Electron Generation Rate Research Articles

Partial discharge within a spherical cavity in a dielectric material as a function of cavity size and material temperature

For high-voltage components, the measurement of partial discharge (PD) is a useful tool for performance assessment of electrical insulation. In this study, experimental measurements of PD activity for different spherical cavity sizes and material temperatures have been performed. A simulation model representing PD behaviour within spherical cavities in homogeneous dielectric materials has also been developed. The model has been used to study the influence of cavity size and material temperature on PD activity. Comparison of measurement and simulation results has been undertaken. The model uses a finite element analysis (FEA) method along with MATLAB code. It has been found that certain parameters in the model are both cavity size and temperature dependent. Thus, critical parameters influencing PD behaviour for different cavity sizes within the material and material temperatures can be identified; these are the charge decay time constant, cavity surface conductivity, electron generation rate (EGR), PD inception and extinction fields and the cavity temperature decay time constant.

Effect of ion bombardment on the a-Si:H based surface passivation of c-Si surfaces

We have found that controlled Ar ion bombardment enhances the degradation of a-Si:H based surface passivation of c-Si surfaces. The decrease in the level of surface passivation is found to be independent on the ion kinetic energy (7–70 eV), but linearly proportional to the ion flux (6×1014–6×1015 ions cm−2 s−1). This result suggests that the ion flux determines the generation rate of electron–hole pairs in a-Si:H films, by which metastable defects are created at the H/a-Si:c-Si interface. Possible mechanisms for the ion induced generation of electron–hole pairs are discussed.

Partial discharge behavior within a spherical cavity in a solid dielectric material as a function of frequency and amplitude of the applied voltage

Modeling of the partial discharge (PD) process allows a better understanding of the phenomena. In this paper, a simulation model for spherical cavities within a homogeneous dielectric material has been developed. The model is implemented using Finite Element Analysis (FEA) software in parallel with a mathematical package. This method provides many advantages over previous PD models because discharge events can be simulated dynamically and the electric field in the cavity can be calculated numerically. The model has been used to study the effect of different amplitudes and frequencies of the applied voltage and simulation results have been compared with experimental measurement results. It is found that certain model parameters are dependent on the applied stress and parameters that clearly affect PD activity can be readily identified, these parameters include; the electron detrapping time constant, the cavity surface conductivity, the initial electron generation rate and the extinction voltage. The influence of surface charge decay through conduction along the cavity wall on PD activity has also been studied.

Investigation of the delay time distribution of high power microwave surface flashover

Characterizing and modeling the statistics associated with the initiation of gas breakdown has proven to be difficult due to a variety of rather unexplored phenomena involved. Experimental conditions for high power microwave window breakdown for pressures on the order of 100 to several 100 torr are complex: there are little to no naturally occurring free electrons in the breakdown region. The initial electron generation rate, from an external source, for example, is time dependent and so is the charge carrier amplification in the increasing radio frequency (RF) field amplitude with a rise time of 50 ns, which can be on the same order as the breakdown delay time. The probability of reaching a critical electron density within a given time period is composed of the statistical waiting time for the appearance of initiating electrons in the high-field region and the build-up of an avalanche with an inherent statistical distribution of the electron number. High power microwave breakdown and its delay time is of critical importance, since it limits the transmission through necessary windows, especially for high power, high altitude, low pressure applications. The delay time distribution of pulsed high power microwave surface flashover has been examined for nitrogen and argon as test gases for pressures ranging from 60 to 400 torr, with and without external UV illumination. A model has been developed for predicting the discharge delay time for these conditions. The results provide indications that field induced electron generation, other than standard field emission, plays a dominant role, which might be valid for other gas discharge types as well.

Large Integrated Absorption Enhancement in Plasmonic Solar Cells by Combining Metallic Gratings and Antireflection Coatings

We describe an ultrathin solar cell architecture that combines the benefits of both plasmonic photovoltaics and traditional antireflection coatings. Spatially resolved electron generation rates are used to determine the total integrated current improvement under AM1.5G solar illumination, which can reach a factor of 1.8. The frequency-dependent absorption is found to strongly correlate with the occupation of optical modes within the structure, and the improved absorption is mainly attributed to improved coupling to guided modes rather than localized resonant modes.

