Filamentary Current Flow Research Articles

Simulation of longitudinal instabilities in filamentary current flow during low-temperature impurity breakdown in semiconductors

Nonlinear semiconductor transport simulations based on the WIAS-TeSCA code are presented. Various regimes of low-temperature breakdown and current filamentation in n-GaAs are investigated using a drift-diffusion model with nonlinear generation-recombination kinetics. Nonlinear charge density waves are found in two-dimensional simulations of a point contact geometry with and without an additional perpendicular magnetic field. The numerical simulations of the nonlinear spatio-temporal dynamics are complemented by a linear stability analysis which reveals the possibility of undamped longitudinal fluctuations in the filamentary regime.

Transverse spatio-temporal instabilities in the double barrier resonant tunneling diode

We derive a simple self-consistent model for the double-barrier resonant tunneling diode, which describes the Z-shaped current–voltage characteristic and includes the transverse re-distribution of electrons in the quantum well. In particular, we predict different regimes of spatio-temporal dynamics: stable uniform or filamentary current flow, and laterally uniform or nonuniform self-sustained current oscillations. Switching between the two stable uniform steady states ( on and off) is explained in terms of transverse trigger fronts. Complex chaotic scenarios including spatio-temporal spiking are found.

Impact ionization breakdown of n-GaAs in high magnetic fields

The influence of high magnetic field and its orientation on the impact ionization breakdown of n-type GaAs has been investigated experimentally. Hysteretic behaviour of current-voltage characteristics was observed for longitudinal magnetic field orientation to the current direction and no hysteresis was observed for transverse. The threshold voltage was much higher for transverse than for longitudinal magnetic field orientation to the current direction. The dependencies of transverse and longitudinal magnetoresistance on magnetic fields in the electric fields in pre- and post-breakdown regimes were also measured. In the pre-breakdown regime (hopping conduction) there is no great difference between the transverse and longitudinal magnetoresistance in the case when the magnetic length is much higher than the radius of an electron in the ground state on the impurity atom (low magnetic fields). In the opposite case (high magnetic fields) the transverse magnetoresistance is much higher than the longitudinal one. Such a behaviour of magnetoresistance in the pre-breakdown regime is, possibly, due to magnetic-field-induced shrinkage of the electron wavefunction. The observed great difference between the transverse and longitudinal magnetoresistance in the post-breakdown regime can be explained in terms of the Lorentz force influence on the filamentary current flow.

Current filamentation in point contact geometry and its 2D stationary model

Two-dimensional reconstructions of the filamentary current flow are shown, obtained by the spatially resolved photoluminescence of a sample of n-GaAs epitaxial layer with two point contacts. A mathematical model is constructed, based on a bistability of the local conductivity controlled by the local electric field strength. The interface between the high and low conductive states is assumed to occur always at a constant critical value of the field strength. Corresponding mathematical description of the model leads to the nonlinear Laplace free boundary problem. Its two-dimensional numerical solution is shown to satisfactorily reproduce the main characteristics of the filament observed in the experiment.

Influence of external circuits on filamentarycurrent flow during impurity breakdown in n -typeGaAs

Two-dimensional simulations are performed of low-temperature impurity breakdown in an n-type GaAs film connected to a capacitive and resistive external circuit under current controlled conditions. The results confirm that the transition between nearly-insulating and highly-conductive states, where a current filament is formed or decays in the sample, is synchronised with the charging and discharging of a parallel capacitor. Our simulations also indicate a mechanism for self-sustained relaxation oscillations.

Impurity-breakdown-induced current filamentation in a dipolar electric field.

Filamentary current flow has been investigated in an epitaxial n-type GaAs layer with two Ohmic point contacts. A scanning laser microscope technique has revealed two characteristic regimes of filamentary structure: a large-area filament typical at higher sample currents, and a bendable filament arising at low currents and becoming curved in perpendicular magnetic fields. Self-organization is supposed to be significant upon forming the bendable filament, whereas the large-area filamentary structure is predominantly formed by the electric field distribution due to the point contact geometry. Numerical simulations have been carried out in the second case, showing a good agreement with experimental results.

