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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.
- Electrical AC behaviour of MPCVD diamond Schottky diodesPublication . Pereira, L.; Rodrigues, A.; Gomes, Henrique L.; Pereira, E.The present work reports some experimental results on the electrical AC behaviour of metal-undoped diamond Schottky diodes fabricated with a free-standing MPCVD diamond film (5 mum thick). The metals are gold for the ohmic contact and aluminium for the rectifier. The capacitance and loss tangent vs, frequency shows that capacitance presents a relaxation maximum at frequencies near 10 kHz at room temperature. Although the simple model (small equivalent circuit) can justify the values for the relaxation, it cannot justify the departure from the Debye model, also verified in the Cole-Cole plot. Taking into account the existence of traps in the depletion region, a best fit to the experimental results was obtained. The difference between the Fermi level and the band edge of 0.2-0.3 eV is in agreement with the activation energy found from the loss tangent analysis. The capacitance with applied voltage (Mott-Schottky plots) gives a defect density of 10(16) cm(-3) with contact potentials near 0.5 V and the profile of defect density obtained shows a major density (approx. 10(17) cm(-3)) in a layer with a thickness less than 50 nm from the junction, decreasing by one order of magnitude with increasing distance. Finally a structural model is proposed to explain the AC behaviour found. (C) 2001 Elsevier Science B.V. All rights reserved.
- Magnetic and transport properties of diluted granular multilayersPublication . Silva, Hugo G.; Gomes, Henrique L.; Pogorelov, Y. G.; Pereira, L. M. C.; Kakazei, G. N.; Sousa, J. B.; Araújo, J. P.; Mariano, José F. M. L.; Cardoso, S.; Freitas, P. P.The magnetic and transport properties of Co80Fe20t /Al2O34 nm multilayers with low nominal thicknesses t=0.7 and 0.9 nm of Co80Fe20 granular layers are studied. Magnetic studies find a superparamagnetic state above the blocking temperature Tb of field-cooled/zero-field-cooled splitting that grows with t and decreases with H. The low-voltage Ohmic tunnel transport passes to non-Ohmic IV3/2 law for applied fields above 500 V/cm. At fixed V, the temperature dependence of conductance reveals an anomalous dip around 220 K, which can be attributed to the effect of surface contamination by supercooled water. Current-in-plane tunnel magnetoresistance MR ratio tends, at lower t, to higher maximum values 8% at room temperature but to lower field sensitivity. This may indicate growing discorrelation effect e.g., between shrinking areas of correlated moments in this regime and corroborates the deficit of granule magnetization estimated from the Inoue–Maekawa MR fit, compared to that from direct magnetization measurements. MR displays a mean-field-like critical behavior when t approaches the point of superparamagnetic/ superferromagnetic transition tc1.3 nm at room temperature from below, different from the formerly reported percolationlike behavior at approaching it from above.With growing temperature, MR reveals, beyond the common decrease, an anomalous plateau from Tb30–50 K up to some higher value T150–200 K, not seen at higher t.
- 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.
- Detection of chloroform with a sensor array consisting of electrochemically deposited polythiophenes films: Processes governing the electrical responsePublication . Carvalho, E. R.; Correa, A. A.; Consolin Filho, N.; Oliveira, O. N.; Gomes, Henrique L.; Mattoso, L. H. C.; Neto, L. MartinA sensor array made up with electrodeposited polythiophene films onto interdigitated gold electrodes has been used to detect small concentrations of chloroform in water, down to 0.1 mg L-1. An analysis of the impedance data revealed two relaxation processes, one attributed to the polymer coating and associated with the double layer and the other ascribed to the electrolyte. Because these processes depended on the type of polythiophene derivative and on doping, the sensitivity of the electrodes varied. The most sensitive to chloroform was the poly(methyl thiophene) oxidized at 5 mC. Also, doped layers were more sensitive, from which one may infer that a plausible mechanism for detection is the removal of dopant ions by the solvent. Surprisingly, a bare gold metal electrode was more sensitive than some electropolymerized samples, probably because the relaxation processes in the latter were not strongly affected by chloroform.
- 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.
- Dynamics of threshold voltage shifts in organic and amorphous silicon field-effect transistorsPublication . Mathijssen, S. G. J.; Colle, M.; Gomes, Henrique L.; Smits, E. C. P.; de Boer, B.; McCulloch, I.; Bobbert, P. A.; de Leeuw, D. M.The electrical instability of organic field-effect transistors is investigated. We observe that the threshold-voltage shift (see figure) shows a stretched-exponential time dependence under an applied gate bias. The activation energy of 0.6 eV is common for our and all other organic transistors reported so far. The constant activation energy supports charge trapping by residual water as the common origin.
- Voltage- and light-induced hysteresis effects at the high-k dielectric- poly(3-hexylthiophene) interfacePublication . Lancaster, J.; Taylor, D. M.; Sayers, P.; Gomes, Henrique L.Capacitance-voltage (C-V) measurements have been undertaken on metal-insulator-semiconductor capacitors formed from atomic-layer-deposited films of aluminium titanium oxide as the insulator and poly(3-hexylthiophene) as the insulator. Upon cycling from -30 to +30 V in the dark, the C-V plots show large, temperature-dependent, reversible shifts in the flatband voltage to more negative voltages consistent with reversible, shallow hole trapping at or near the insulator-semiconductor interface. When illuminated with photons of energy exceeding the polymer band gap, even larger shifts to positive voltages are observed accompanied by inversion layer formation. This latter effect has potential applications in optical sensing. (c) 2007 American Institute of Physics.
- 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.
- Polycrystalline diamond thin film as wide bandgap material: the optoelectronic behaviour and the relationship with the structurePublication . Pereira, L.; Pereira, E.; Rodrigues, A. M.; Gomes, Henrique L.In this work metal - Microwave Plasma CVD diamond Schottky devices were studied. The current density vs. applied voltage reveals rectification ratios up to 10(4) at \ +/- 2V \. Under illumination an inversion and increase of the rectification is observed. The carrier density is 10(15) cm(-3) and the ideality factors near 1.5. The dark current vs. temperature shows that below 150 K the bulk transport is controlled by a hopping process with a density of defects of 10(16) cm(-3). For higher temperatures an extrinsic ionisation with activation energy of 0.3 eV takes place. The correlation with the polycrystalline nature of the samples is focused.