Time-dependent Charge Distribution Research Articles

Transient performance analysis of graphene FET gated via ionic solid by numerical simulations based on tight-binding method and Nernst–Planck–Poisson equations

We investigate the electrical transport characteristics of graphene channel field-effect transistors (FETs) gated via ionic solid (IS), where the conventional gate insulator, such as SiO2, has been replaced by solid electrolytes, such as LiP3O4. In this study, we focus on (i) the gate controllability of the current in comparison to conventional graphene FETs with SiO2 as an insulating material and (ii) the transient characteristics of the drain current and time required to switch on the current. We employ the tight-binding formalism and Boltzmann equation to calculate the electronic band structure and the electronic transport in graphene, while the Nernst–Planck–Poisson equations have been employed to calculate the time-dependent charge distribution in solid electrolytes and the resulting electric double layer formation at the graphene/IS and IS/gate interfaces. Our simulations have shown that graphene FET gated via IS shows superior gate controllability more than SiO2-gated graphene FET with the insulator thickness of 1 nm, and the saturated drain current is insensitive to the IS thickness. Moreover, the time required to switch on the drain current is proportional to the thickness of IS, and the limited number of Li+ ion vacancies in IS is preferable in obtaining faster switching than the case of unlimited vacancy cases while keeping the superior gate controllability.

Empirical Conductivity Equation for the Simulation of the Stationary Space Charge Distribution in Polymeric HVDC Cable Insulations

Many processes are involved in the accumulation of space charges within the insulation materials of high voltage direct current (HVDC) cables, e.g., the local electric field, a conductivity gradient inside the insulation, and the injection of charges at both electrodes. An accurate description of the time dependent charge distribution needs to include these effects. Furthermore, using an explicit Euler method for the time integration of a suitably formulated transient model, low time steps are used to resolve fast charge dynamics and to satisfy the Courant–Friedrichs–Lewy (CFL) stability condition. The long lifetime of power cables makes the use of a final stationary charge distribution necessary to assess the reliability of the cable insulations. For an accurate description of the stationary space charge and electric field distribution, an empirical conductivity equation is developed. The bulk conductivity, found in literature, is extended with two sigmoid functions to represent a conductivity gradient near the electrodes. With this extended conductivity equation, accumulated bulk space charges and hetero charges are simulated. New introduced constants to specify the sigmoid functions are determined by space charge measurements, taken from the literature. The measurements indicate accumulated hetero charges in about one quarter of the insulation thickness in the vicinity of both electrodes. The simulation results conform well to published measurements and show an improvement to previously published models, i.e., the developed model shows a good approximation to simulate the stationary bulk and hetero charge distribution.

Time-dependent charge distributions in polymer films under electron beam irradiation

The time-dependent charge distribution in polymer film under electron beam irradiation is studied by both experiment and numerical simulation. In the experiment, the distribution is measured with the piezoinduced pressure wave propagation method. In the simulation, the initial charge distribution is obtained by the Monte Carlo method of electron scattering, and the charge drift in the specimen is simulated by taking into account the Poisson equation, the charge continuity equation, Ohm’s law, and the radiation-induced conductivity. The results obtained show that the negative charge deposited in the polymer film, whose top and bottom surfaces are grounded, drifts toward both grounded electrodes and that twin peaks appear in the charge distribution. The radiation-induced conductivity plays an important role in determining the charge distribution in the polymer films under electron beam irradiation.

Dynamic admittance of carbon nanotube-based molecular electronic devices and theirequivalent electric circuit

We use first-principles quantum mechanics to simulate the transient electrical responsethrough carbon nanotube-based conductors under time-dependent bias voltages. Thedynamic admittance and time-dependent charge distribution are reported and analyzed.We find that the electrical response of these two-terminal molecular devices canbe mapped onto an equivalent classical electric circuit and that the switchingtime of these end-on carbon nanotube devices is only a few femtoseconds. Thisresult is confirmed by studying the electric response of a simple two-site modeldevice and is thus generalized to other two-terminal molecular electronic devices.

Analysis of internal charge distribution in electron irradiated polyethylene and polyimide films using a new experimental method

The charge distribution in low density pyethylene and polyimide films under a quasi-mono-energetic electron beam irradiation in the range [5–400 keV] is analysed using a new experimental set-up based on the pulsed electro-acoustic (PEA) method. The device allows measurements without any physical contact between the excitation electrode used in the PEA method and the irradiated surface which remains at a floating potential during the entire duration of the experiment. This special configuration makes it possible to study bulk as well as interface phenomena. In particular, emission of charges from the irradiated surface to the vacuum is in evidence as well as charge injection from the ground electrode to the bulk. The time-dependent charge distribution during irradiation is approached by combining a Monte Carlo toolkit for modelling radiation transport in matter in conjunction with a macroscopic description of charge transport based on the concept of a radiation-induced conductivity generated by the injected electrons. Although the macroscopic description yields a fair agreement with the experimental data, it fails to depict the detailed shape of the charge distribution, thereby calling for a microscopic, bi-polar model.

