Concentration Of Trapped Carriers Research Articles

Effect of contact phenomena on the electrical conductivity of reduced lithium niobate

Lithium niobate is a ferroelectric material finding a wide range of applications in optical and acoustic engineering. Annealing of lithium niobate crystals in an oxygen-free environment leads to appearance of black coloration and concomitant increasing electrical conductivity due to chemical reduction. There are plenty of literary data on the electrophysical properties of reduced lithium niobate crystals though contact phenomena occurring during electrical conductivity measurement as well as issues of interaction between the electrode material and the test specimens are almost disregarded. The effect of chromium and indium tin oxide electrodes on the results of measurements of electrophysical parameters at room temperature for lithium niobate specimens reduced at 1100 °C has been investigated. It was found that significant nonlinearities in the VACs of the specimens at below 5 V distort the specific resistivity readings for lithium niobate. This requires measurements at higher voltages. Impedance spectroscopy studies have shown that the measurement results are largely affected by capacities including those probably induced near the contacts. It has been shown that the experimental results are described adequately well by a model implying the presence of near-contact capacities that are parallel to the specimen’s own capacity. Possible mechanism of the induction of these capacities has been described and a hypothesis has been proposed of the high density of electron states at the electrode/specimen interface that can trap carriers, the concentration of trapped carriers growing with an increase in annealing duration.

Surface-potential-based drain current model for two-dimensional organic TFTs using the multiple trapping and release conduction theory

A new analytical surface-potential-based drain current model is presented for the organic thin-film transistors (TFTs) when carriers are confined in two dimensions. Following the carrier multiple trapping and release (MTR) conduction theory, i.e., the assumption that the trapped carrier concentration is much higher than the free carrier concentration, the model is developed. The presented model can account for the linear regime and saturation regime by a single formulation. The calculated results of the presented model are verified by the available experimental drain current considering the temperature characteristics. Comparing with the previous model using the variable range hopping and percolation (VRH) conduction theory, although the presented model and the previous model are similar in mathematics, the presented model is more efficient to estimate the density of trap states for the organic TFTs.

Simulation Analysis of Increase in ON-State Voltage of 4H-SiC Bipolar Devices Due to Single-Shockley-Stacking Faults

Mechanism of increase in the ON-state voltage of 4H-SiC PiN diodes due to the expansion of single-Shockley-stacking faults (1SSFs), so-called bipolar degradation, was investigated by performing a technology computer-aided design (TCAD) simulation. Electrical characteristics in 15-kV class PiN diodes with a 1SSF monolayer in a voltage-blocking layer were calculated while modeling the 1SSF as a quantum well (QW) for electron conduction. The simulation results clarified that a potential barrier formed by electrons trapped in the QW does not significantly deteriorate the current conduction under a low applied voltage due to hole current and recombination current paths. However, since the concentration of trapped carriers in the QW becomes extremely high especially when increasing the applied voltage, direct and Auger recombination occurs considerably around the 1SSF. This suppresses the conductivity modulation, and then the contribution of the hole current decreases, leading to significant disturbance of the current flow due to the potential barrier and large voltage drop at the 1SSF. Accordingly, the increase in the ON-state voltage of SiC bipolar devices is attributed to both electron trapping to the QW and enhanced carrier recombination around the 1SSF.

Modeling of Both Arrhenius and Non-Arrhenius Temperature-Dependent Drain Current for Organic Thin-Film Transistors

To describe both Arrhenius and non-Arrhenius temperature-dependent drain currents for the organic thin-film transistors, the effective trapped carrier concentration expression is presented. Based on the expression, following the Shur and Hack's trap limited carrier conduction theory, the analytical drain current model is developed. The temperature-dependent trap states' density is presented to explain the origin of the effective trapped carrier concentration expression.

