Formation Of Current Filaments Research Articles

Investigation of repetitive short-circuit operation of 1200 V IGBTs in the IC-VCE phase space

This work shows an investigation of repetitive Short-Circuit (SC) operation of 1200 V–15 A IGBT far above the Short-Circuit Safe Operating Area (SC-SOA). The goal behind this work was to perform repetitive SC tests with test conditions provoking the formation of current filaments in the short-circuit pulse, which leads to a non-homogeneous heating but not to a destruction. The target was to investigate unknown physical effects that lead to device degradation during the repetitive SC test. The IGBT device shows strong deterioration in its blocking characteristic after it was subjected to several 1000 SC pulses at extremely high SC levels.

Trap‐Related Breakdown and Filamentary Conduction in Carbon Doped GaN

A breakdown phenomenon is studied in thin carbon doped GaN layers (GaN:C; carbon concentration 1019 cm−3) embedded between a top metal electrode and bottom n‐doped GaN (n‐GaN). When slowly sweeping positive bias V at the top electrode up and down, a hysteresis is found with transitions to on‐ and off‐states at voltages and (<Vbd,up), respectively, and on‐ to off‐current ratios exceeding 103. Breakdown at occurs at an electric field of about 0.5 MV cm−1 in GaN:C. For , transition to on‐state is time‐dependent with random time‐to‐breakdown in the ms to 10s range. Electroluminescence measurements in on‐state show conduction via multiple standing or slowly moving current filaments (CFs) which number and size increases with total current. The origin of an S‐shape I–V curve leading to hysteresis behavior is discussed in terms of trap‐related nonlinear generation‐recombination processes. Formation of multiple CFs is explained by spontaneous CF formation in a homogeneous spatially extended system composed of an active medium with S‐shape I–V curve (here GaN:C) connected in series with a passive layer (here n‐GaN). Unlike previous studies in various III‐N opto‐electronic systems, extended defects are supposed not to be responsible for the CF origin but just for their pinning.

Formation of current filaments and magnetic field generation in a quantum current-carrying plasma

The nonlinear dynamics of filamentation instability and magnetic field in a current-carrying plasma is investigated in the presence of quantum effects using the quantum hydrodynamic model. A new nonlinear partial differential equation is obtained for the spatiotemporal evolution of the magnetic field in the diffusion regime. This equation is solved by applying the Adomian decomposition method, and then the profiles of magnetic field and electron density are plotted. It is shown that the saturation time of filamentation instability increases and, consequently, the instability growth rate and the magnetic field amplitude decrease in the presence of quantum effects.

Simulation of current filamentation in a dc-driven planar gas discharge–semiconductor system

We have performed a theoretical study of self-organized current filamentation in a dc-driven planar gas discharge–semiconductor system at very low currents and under cryogenic conditions. The discharge instability and the observed formation of current filaments are explained by a thermal mechanism, as proposed in our previous paper. We have found, for the first time, a stationary periodic current structure in a two-dimensional Cartesian geometry from first principles, by numerically solving the general system of continuity equations for ions and electrons, the Poisson equation for the electric field in the gas, together with the equation for gas temperature and the equation for electric field in the semiconductor. The space charge induced electric field redistribution, which usually leads to a discharge instability and is automatically included in the first three equations of the system, is practically absent at the very low currents considered, and thus it cannot be responsible for the discharge instability. This is why another mechanism of filamentation (thermal) should be considered. The calculated periodic current structure agrees with the hexagonal current pattern observed in the experiment, as well as with the periodic current structure found in the frame of the previously developed simple model. This serves as a corroboration of the fact that the thermal effect is essential for pattern formation under the conditions considered.

Localization of magnetized electrons in current filaments as a fundamental cause of Coulomb explosion

Mechanisms for generating current filaments in a dense plasma under the action of focused laser pulses and in a Z-pinch configuration are discussed. The main properties of current filaments with a zero and nonzero electron vorticity Ωe=B−(c/e)∇×pe that originate at magnetic fields in the range 4πnemec2≪B2≪4πnimic2 are investigated under the conditions of Coulomb explosion at currents below the ion Alfven current. A study is made of the equilibrium configurations of nonquasineutral current filaments in a purely longitudinal (Bz) and a purely azimuthal (Bθ) magnetic field and also in a more general case of a helical magnetic field, having two components, under conditions such that the charge separation occurs on a spatial scale on the order of the magnetic Debye radius rB ≃ |B|/(4πene. It is shown that strong electric fields generated in the current filaments are comparable in magnitude to the atomic field and are capable of accelerating ions to energies of several tens of megaelectronvolts. The ion dynamics in strong electric fields of the filaments is calculated numerically and is shown to lead to the formation of collisionless shock waves on time scales on the order of several inverse ion plasma frequencies ωpi−1. The possible formation of current filaments on different spatiotemporal scales is considered.