Short-circuit current density and spectral response modelling of bulk-heterojunction solar cells

A general model to calculate short circuit current and spectral response of the bulk-heterojunction solar cell is discussed here. In order to get the short circuit current density at each wavelength the generation rate of electron and holes are calculated by the optical modelling based on transfer matrix approach. The drift–diffusion equation is then solved to get the current density at each wavelength using the calculated generation rate. Spectral response is found to be very similar to the experimental data of (3-hexylthiophene) (P3HT): [6,6]-phenyl C61-butyric acid methylester (PCBM) based bulk-heterojunction solar cell. The variation of short circuit current as a function of active layer thickness has been calculated using the model which matches with the experimental observations taken from literature. The sub-linear dependence of short circuit current on incident light intensity has also been explained by the model.

Diffusion-Controlled Glow Discharge Pressure Extended by the Multi-Dual Microhollow Cathode Configuration for Flat Lighting

We investigated pure Xe discharge primarily as a source of vacuum ultraviolet for discharge lamps, using a multi-dual hollow cathode. One of the problems associated with the discharge properties of Xe gas is its easy constriction. We prevented the problem by using an electrode configured as a multi-dual microhollow cathode. This result originated from auxiliary charge particles generated in a lamp discharge space and less variation of electron generation rate, which reduced thermal instability and as a result sequentially suppressed the constriction of Xe discharge. Furthermore, we observed that a six-channel flat-panel lamp could be driven with a pulse power supply.

Gas-Insulated Self-Breakdown Spark Gaps: Aspects on Low-Scattering and Long-Lifetime Switching

The influence of the electric field distribution between the electrodes and the seed electron generation rate on the scattering of the breakdown voltage of SF6-insulated spark gaps was investigated. The breakdown voltage scattering considerably can be reduced by applying large-gap-volume, uniform-field electrode profiles instead of spherical shaped electrodes. Moreover, uniform field electrode profiles exhibit an uniform discharge probability in the entire gap volume and following an almost uniform erosion of electrode material along the electrode’s surface. This preserves electrode shape and switching performance of the spark gap for a long maintenance-free lifetime. Breakdown voltage scattering further can be reduced by increasing the seed electron generation in the gap by an auxiliary corona discharge adjacent to the main gap. The experimental observations are discussed on the basis of the volume time law for discharge initiation.

Mechanoluminescence induced by elastic deformation of coloured alkali halide crystals using pressure steps

During the elastic deformation of coloured alkali halide crystals, the bending segments of dislocations capture F-centre electrons lying in the expansion region of edge dislocations, to the states of dislocation band. After the separation from interacting F-centres, the captured electrons move together with the bending segments of dislocations and also drift along the axis of dislocations and subsequently the radiative electron–hole recombinations, owing to both the processes of captured-electron movement, give rise to the light emission. The generation rate of electrons in the dislocation band and the mechanoluminescence (ML) intensity initially increase with time, attain maximum value at a particular time, and then they decrease with time. The intensity I m corresponding to the peak of ML intensity versus time curve and the total intensity I T of ML increase with the applied pressure and also with the density of F-centres in the crystals. At low temperature, both I m and I T increase with temperature and at higher temperature they decrease with increasing temperature due to the thermal bleaching of F-centres and also due to the decrease in luminescence efficiency. Thus, both I m and I T are optimum for a particular temperature of the crystals. For longer time duration, the ML intensity decreases exponentially with time in which the decay time is equal to the lifetime of interacting F-centres. Expressions derived for the different characteristics of ML are able to explain the experimental results. It is shown that the time constant for rise of pressure, lifetime of the interacting F-centres or damping time of dislocation segments, and the activation energy can be determined from the ML measurements.

