Model Of Charge Carrier Transport Research Articles

Quadrupolar glass as a model for charge carrier transport in nonpolar organic materials

AbstractMonte Carlo simulation of the charge carrier transport in disordered nonpolar organic materials has been carried out. As a suitable model we considered the model of quadrupolar glass. A general formula for the temperature and field dependence of the mobility was suggested. A comparison with experimental data has been carried out.

Electron transport in pristine and alkali metal doped perylene-3,4,9,10-tetracarboxylicdianhydride (PTCDA) thin films

The controlled adjustment of the conductivity of molecular organic semiconductors is essential for their technical application. As the $n$-type organic semiconductor perylene-3,4,9,10-tetracarboxylicdianhydride (PTCDA) is known to be $n$ doped by alkali metals, pristine and sodium doped PTCDA was chosen as a model system to investigate the charge carrier generation and transport in organic semiconductors. Temperature-dependent conductivity measurements were performed to obtain information about the transport mechanism. Measurement of the thermoelectric force, i.e., the Seebeck coefficient, gives a closer insight into the energy levels, participating in charge carrier generation. Based on the performed measurements a model for charge carrier transport in pristine and in alkali metal doped PTCDA is suggested.

Optimal design of semiconductor opening switches for use in the inductive stage of high power pulse generators

Semiconductor opening switches (SOS) are able to interrupt currents at density levels of up to 10 kA/cm2 in less than 10 ns, operate at repetition rates up to 1 kHz, and possess lifetimes of more than 1011 pulses. If stacked, SOS diodes can hold off voltage levels up to several 100 kV. They are therefore ideal for the design of compact high voltage pulse generators of the GW-class for industrial applications. The aim of this work was to improve our understanding of the opening process in a semiconductor diode of SOS-type with a doping profile of p+pnn+ structure, obtainable through diffusion from the surfaces. To simulate the physical processes inside this diode the code POSEOSS was developed. It contains a detailed physical model of charge carrier transport under the influence of density gradients and electric fields and considers all relevant generation and recombination processes. It possesses a large degree of flexibility and is easy to use, and thus allows to carry out parameter studies to determine the influence of different physical quantities, such as doping and impurity levels, on the performance of the device. When applying the code some interesting results concerning the plasma dynamics during the opening process in the switch have been found. In particular, using realistic values for the charge carrier mobility, it was found that the opening process starts first at the n–n+ boundary. Also it has been possible to derive the physical conditions for the occurrence of the SOS-effect. Based on the simulation results a simplified SOS equivalent circuit model has been developed. This model can be used in the circuit simulation program PSPICE. A pulse generator scheme based on inductive storage is proposed, in which power multiplication is achieved by unloading the inductors, previously charged in series, in parallel. This scheme can be considered as the inductive equivalent of a Marx-generator. PSPICE simulations of such a scheme based on semiconductor opening switches are presented. The theoretical results have been compared to measurements obtained with a simple experimental set-up using two 100 kV SOS-switches. The measurements showed good agreement with the simulation results. Further improvements seem possible by adapting the SOS device structure to the specific generator circuit.

Charge effects controlling the current hysteresis and negative differential resistance in periodic nanodimensional structures Si/CaF2

A kinetic model for charge carrier transport in periodic nanodimensional Si/CaF2 structures via localized states in the insulator was suggested. The appearance of the built-in electric charge in the insulator due to the polarization of the charge trapped by localized charge centers and the subsequent discharge of these centers were investigated. It was demonstrated that these phenomena explain the hysteresis of current-voltage (I–V) characteristics with a change of polarity of the applied external voltage. These phenomena bring about the portion of negative differential resistance (NDR) in these characteristics. Major factors ensuring the NDR appearance for the structures under investigation are the charge carrier density at the contacts and the charge voltage. At temperatures below 250 K, the NDR portion disappears. It was demonstrated that, in the course of recording the experimental I–V characteristics, the effect of the charging-discharging of localized centers should decrease. This decrease is in accordance with an increase in the time interval of measuring the current at a constant voltage and with an increase in the step of the applied voltage. This effect actually disappears for the measurement time of 20 s and the voltage step of 0.6 V.