Characteristics of an optically activated pulsed power GaAs(Si:Cu) switch obtained by two-dimensional modeling

Photoconductive semiconductor switches (PCSS) have high-voltage hold-off (many to tens of kilovolts) and fast current rise times (<1 ns). However, lock-on, nonuniformities in the electric field, and filamentary current flow across the device when switching at high fields (∼10 kV/cm) have been reported. These observations raise concerns about the scaling of PCSS to high currents (tens of kiloamperes). To investigate these issues a two-dimensional time dependent computer model of a GaAs PCSS with Si and Cu doping has been developed. The model solves the continuity equations, the bulk energy equation, Poisson’s equation for the electric field, and a circuit equation for external currents. Physical effects in the model include band-to-band impact ionization, trap impact ionization, photoionization, and negative differential resistance. Computed characteristics of GaAs(Si:Cu) switches will be reported. Experimentally observed electric-field distributions are explained.

Uniform and filamentary transport in d.c. thin-film ZnS: Mn electroluminescent devices

Abstract Thin-film ZnS is highly insulating until the applied field rises above approximately 1 MVcm−1. The current then rises almost exponentially with increasing voltage, and current densities in excess of 1 A cm−2 2 can be maintained. If the ZnS is doped with Mn, the resulting efficient electroluminescence provides a means of spatially and temporally mapping the local current density. Device behaviour under both steady-state and non-equilibrium conditions has been investigated including laterally uniform current, filamentary current flow, kinetic electroluminescence and negative-capacitance behaviour. This behaviour is inconsistent with previously reported transport models but can be understood, at least qualitatively, within the framework of the novel high-field transport mechanism proposed. This is based on tunnelling from states within the bandgap and the dynamics of interacting regions of space charge within the ZnS.

Magnetoresistance of n-GaAs at filamentary current flow

A large number of sharp structures are observed in the 4.2 K magnetoresistance of n-GaAs biased above impurity breakdown in a regime where current flow is filamentary. Most of the structures cannot be attributed to spectral properties of the semiconductor such as impact excitation of shallow donors or the magnetoimpurity effect. Experimental results give evidence that these structures are caused by a redistribution of the filamentary current flow when one filament border is swept across an imperfection in the material.

Simulation studies of biomagnetic computed tomography.

The reconstruction of planar and three-dimensional current distributions from measured biomagnetic signals is a new field of research, known as biomagnetic computed tomography. This noninvasive imaging technique promises to provide precise, millimeter-sized resolution images of the electrical currents in tissues or organs. We performed simulation studies on phantom models of electrical sources. As a first step towards the development of an imaging algorithm, we addressed a simplified problem to identify the shape and direction of current flow in a planar surface. The problem was formulated by identifying a space in which the image was to be reconstructed. The space was segmented into a grid. Each grid space represented a current element. The magnetic field at a sampling point due to the current elements was computed using the Biot-Savart law. Since there were many more current elements than sample points, the problem was undetermined and had an uncountable number of solutions. The projection theorem was used to define an analytic solution for the magnitude and orientation of the current elements in the grid space. The solution required the inversion of large matrices in double precision. Such arrays were preprocessed on a mainframe computer, which permitted them to be rendered on any workstation. The accuracy of the image was determined by comparing it with the known location of the sources. Our results show that shape of the filamentary current flow can be imaged with our techniques. The resolution of images based on the sampling of the field, number of voxels in the reconstruction space, and noise is also analyzed.

Regular and chaotic current oscillations in n-type GaAs in transverse and longitudinal magnetic fields.

Self-sustained current oscillations due to impurity breakdown in n-type GaAs epitaxial layers are reported for various magnetic-field strengths and orientations of the field with respect to the direction of current. In all cases relaxation oscillations at the onset of breakdown and formation of a current filament in the postbreakdown regime are observed. A magnetic field normal to the epitaxial layer destabilizes the filament and causes multifrequency oscillations and chaotic fluctuations. On the other hand, if the magnetic field lies in the plane of the layer the filamentary current flow is stable up to field strengths of 2 T.