A Mobility Model Considering Temperature and Contact Resistance in Organic Thin-Film Transistors

Based on the device physics, a mobility model for organic thin-film transistors (OTFTs) is presented considering temperature and contact resistance. As a function of the surface potential, the mobility model including hopping mechanism is able to explain the dependence of temperature and gate bias. The contact resistance is also considered in order to extract the correct mobility. Furthermore, with the assumption that the trapped carrier concentration dominates Poisson’s equation, and combining the mobility model, a DC compact model accounting for contact resistance and temperature is proposed suitable for the temperature scaling from 83 to 295K. Through the extensive comparisons between the model results and the numerical iteration or experimental data, the validity of the mobility and current models is strongly supported.

The Study of the effects of Free Carriers Trap on the Recombination Mechanism through the Localized States

In this study, the effects of free carriers trap on the recombination mechanism through the localized states in gallium antimonide (GaSb) is investigated through the simulation of excess carrier traps corresponding to the non-radiative recombination mechanism. Fist, the trapping effects (the ratio of the concentration of trapped carriers to the corresponding concentration of free carriers) is described as a function localized state energy at room and different temperatures inside the sample. Then, the radiative, Auger and Shockley-Read-Hall recombination mechanisms are compute using the relations describing the lifetimes of the excess majority and minority carriers in the band gap of semiconductor(GaSb). Finally, the temperature dependence of the radiative, Auger and Shockley-Read-Hall excess carrier lifetimes in the localized regions are investigated by considering the variation of localized energy positions. Since trap is indirectly related to the temperature, the description of the trapping effect as functions of localized state energies is found to be the important. The analysis of the results also shows that, the under localized regions are divided in to five based on the interaction of localized state energy with the conduction band energy or the valance band energy and decreases with temperature. The trap in shallow levels (donor and acceptor regions) tends to zero and deep levels are dominated at high temperature. The Auger lifetime is dominated than the other (radiative and Shockley-Read-Hall) lifetimes for the sample semiconductor (GaSb). The temperature affects both majority and minority carriers trap and their lifetimes in the three recombination mechanisms as functions of the localized state energies and the Auger lifetime is dominated in the semiconductor gallium antimonide sample. Keywords: Free Carriers Trap, localized States, Shockley-Read-Hall, Auger band-to-band recombination DOI : 10.7176/JNSR/9-5-03 Publication date :March 31 st 2019

Analytical Drain Current Model for Amorphous InGaZnO Thin-Film Transistors at Different Temperatures Considering Both Deep and Tail Trap States

Surface-potential-based drain current model is presented for amorphous InGaZnO thin-film transistors considering both exponential deep and tail trap states densities in the energy gap. The trap states densities are determined by the numerical calculation on the basis of the assumption that the trapped carrier concentration is much higher than the free carrier concentration. The analytical drain current model is developed consistent with the numerical calculation, and verified by the experimental data at different temperatures.

Obtainment of the density of states in the band tails of hydrogenated amorphous silicon

In this work, we present two new pairs of formulas to obtain a spectroscopy of the density of states (DOS) in each band tail of hydrogenated amorphous silicon (a-Si:H) from photoconductivity-based measurements. The formulas are based on the knowledge of the small-signal recombination lifetime τ′, the characteristic decay time of the concentration of trapped carriers generated in excess by the illumination, and that can be measured by methods like the Oscillating Photocarrier Grating (OPG) or Moving Grating Technique (MGT). First, we deduce the formulas and test their accuracy by numerical simulations using typical a-Si:H parameters. Next, we characterize an a-Si:H sample using well-known methods, like Fourier transform photocurrent spectroscopy to evaluate the valence band tail and modulated photoconductivity to measure the conduction band tail. We also performed measurements of steady-state photoconductivity, steady-state photocurrent grating and MGT, for a range of generation rates. From these measurements—and taking typical values for the capture coefficients, the extended states mobilities and the DOS at the band edges—we apply the new formulas to get the band tails. We find that the results obtained from the application of our formulas are in good agreement with those found with the traditional methods for both band tails. Moreover, we show that MGT/OPG measurement to get τ′ can be avoided if one of the band tails is measured by one of the traditional methods, since the known band tail can be used to evaluate τ′ with one pair of equations, and then the other pair can be applied to get the other band tail.