Estimation of Charge Mobility and Charge Density in Polyimide Film by Charge Chromatography

To investigate the electrical conduction properties in polyimide affected by moisture absorption, charge mobility was measured by “charge chromatography”. A time-dependent charge distribution was monitored by an electroacoustc method when a pulse voltage was superposed on a dc bias voltage. In this newly proposed technique, the generation and traveling of charges were visualized. Charge density and charge mobility were individually determined from the measurement results of both charge chromatography and conductivity measurement. The conductivity of a moisturized specimen was 125-hold higher than that of a dry sample. Although both charge mobility and charge density increased by moisturizing the sample, it was concluded that an increase in charge density plays a dominant role in an increase in conductivity. It is considered that the dissociation of impurities in polyimide by moisture absorption leads to the increase in mobile charges.

Infrared catastrophe and tunneling into strongly correlated electron systems: Beyond the x-ray edge limit

We develop a nonperturbative method to calculate the electron propagator in low-dimensional and strongly correlated electron systems. The method builds on our earlier work using a Hubbard-Stratonovich transformation to map the tunneling problem to the x-ray edge problem, which accounts for the infrared catastrophe caused by the sudden introduction of a new electron into a conductor during a tunneling event. Here we use a cumulant expansion to include fluctuations about this x-ray edge limit. We find that the dominant effect of electron-electron interaction at low energies is to correct the noninteracting Green's function by a factor ${e}^{\ensuremath{-}S∕\ensuremath{\hbar}}$, where $S$ can be interpreted as the Euclidean action for a density field describing the time-dependent charge distribution of the newly added electron. Initially localized, this charge distribution spreads in time as the electron is accommodated by the host conductor, and during this relaxation process action is accumulated according to classical electrostatics with a screened interaction. The theory applies to lattice or continuum models of any dimensionality, with or without translational invariance. In one dimension the method correctly predicts a power-law density of states for electrons with short-range interaction and no disorder, and when applied to the solvable Tomonaga-Luttinger model, the exact density of states is obtained.

Carrier mobility in ethylene-vinylacetate copolymer estimated by transient space charge

Carrier mobility in an ethylene vinyl acetate (EVA) copolymer film was measured by a new technique. The time-dependent charge distribution was monitored by the pulsed-electroacoustic method when a pulse voltage was superposed on a dc bias voltage. A positive charge packet was injected from the anode soon after the pulse application, and moved towards the cathode. It was suggested from the movement of the charge packet that the mobility of the positive carrier at 10 to 60 MV/m was in the order of 10/sup -13/ m/sup 2//Vs at room temperature. When the pulse voltage was very high, a charge packet appeared within the bulk, suggesting that carrier dissociation takes place.

Contactless evaluation of semi-insulating GaAs wafer resistivity using the time-dependent charge measurement

A method based on time-dependent measurement of charge transients has been developed to evaluate the specific resistivity of semi-insulating wafers quickly, non-destructively and with good lateral resolution. The material is inserted between capacitive electrodes. The time-dependent charge distribution after application of a voltage step allows evaluation of the resistivity with high accuracy in the range 106-109 Omega cm. The technique has been elaborated to allow rapid contactless scanning of wafers for routine measurement of the lateral variation of resistivity with a resolution of about 2 mm2. The results are in agreement with conventional Hall measurements. The mechanical and electronic systems are described in detail. Scans across wafers cut from as-grown as well as annealed ingots are presented.

Calculations of bipolar charging of aerosols

The acquisition of electric charge by the particles of an aerosol is a subject which heretofore has received only incomplete treatment, much of it based upon intuitive considerations. In this paper we present a rigorous treatment of the subject based upon the difference equations for charging of monodisperse and polydisperse aerosols in the presence of ions of both signs but unequal mobilities. The stochastic equations are written and the conditions of their solution are described for positive and negative small ions of mobilities 1.4 cm 2/V-sec and 1.9 cm 2/V-sec, respectively. Numerical calculations are presented of the time-dependent charge distributions and small ion number densities. Their relation to small-ion production, aerosol particle number concentration, and polydispersity is discussed. These results are compared where possible with calculated and experimental results of other workers. These calculations demonstrate for the first time that for bipolar charging a significant difference arises in polarity between the larger and smaller particle components of a polydisperse aerosol owing to unequal charging rates.