Source of runaway electrons in a thundercloud field caused by cosmic radiation

The volumetric generation rate of secondary electrons, produced by cosmic radiation in the Earth’s atmosphere and able to accelerate in a thundercloud electric field, has been calculated as a function of height above sea level. It is recommended to use the obtained function as a source when modeling atmospheric breakdown in thunderstorm fields with the participation of relativistic runaway electron avalanches. It has been indicated that ionization of the atmosphere by a cosmic particle with an energy of 1016 eV cannot initiate lightning.

Improvement of solar cell efficiencies by impurity transitions

Abstract Transitions via impurities, in addition to transitions from the valence band to the conduction band, increase the generation rate of electron–hole pairs. By the presence of impurities, however, the recombination rate is also increased. In this study, the recombination process is assumed to be entirely radiative. The probability for photon absorption in generation and for photon emission in recombination depend on the occupation probability of the impurity states. The variation of the occupation resulting in a variation of the optical properties along a current–voltage characteristic is taken into account. With a single impurity state a maximal efficiency of 46% for the AM0 spectrum is found for a band gap of 2.3 eV and an impurity level at 0.88 eV. No improvement is found for band gaps less than 1.2 eV. For two different impurity levels, transitions between these levels must be excluded, otherwise the lower level is mostly occupied and the higher level is mostly empty leading to poor optical absorption for some of the possible impurity transitions.

Study of surface recombination velocity of Cd1−xZnxTe radiation detectors by direct current photoconductivity

The surface recombination velocity of a Cd1−xZnxTe (CZT) radiation detector treated by mechanical polishing and by a standard 5% bromine in methanol chemical etch is reported. The light power dependence of the surface recombination velocity was measured using dc photoconductivity. The results reveal that the surface recombination velocity is a function of the electron generation rate, which can be described by a Shockley–Read one-center model. It was observed that the surface recombination velocity of the CZT detector treated by polishing only is much larger than that treated with polishing followed by chemical etching. The correlation of dc photoconductivity and low-temperature photoluminescence measurements of the CZT detector is also discussed.

A comprehensive study of inversion current in MOS tunneling diodes

The gate current of MOS tunneling diodes biased at inversion region with different substrate doping is investigated. For p-type substrate (1-5 /spl Omega/-cm) devices, the tunneling diode works in the deep depletion region and the inversion current is dominated by the thermal generation rate of minority electrons via traps at Si/SiO/sub 2/ interface and in the deep depletion region. The activation energy is approximately equal to half of the silicon bandgap independent of gate voltage. For devices on p/sup +/ substrate (0.01-0.05 /spl Omega/-cm), the band-to-traps tunneling and band-to-band tunneling are the dominating current components at inversion bias, and reveal a strong field dependence and a weak temperature dependence. The band-to-traps and band-to-band current components are even more significant in the devices on the p/sup ++/ substrate (0.001-0.0025 /spl Omega/-cm). Finally, the effects of temperature and light illumination on inversion current of MOS tunneling diodes will be also discussed.

Effect of the duration of the front edge of the voltage pulse on the electric breakdown of ammonium perchlorate

The results of a study of the effect of the front edge duration of a voltage pulse (τ) on the electric breakdown of ammonium perchlorate single crystals are submitted. Experiments have shown that at τ 1.5 µs. It is shown that the electric breakdown of ammonium perchlorate results from an impact generation of electrons. The impact generation rate of electrons versus electric field and the diameter of the through channel being formed during electric breakdown in ammonium perchlorate are estimated.

Role of the charge state of deep vanadium impurities and associations of defects in photoelectric and optical properties of semi-insulating CdTe crystals

Photoexcited carrier and space charge field subnanosecond dynamics has been investigated in bulk vanadium-doped and Cl- (or As-) co-doped CdTe crystals by using a degenerate four-wave mixing technique. Time-resolved measurements of picosecond grating decay at various illumination intensities revealed peculiarities of defect transformation with doping or under illumination. Novel features in carrier generation and dynamics were observed for the first time and allowed parameters of the defects to be extracted. A fast electron capture and its saturation with increasing intensity of illumination were observed in CdTe:V and attributed to a Cd divacancy, which is known as a deep double acceptor. Numerical modelling by using the two-deep-trap model allowed us to extract the concentration and recombination activity of Cd divacancies. An enhanced electron generation rate by factor of 3-4 was found in Cl-co-doped CdTe:V crystals in spite of a decreased density of donor vanadium states after the co-doping. We attribute this additional channel of electron generation to photoexcitation of the DX centre, which activation energy of 1-1.2 eV is close to a quantum energy of the YAG:Nd laser used ( eV).