Formation of current filaments in n-type GaAs under crossed electric and magnetic fields

We present two-dimensional simulations of current filaments in the regime of low-temperature impurity breakdown in thin n-GaAs films under the influence of a magnetic field perpendicular to the sample plane. Our simulations show that current filaments are bent by the Lorentz force and their width and curvature increase with magnetic-field strength. Significant asymmetry of the electric fields is observed at opposite filament boundaries. These results agree well with experiments. It is also revealed by simulations that impurity breakdown is accelerated by the presence of an external magnetic field. This is attributed to a finite Hall field inside the filaments.

Dynamics of nascent current filaments in low-temperature impurity breakdown.

We present two-dimensional simulations showing the spatiotemporal dynamics of the formation of current filaments in n-type GaAs films in the regime of impurity impact ionization breakdown. From the spatial distribution of carrier densities, electron temperature, current density, and electric field for nascent and for fully developed filaments, we find a three-stage scenario for breakdown: (i) front creation and propagation from the cathode, (ii) stagnation in the phase of a rudimentary filament, (iii) filament growth. Our model combines semiclassical rate equations with microscopic transport parameters, which are obtained from Monte Carlo simulations. \textcopyright{} 1996 The American Physical Society.

Magnetic-field-induced lateral displacements of current filaments in n-GaAs

The formation of current filaments in n-type GaAs due to the low temperature impurity breakdown has been investigated in crossed electric and magnetic fields with a laser scanning microscope. In a highly compensated, low mobility, epitaxial layer the lateral displacement and stretching of a filament due to the Lorentz force has been observed at currents slightly larger than those required to form a filament between point contacts. The transverse elongation of the filament increases linearly with rising magnetic field strength until the filament is disrupted. In this case periodic current oscillations are generated by a repetitive filament ignition and extinction. The elongation decreases with rising current and finally vanishes even at high magnetic fields. This stiffening of a current filament is attributed to an increased electron temperature and mobility in the filament, which cause the charges of the Hall field to be accumulated at the filament borders rather than at the edges of the sample.

Time-resolved OBIC for probing current filamentation in GaAs/AlxGa1-xAs heterostructures

We developed a time-resolved optical beam induced current (TROBIC) technique to image current filamentation in a GaAs/AlxGa1-xAs heterostructures under high electric field conditions. The TROBIC method combines a laserscan microscope in the OBIC-mode with a sampling oscilloscope to time-resolve the OBIC signal and allowing to operate under high electric field conditions. We attribute the current patterns observed in the TROBIC images to the formation of current filaments in the AlxGa1-xAs layer, parallel to the two-dimensional electron gas (2DEG). We show that even in samples where the two-dimensional electron gas and the contacts to the 2DEG are perfectly ohmic and homogeneous, current filaments can still develop at high electric fields.

Observation of current filaments in GaAs/AlxGa1−xAs heterostructures using a time-resolved imaging technique

We developed a time-resolved optical beam induced current (TROBIC) technique, and performed time-resolved current imaging experiments on GaAs/AlxGa1−xAs heterostructures under high electric field conditions. These experiments are the first time-resolved imaging experiments of current patterns in a two-dimensional semiconductor structure. We attribute the current patterns observed in the TROBIC images to the formation of current filaments in the AlxGa1−xAs layer, parallel to the two-dimensional electron gas (2DEG). We show that even in samples where the two-dimensional electron gas and the contacts to the 2DEG are perfectly ohmic and homogeneous, current filaments can still develop in high electric fields. These temporal and spatial instabilities in the AlxGa1−xAs layer strongly affect the high-field transport properties of the heterostructure.

