parasites, they cause a defensive reaction and the parasites start to aggregate in clusters. The level of aggregation is a measure of the host-parasite interaction, and provides information about the efficiency of the immune system response. The cell aggregation is monitored using a set of microelectrodes. The impedance spectrum is measured between each individual microelectrode and a large reference electrode. As the cells starts to aggregate and settle down towards the microelectrode array the impedance of the system is changed. It is shown that the system impedance is very sensitive to the level of cell aggregation and can be used to monitor in real time the interaction between hemocyte cells and parasites.
  • Study of trap states in zinc oxide (ZnO) thin films for electronic applications
    Publication . Casteleiro, C.; Gomes, Henrique L.; Stallinga, Peter; Bentes, L.; Ayouchi, R.; Schwarz, R.
    The electrical properties of ZnO thin films grown by pulsed laser deposition were studied. Field-effect devices with a mobility reaching 1 cm2/V s show non-linearities both in the current–voltage and in the transfer characteristics which are explained as due to the presence of trap states. These traps cause a reversible threshold voltage shift as revealed by low-frequency capacitance–voltage measurements in metal insulator semiconductor (MIS) capacitors. Thermal detrapping experiments in heterojunctions confirm the presence of a trap state located at 0.32 eV.
  • Metal contacts in thin-film transistors
    Publication . Stallinga, Peter; Gomes, Henrique L.
    The effects of metal contacts on the electrical characteristics in thin-film transistors are discussed. It is found that the effects of these contacts are twofold. First, a constant potential that can range from zero to some volts (half the bandgap) is added to the entire channel. Second, a residual barrier is formed with a height that depends on the bias, and is in the order of tens of meV when a current is present. It is shown that these predicted effects are in agreement with experimental observations. (c) 2006 Elsevier B.V. All rights reserved.
  • Determining carrier mobility with a metal–insulator–semiconductor structure
    Publication . Stallinga, Peter; Benvenho, A. R. V.; Smits, E. C. P.; Mathijssen, S. G. J.; Cölle, M.; Gomes, Henrique L.; De Leeuw, Dago M.
    The electron and hole mobility of nickel-bis(dithiolene) (NiDT) are determined in a metal– insulator–semiconductor (MIS)structure using admittance spectroscopy. The relaxation times found in the admittance spectra are attributed to the diffusion time of carriers to reach the insulator interface and via Einstein’s relation this yields the mobility values. In this way, an electron mobility of 1:9 104 cm2=Vs and a hole mobility of 3:9 106 cm2=Vs were found. It is argued that the low mobility is caused by an amphoteric mid-gap trap level. The activation energy for electrons and holes from these traps is found to be 0.46 eV and 0.40 eV, respectively.
  • Interface state mapping in a Schottky barrier of the organic semiconductor terrylene
    Publication . Stallinga, Peter; Gomes, Henrique L.; Murgia, M.; Mullen, K.
    In this work we quantitatively map interface states in energy in a Schottky barrier between aluminum and the vacuum sublimed organic semiconductor terrylene. The density map of these interface states was extracted from the, admittance spectroscopy data. They revealed an interface state density of 2 x 10(12). cm(-2)eV(-1) close to the valence band which decreases slightly towards midgap. Additional do measurements show that the semiconductor bulk activation energy is 0.33 eV which may correspond to an acceptor level. (C) 2002 Elsevier Science B.V. All rights reserved.
  • Modeling electrical characteristics of thin-film field-effect transistors II: Effects of traps and impurities
    Publication . Stallinga, Peter; Gomes, Henrique L.
    Based on a new model for thin-film field-effect transistors, in which the active layer is treated as purely two-dimensional, the effects of impurities on the electrical characteristics are discussed. Localized electronic levels are introduced into the model. It is shown that the presence of traps readily accounts for the non-linearities in the current-voltage curves. Trap states can also explain the temperature dependence of the current and mobility, including the so-called Meyer-Neldel Rule. Finally, transients are qualitatively discussed. (c) 2006 Elsevier B.V. All rights reserved.
  • Minority-carrier effects in poly-phenylenevinylene as studied by electrical characterization
    Publication . Stallinga, Peter; Gomes, Henrique L.; Rost, H.; Holmes, A. B.; Harrison, M. G.; Friend, R. H.
    Electrical measurements have been performed on poly[2-methoxy, 5 ethyl (2' hexyloxy) paraphenylenevinylene] in a pn junction with silicon. These included current-voltage measurements, capacitance-voltage measurements, capacitance-transient spectroscopy, and admittance spectroscopy. The measurements show evidence for large minority-carrier injection into the polymer possibly enabled by interface states for which evidence is also found. The shallow acceptor level depth (0.12 eV) and four deep trap level activation energies (0.30 and 1.0 eV majority-carrier type; 0.48 and 1.3 eV minority-carrier type) are found. Another trap that is visible at room temperature has point-defect nature. (C) 2001 American Institute of Physics.
  • Electronic levels in MEH-PPV
    Publication . Stallinga, Peter; Gomes, Henrique L.; Rost, H.; Holmes, A. B.; Harrison, M. G.; Friend, R. H.
    pn-Junctions of MEH-PPV on top of heavily doped n-type silicon were used in electrical measurements. Through deep-level transient-spectroscopy (DLTS)-like measurements, four traps (two majority and two minority traps) could be identified on top of the shallow acceptor level responsible for conduction. Furthermore, evidence is found for interface states. (C) 2000 Published by Elsevier Science S.A. All rights reserved.