Perturbation of atmospheric conductivity as a cause of the seismoionospheric interaction

Two possible channels of an influence of atmospheric current on the ionospheric D-layer has been considered: 1) galvanic mechanism due to current flowing through nonhomogeneous media and 2) Joule heating of the low ionosphere. An estimation of the electron generation rate proves that the increasing of the background vertical atmospheric electric current by two orders produces ∼ 10 −2cm −3sec −1 electron influx into D-region above seismic area. Joule heating increases the integral atmospheric conductivity in the seismic area by 1 ÷ 2 order thus resulting enhancement of the electron ionospheric temperature up to 10 3 ÷ 10 4 K. Such electron heating leads to growing of the electron collision frequency v en ∼ T e by 1 ÷ 1.5 order and, as an effect, to essential drop of the lower D-layer conductivity and growing of the Pedersen conductivity of the lower E-layer above the place of developing earthquake.

Impact of tunnel-oxide nitridation on endurance and read-disturb characteristics of Flash E 2PROM devices

Threshold-voltage window closure in non-volatile memory (NVM) devices is known to originate from charge trapping in the dielectric underneath the floating gate (FG dielectric). In this paper, it is shown that oxide nitridation lowers the generation rate of neutral electron traps and, consequently, reduces the amount of trapped charge. Therefore, FG dielectric nitridation improves the endurance characteristics of NVM devices. Also, write/erase ( W E ) degradation of NVM devices results in a Stress Induced Leakage Current (SILC) through the FG dielectric, enhancing read-disturb. The conduction mechanism of the SILC being trap-assisted tunnelling, FG dielectric nitridation is expected to reduce the SILC. However, the lower trap generation rate in the nitrided oxides is compensated by an enhanced trap-assisted conduction efficiency, resulting in nearly the same SILC as compared to conventional oxides. Therefore, read-disturb is barely affected by nitridation of the FG dielectric.

Noise performance of bound-to-miniband transition III-V quantum-well infrared photodetectors

Dark current noise measurements between 10 and 105 Hz were carried out at T=77 K on three different types of III-V quantum-well infrared photodetectors designed for 8–12 μm IR detection. These devices have superlattice barriers leading to miniband transport in the extended conduction band. For frequencies between 102 and 104 Hz, noise plateau levels stemming from the trapping and detrapping of electrons in the quantum-well bound states are observed. From the measured noise data the low-bias electron diffusion length, the bias-dependent noise gain, the electron trapping probability, and the thermal electron generation rate are calculated.

On the dark current noise of quantum well infrared photodetectors

Dark current noise measurements on a bound to miniband transition quantum well infrared photodetector were carried out at 77 K to either prove or disprove the correctness of three different, recently published noise expressions. The noise data indicate that the bound electron generation rate varies exponentially with applied voltage.

Radiation-induced conductivity in alumina from 100 Hz to 10 MHz during proton irradiation

Changes in the dielectric constant and loss tangent of alumina have been measured in situ during proton irradiation. In these experiments, single crystal sapphire specimens were irradiated with 3 MeV protons which passed through the sample and were stopped in a copper-block heat sink. Dielectric properties were measured between 100 Hz and 10 MHz using a guard-ring capacitor configuration. The proton irradiation caused an immediate increase in loss tangent from about 10 −4 to more than 1.0. We have evaluated these changes at 300 and 373 K, vs irradiation time, flux and frequency. While the in situ radiation-induced conductivity (RIC) depends on these variables as well as on the history of previous irradiation, we believe that it is caused by a balance between the generation rate of electrons and holes, and their trapping and annihilation at displacement-type defects.