X-ray diffraction studies of microstructural transformations in single-crystal silicon during current filamentation

The (111) Bragg peak of p- and n-Si single crystals is studied when a large dc current (average current density 10 A/cm2) is passing through the specimen. The integrated intensity of the peak changed drastically with time after current filamentation, showing existence of microstructural transformations. The fresh samples (type A) showed a decrease in integrated intensity by a factor of about 50, 15 min after current filamentation, with a limiting current of 330 mA passing through the sample. The intensity partially recovered when the current was switched off, and, after cooling to room temperature, the specimen behaved differently when the electric field was again applied. The specimen (type B) then showed an increase in integrated intensity a couple of minutes after current filamentation so that the diffracted intensity became comparable to the type A specimen. Thereafter, with continued high current passing through the specimen, it behaved like a fresh silicon sample. The results are interpreted by considering formation of current filaments and consequent local heating.

Quasilinear evolution of the self-filamentation instability

A thermally inhomogeneous plasma immersed in a strong uniform magnetic field may be unstable to the formation of current filaments aligned to the field propagating at the diamagnetic drift frequency. The quasilinear evolution of this self-filamentation is studied. Transverse magnetic fields generated by the currents bring into thermal contact unequal temperature surfaces leading to a relaxation of the thermal gradient. For weak gradients, the instability is shown to saturate at a constant amplitude while, for stronger gradients, periodic behavior is exhibited. The transition from saturation to periodic behavior is predicted analytically from a three-wave model. The anomalous heat transport resulting from the instability is assessed.

Impact ionization phenomena initiated by the acoustoelectric effect in n-GaAs

Acoustic flux amplified from thermal background in semidegenerate n-GaAs at liquid-air temperature is accompanied by electric fields that initiate impact ionization. When the acoustic flux is concentrated in a narrow domain the interplay between the impact and the acoustoelectric effect brings about current oscillations of frequencies in the MHz range that increase monotonically with the net gain factor of the acoustic flux measured prior to the onset of impact. When wide stationary acoustic domains are generated in such samples, the impact causes an increase of the saturation current typical of the acoustoelectric effect. For initially applied fields higher than the threshold for impact, current modulation is observed when the acoustic flux is still in the embryonic stage and its incubation time is increased. The threshold field for impact in epitaxial layers is about 1500 V/cm, while in bulk samples it is much higher. At high power levels these instabilities may bring about the formation of current filaments that burn channels across the samples. The experimental results and a semiquantitative analysis of the instabilities are presented in detail in this work.

Double injection in light emitting (Ga, In) As p‐i‐n structures

AbstractThe double injection behavior of (Ga, In)As p‐i‐n light emitting diodes (DILED's) is investigated. Using pulsed driving current the formation of current filaments is studied. As explanation is suggested the filling of deep‐lying traps. The optoelectronical switch properties of the DILED's are included in the discussion.

Switching and Memory Characteristics of ZnSe - Ge Heterojunctions

Heterojunctions of nZnSe-pGe in which the ZnSe layer is thin and contains a considerable defect density exhibit switching between a high-impedance state of typically 106−107 Ω and low-impedance states three to five orders of magnitude lower, and have the ability to retain either state at zero bias for long periods of time. Switching speeds of 100 nsec or less are observed at repetition rates of 500 KHz, as long as sufficient overdrive is applied to reduce delay times for switching in both directions to negligible values. The effect of temperature on the device parameters is presented, and a model consisting of trap emptying by impact ionization, the formation of current filaments, and the refilling of traps by electron injection is proposed.

Current filaments in acoustically amplifying CdS

Electrical breakdown and the appearance of thin, hollow channels in single crystals of photoconducting CdS are explained in terms of current filamentation. The formation of current filaments is attributed to an S-type negative resistance of an acoustoelectric origin.

Negative resistance and filamentary currents in avalanching silicon p+-i-n+junctions

Space-charge effects in avalanching p+-i-n+diodes give rise to a current-controlled bulk negative resistance. It is shown that this negative resistance gives rise to an instability which tends to lead to the formation of current filaments. A steady state can be found in which the generation of carriers in the filament by impact ionization is balanced by radial diffusion of carriers. We present the results of approximate numerical calculations of filament current-density profiles and total filament current as a function of applied voltage. The total filament current is a decreasing function of the applied voltage; thus, the diode exhibits a quasistatic negative external resistance. It is suggested that this negative resistance may be used to interpret observed sub-transit-time oscillations of p+-i-n+structures.