Study of the effect of trap levels on steady-state dark I–V characteristics in Safranine-T-based solid-state thin film photoelectrochemical cell

In this work we describe the steady-state current–voltage (I –V) characteristic of solid-state photoelectrochemical cells (PEC). The cell contains a blend made of Safranine-T dye dispersed in polyvinyl alcohol (PVA), polyethylene oxide (PEO) complexed with ammonium perchlorate (NH4ClO4), ethylene carbonate (EC) and propylene carbonate (PC). A thin film (20 μm) of this blend is sandwiched between two transparent indium-tin-oxide (ITO)-coated glass plates, which act as the two contact electrodes. Addition of ion salt in the active materials alters the device characteristics in a significant way. In the present work we have concentrated on the steady-state dark I –V characteristics of the device. The trap-assisted charge carrier transport model is used to satisfactorily describe the I –V characteristics. The experimental results are explained by the transport of carriers having traps with exponential distribution. The characteristic temperature of the traps and trap energy level have been estimated to be 1164 K and 0.1 eV, respectively, within the experimental limit. The present experimental data will be helpful to understand the steady-state operation of a photoelectrochemical cell.

Charge induced energy fluctuations in thin organic films: effect on charge transport

Effect of static charges on transport properties of disordered organic materials is considered. Long range nature of Coulomb interaction requires to consider a transport layer of the finite thickness. Presence of conducting electrodes significantly modifies properties of organic medium, removes a long range Coulomb divergence, and makes it possible to calculate in finite form statistical properties of organic medium (with nonzero total charge density), relevant for transport characteristics. We present also a generalization of ID model of charge carrier transport to the case of inhomogeneous energetic disorder that realizes for the case of charge-induced disorder.

Modeling the electrooptic evolution in thermally poled germanosilicate fibers

We formulate a simple quantitative three-species charge-carrier transport model, consisting of two distinct positive ions and a single negative ion, to describe the dynamics during thermal poling of a germanosilicate optical fiber. We numerically solved the equations and report one-dimensional space-time solutions for the electrooptic (EO) coefficient. In the two-cation model, our findings show the EO coefficient initially dips near the anode and then monotonically rises to a steady-state value, higher than that produced by the initial applied poling field. However, at the cathode, the electric field quickly dropped to zero where it remained zero for the poling duration. The introduction of a moving negative ion clearly shows the existence of a dead time characteristic appearing at the cathode, resulting in a gain in the initial EO coefficient. This model also reveals that the resulting EO evolution in a thermally poled germanium-boron codoped fiber can he attributed to the movement of just two ions of opposite polarity. To explain the increase in the EO coefficient in boron codoped germanosilicate fiber, we found it necessary to allow for an increase in the third-order susceptibility by a factor of /spl sim/3.4.

Combined optical and electric simulation of metal-semiconductor-metal photodetectors

The combined optical and electric simulation of a metal-semiconductor-metal (MSM) photodetector is presented. The generalised multipole technique (GMT) for rigorous modelling of the light propagation is combined with electric device simulation methods based on a drift-diffusion charge carrier transport model. Different optical designs are compared with respect to the detector performance that can be achieved. It is demonstrated that the use of incoupling gratings can significantly increase the detector efficiency, compared to designs using antireflective layers. Numerical experiments carried out for illumination under varying angles of incidence also show that not only the amount of light that reaches the semiconducting substrate but also the exact spatial intensity distribution have non-negligible effects on the detector efficiency and response times. This in turn proves the necessity of rigorous optical simulation.

