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- Modeling electrical characteristics of thin-film field-effect transistors III. Normally-on devicesPublication . 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.
- Analysis of deep levels in a phenylenevinylene polymer by transient capacitance methodsPublication . Gomes, Henrique L.; Stallinga, Peter; Rost, H.; Holmes, A. B.; Harrison, M. G.; Friend, R. H.Transient capacitance methods were applied to the depletion region of an abrupt asymmetric n(+) -p junction of silicon and unintentionally doped poly[2-methoxy, 5 ethyl (2' hexyloxy) paraphenylenevinylene] (MEH-PPV). Studies in the temperature range 100-300 K show the presence of a majority-carrier trap at 1.0 eV and two minority traps at 0.7 and 1.3 eV, respectively. There is an indication for more levels for which the activation energy could not be determined. Furthermore, admittance data reveal a bulk activation energy for conduction of 0.12 eV, suggesting the presence of an additional shallow acceptor state. (C) 1999 American Institute of Physics. [S0003-6951(99)02308-6].
- Light-emitting thin-film field-effect transistorsPublication . 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.
- Determination of deep and shallow levels in conjugated polymers by electrical methodsPublication . Stallinga, Peter; Gomes, Henrique L.; Rost, H.; Holmes, A. B.; Harrison, M. G.; Friend, R. H.; Biscarini, F.; Taliani, C.; Jones, G. W.; Taylor, D. M.Conjugated organic semiconductors have been submitted to various electrical measurement techniques in order to reveal information about shallow levels and deep traps in the forbidden gap. The materials consisted of poly[2-methoxy, 5 ethyl (2' hexyloxy) paraphenylenevinylene] (MEH-PPV), poly(3-methylthiophene) (PMeT), and alpha-sexithienyl (alpha T6) and the employed techniques were IV, CV, admittance spectroscopy, TSC, capacitance and current transients. (C) 1999 Elsevier Science B.V. All rights reserved.
- A microelectrode impedance method to measure interaction of cellsPublication . Gomes, Henrique L.; Leite, Ricardo; Afonso, R.; Stallinga, Peter; Cancela, M. LeonorAn 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 applicationsPublication . 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.
- Electrical study of impurity states in conjugated polymersPublication . Stallinga, Peter; Gomes, Henrique L.; Jones, G. W.; Taylor, D. M.Schottky diodes resulting from an intimate contact of aluminum on electro-deposited poly(3-methylthiopene), PMeT, have been studied by admittance spectroscopy, capacitance-voltage and current-voltage measurements, and optically-induced current transients. The loss-tangents show the existence of interface states that can be removed by vacuum annealing, also visible in the transients. Furthermore, the CV curves don't substantiate the idea of movement of the dopant ions.
- Metal contacts in thin-film transistorsPublication . 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 structurePublication . 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 terrylenePublication . 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.
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