Thermoluminescence and Mechanoluminescence Properties of UV- Irradiated Ca2Al2SiO7:Ce3+, Tb3+ Phosphor

Rare earth activated, white light emitting Ca2Al2SiO7: Ce3+, Tb3+ phosphor was synthesized by combustion method at initiating temperature of 600 0C, using urea as a reducer. X – Ray diffraction, absorption spectra, thermoluminescence(TL) and mechanoluminescence (ML) were studied. The particle size was found to be 50.85nm. The absorption spectra shows that the absorption edge is at λ = 240nm. Two peaks were observed in the plot of time vs ML intensity. ML intensity increases with increasing droping height of the piston. The TL intensity was increased with increasing UV exposure time indicating the increase in concentration of trapped carriers with UV exposure. The TL emission spectra was found at 400nm (UV-violet band) and 540nm (green band).

Investigation of the trap state of Sr2MgSi2O7: Eu2+, Dy3+ phosphor and decay process

The thermoluminescence glow curves of Sr 2 MgSi 2 O 7 : Eu 2+ , Dy 3+ phosphor were measured after various delay times. A single trap center is confirmed that conforms to a kinetics model with order greater than 1, leading to a suppression of TL intensity and a high temperature shift of the TL peak with longer delay times. A constant trap depth supports this phenomenon. Further, the decay curve of the afterglow and the change in initial trapped carrier concentration can be fitted using general-order kinetics and the fitting results show that the afterglow is close to a second-order kinetics process, which implies that most of the released carriers are retrapped.

An insight into the localization of charge carriers in conducting polyaniline by analyzing thermally stimulated depolarization signals

A dielectric relaxation mechanism, which is attributed to the localized motion of trapped polarons, has been recorded in conducting polyaniline by employing the thermally stimulated depolarization current technique. The signal was analyzed within the frame of the normal distribution in the activation energy value. The experimental dielectric relaxation results were manipulated in order to evaluate the length of the jump distance that the trapped polarons transfer along and the concentration of trap centers. The concentration of trapped carriers is calculated from two different viewpoints: the pair approximation that assumes phonon-assisted tunneling through the barrier separating two adjacent sites and the pinning model that considers the trapped polaron oscillating around its pinning point. Both models provide compatible results.

Electrical properties of nearly stoichiometric GaAs grown by molecular beam epitaxy at low temperature

Nearly stoichiometeric GaAs epilayers doped with Si or Be were grown by molecular beam epitaxy at a low temperature with closely controlled fluxes near the stoichiometric conditions. Excess As point defects in the epilayers which acted as trap sites of free carriers were studied by means of the Hall effect measurements. Under the same As/Ga flux condition, concentrations of excess As point defects which trap free carriers increase with increase of concentrations of Si or Be. With the same concentration of the dopants, on the other hand, concentrations of excess As point defects increase linearly with the As/Ga flux ratio. The dependence of carrier mobility on the trapped carrier concentrations indicates that free carriers are trapped by neutral point defects and change them into ionized ones. In the epilayers which are very close to the stoichiometric composition, Si exhibits its amphoteric nature, occupying both donor and acceptor sites and results in the self-compensation.

Theoretical background of the optical charging spectroscopy method used for investigation of trapping levels

An analytical formula for the optical charging spectroscopy (OCS) discharging current in the case of homogeneous semiconductors is deduced. The computation shows that the OCS current is proportional with the square of the initially trapped carrier concentrations. This result leads to a higher sensitivity in case of the OCS method compared with the thermally stimulated current method. The OCS current formula obtained for one trapping level is generalized in the case of many trapping levels. From the generalized formula it can be seen that the OCS current can change the sign if both electron and hole traps exist in the sample.

Internal and external characteristics in transient forward bias simulations of a-Si:H devices

A numerical program for transient modeling of multilayer a-Si:H devices is described. Using a computer simulation, transient currents in a-Si:H PIN and NIN devices using different magnitudes of step voltage excitations were studied. The results of calculations show first a rapid drop in electric current, followed by an intermediate interval of current rise in both devices. During longer times after step excitation, the transient current in a NIN device decays monotonically towards the new steady-state value, while the current in a PIN device shows a sudden rise to its final value. Such behaviour is explained on the basis of calculated results for different internal quantities such as free and trapped carrier concentrations, space charge density, electric field, potential, net recombination rates and others. It is shown that initially the current in both devices is space-charge limited. The presence of both types of carrier in a PIN diode enables space-charge neutralization at the end of the transient, causing the final abrupt current increase and transition to recombination-limited current flow. The influence of varying the density of localized states (DOS) on simulated transient currents is analyzed and discussed.