Investigation into the effect of Auger recombination on charge carrier transport and static characteristics of silicon multilayer structures

A new model for charge carrier transport through a silicon multilayer structure at high current density is presented. This model accounts for a number of nonlinear physical phenomena (electron-hole scattering, Auger recombination, high doping effects) which become of significance at high current density. The injected charge carrier distribution in the lightly doped layer of the structure at high current density was investigated on the basis of this model and previously predicted phenomenon of injected carrier saturation is confirmed. The dependence of injected carrier limiting density on the electrophysical parameters of the silicon structure was investigated. Within the framework of the suggested model the current-voltage characteristics of a p +- n- n + structure was studied. It is demonstrated that injected carrier saturation phenomenon results in linear current-voltage characteristics at high current densities. A characteristic ratio ( W L ) c (where W is the width of the n-base layer and L is the ambipolar diffusion length of charge carriers in the n-base layer) was found to divide the diode structures into two groups. In the first group with W L <( W L ) c the recombination of injected carriers in the n-base layer at high current density is provided by Auger processes only and therefore the current-voltage characteristics does not depend on lifetime τ, conditioned by the Shockley-Read-Hall recombination processes. The second group of structures with W L >( W L ) c retains a dependence on τ at all current densities. Experimental data presented in the last section of the article confirm the results of device modeling.

The effect of polar additives on charge carrier transport in polysilylenes

The effect of polar additives is introduced into the theoretical model of charge carrier transport by hopping across disordered rigid polymer matrices. The electrostatic charge carrier–dipole interaction causes a broadening of the energy distribution of transport states which results in a decrease in the charge carrier mobility. The mobility depends on the dipole moment, the molecular dimensions of the additive, and the additive concentration. For geometrically similar additives, the mobility decreases with increasing dipole moment. The theory explains well the observed decrease in the mobility of holes in poly(methyl-phenylsilylene) with increasing concentration of 1-(benzamido)-4-nitronaphthalene.

Photoconductive gain and generation-recombination noise in quantum well infrared photodetectors

Photocurrent and excess current noise in a quantum well infrared photodetector are considered using a drift-diffusion model of charge carrier transport. The effect of quantum well recharge under the influence of the nonuniform generated charge carriers is addressed. The recharging effect drastically changes the dependency of both photoconductive gain and excess current noise gain upon detector parameters. We have found that for uniform generation, both gains coincide. For nonuniform generation, noise gain is essentially different from photoconductive gain. This distinction is of the order of 100% for the real device parameters. The existing discrepancy in formulae for photoconductive gain and excess current noise derived in different models, which implicitly assumed drift transport of electrons, is cleared up.

Autoionization of excited Frenkel states in the anthracene crystal

Autoionization of Frenkel excitons is re-examined as a potential source of the anthracene photocurrent at photon energies higher than 4.4 eV. A crude estimate of the rate constant suggests autoionization as a feasible depletion channel, the resultant photocurrent quantum yield being in reasonable agreement with experiment. Bounds for the thermalization radius of charge carriers produced by autoionization are estimated. The estimates, based on a simple model of charge carrier transport and scattering, are found to agree, as to the order of magnitude, with experimental data. A fit of the experimental photocurrent spectrum indicates that while in the near-threshold region the photocurrent is dominated by charge pairs produced by direct optical excitation, at higher energies the autoionization mechanism generally prevails.

Analysis of time-of-flight transit times based on the multiple-trapping model of charge-carrier transport

Abstract Within the context of the multiple-trapping model, we discuss different theoretical ways of defining a transit time in the time-of-flight experiment. A new definition based on the first moment with respect to space of the free-carrier distribution is proposed. At the same time various experimentally used definitions of transit time are identified and the corresponding operational values are extracted from Monte Carlo simulated time-of-flight current traces for different field strengths and temperatures, with either an exponential or a linear density of tail states. A comparison between the theoretical and Monte Carlo results for the different approaches is made. Our calculations show that definitions of transit time that emphasize the fastest of the drifting carriers exhibit a larger electric field dependence and a smaller apparent activation energy than definitions which are based on the totality of excess charge carriers. Consequently we conclude that, if one analyses drift-mobility data in the...