Bulk and contact effects in p+-SI-n+ semi-insulating GaAs structures

Current–voltage characteristics, free- and trapped-carrier concentrations, electric field, and excess carrier lifetime contours are given for a p+-SI-n+ GaAs structure. This analysis is done by numerical simulation of the two carrier drift-diffusion model of conduction. The dependence of the deep trap parameters and of the semi-insulating (SI) thickness on the J-V characteristic is presented. Particular emphasis will be put on the study of the equivalent resistivity and electric-field overshoots inside the structure. It is shown that, except for very thick samples, contact and near contact effects on the low-voltage electrical conduction mechanism are all important. It is then necessary to take into account these effects for a precise and exact interpretation of transport in a p+-SI-n+ structure. When the deep trap density Nt is high (Nt≫ni), as is the case for compensated (SI) GaAs, the equivalent low current resistivity ρ is higher than the equilibrium (unperturbed bulk) resistivity ρe. The nature, electron trap or hole trap, of the deep level affects the equivalent resistivity at intermediate applied voltage. When the density of deep traps is low, ρ decreases from values higher than ρe to values lower than ρe as the sample thickness increases. Only in the limit of very thick samples, and then regardless of the trap concentration, does the equivalent resistivity tend asymptotically towards its equilibrium value.

Transport and recombination in a-Si:H p-i-n diodes under forward bias conditions

The transport and recombination of injected electrons and holes in a-Si:H p-i-n-solar cells are studied by simulataneous detection of electroluminescence and forward current as a function of temperature. Time resolved electroluminescence and forward current measurements show that at room temperature the forward current is first determined by the space charge limited current of the electrons subsequently followed by the much larger recombination current. Radiative and non-radiative lifetimes of the injected carriers are examined. At lower temperatures the increasing concentration of trapped carriers dominates the transient response of the device.

The probability to avoid optical breakdown at 6943 Å in NaCl

Computer-simulated ’’one-shot-on-one-site’’ damage events in NaCl at the ruby wavelength are illustrated as deterministic survival curves. Multiphoton-assisted avalanche breakdown is considered for varying laser peak flux densities. For a fixed laser output power level, the effects are calculated for various initial free and trapped carrier concentrations in the presence and absence of multiphoton absorption. The results are in reasonable agreement with recent measurements obtained with LiF and Ar and N2 at intermediate pressures. Experimental observations in NaCl are not presently available.

II. Correlations Between Thermoluminescence and Thermally Stimulated Conductivity

Correlations between corresponding thermoluminescence (TL) and thermally stimulated conductivity (TSC) glow curves are investigated in detail for a single trap depth in the presence of thermally disconnected traps (reservoir for trapped charge carriers) and a single type of recombination centers. As a result of our calculations, we conclude that both types of glow curves have identical shapes if the concentration of thermally disconnected traps is much greater than the concentration of thermally connected traps. We find pronounced differences if this inequality is not satisfied, and conclude (a) the customary practice of analyzing TSC data with the aid of methods developed for TL is inadequate if those methods are based solely on the shape of the glow peak; (b) simultaneous measurements of TL and TSC data by themselves do not provide sufficient information on all the relevant parameters entering the kinetic equations derived from the simple basic model; and (c) from correlated TL and TSC experiments, it is possible to decide (1) whether there is a charge carrier transport associated with the thermal release of trapped charged carriers and their subsequent recombination, and (2) whether there is a large concentration of thermally disconnected traps present. The validity of the customary assumptions, that the concentration of free charge carriers is much smaller than the concentration of trapped carriers, is discussed. In addition, the Dussel-Bube theory of TSC is completed by extending it to the case of low retrapping and small concentration of thermally disconnected traps.