FIR photoconductivity at filamentary current flow in n-GaAs in a magnetic field

A large number of spectral structures are observed by far infrared magneto-photo-conductivity in n-GaAs at 4.2K biased above impurity breakdown in a regime where current flow is filamentary. Phase changes are found in the center of the 1 s−2 p +1 line and new structures are observed which shift to higher magnetic fields with increasing bias voltage. The large strength and the intensity dependence of the photosignals indicate that the observed phenomena are due to self-organizing effects of the filament. Small variations of the electron population by infrared irradiation may cause substantial rearrangements of the spatial current distribution.

The behavior of silicon p-n junction-based devices at liquid helium temperatures

In this paper, the forward current-voltage (I-V) characteristics of Si p-n junction diodes, fabricated in different state-of-the-art complementary-metal-oxide-semiconductor (CMOS) technologies, are investigated at liquid helium temperatures. As will be shown, three different I-V regimes can exist: a forward breakdown/hysteresis regime at a turn-on voltage which may be larger than the built-in potential of the junction; a thermionic emission regime, corresponding to a I=A exp(qV/kT) relation and a high-injection space-charge-limited regime, giving rise to a current which is proportional to Vn. The anomalous turn-on behavior can be explained by considering the small barrier for carrier injection, which exists at the ‘‘ohmic’’ contact. It will be demonstrated that the presence of this forward breakdown is strongly determined by the fabrication technology used. In extreme cases (large-area well diodes) multiple breakdown is observed, indicating an inhomogeneous, filamentary current flow. As will be shown, similar breakdown behavior is observed in more complex junction-based Si devices at 4.2 K; the consequences for deep-cryogenic CMOS circuitry will be briefly addressed.

Nascent states of current filamentation in semiconductors governed by negative differential resistance

We report the first two-dimensional imaging of just nucleating filamentary current flow during low-temperature impact ionization breakdown of n-type GaAs. Various nascent stages of transient spatial structures leading to the formation of a stationary minimal-size filament are resolved in the highly nonlinear breakdown regime of current-controlled negative differential resistance.

Hall voltage collapse at filamentary current flow causing chaotic fluctuations in n-GaAs.

Experimental investigations of the Hall effect at the occurrence of autonomous current fluctuations in high-purity n-GaAs epitaxial layers at low temperatures show that the transition to chaos is accompanied by intermittent collapses in the Hall voltage. This may represent the significant third mechanism for a transition into chaos according to the Ruelle-Takens-Newhouse scenario. Basically, space charges accumulated in the filament boundaries yield different impact ionization probabilities at opposite sides of the filament destabilizing the current flow and causing multimodal oscillation phenomena originating at the filament borders.

Fluctuations located at current filament boundaries in n-GaAs

The location of fluctuations occurring in a filamentary current flow is detected by spatially resolved photocurrent measurements using optical interband excitation in n-GaAs at low temperatures. Current fluctuations are only observed in the filament boundaries. In the presence of an external magnetic field, characteristics like the frequency and amplitude of the spontaneous current oscillations are different for opposite edges of the filament pattern. The observed phenomena are discussed in terms of electron generation-recombination kinetics.

A MINIATURIZED APPROACH TO THE CRYOELECTRONIC MAGNETIC FIELD EFFECT TRANSISTOR

Magnetic control of the filamentary current flow during avalanche breakdown in bulk semiconductors at low temperatures represents an interesting operation principle for the design of a magnetic field effect transistor. We have experimentally verified miniaturized device configurations using superconducting lines for the generation of the magnetic control field and all interconnections. The evaluation of the important performance characteristics of our transistor concept looks highly promising.

A new cryoelectronic device family

Analog and digital cryoelectronic applications of a novel hybrid device family combining both semiconducting and superconducting elements have been demonstrated experimentally. The proposed device concept is based on the magnetic control of the filamentary current flow during avalanche breakdown in bulk semiconductors at low temperatures. Power losses are minimized by using superconducting lines for the generation of the magnetic control fields and for the interconnections. Our device configuration represents an interesting concept for the development of ultrafast electronic circuits requiring high packing density and correspondingly low power dissipation.

A magnetic field-effect transistor

The magnetic control of the filamentary current flow during avalanche breakdown in a semiconductor at low temperatures represents an interesting new principle of a magnetic field-effect transistor. The power losses in such a device are minimized by using superconducting lines for the generation of the magnetic control field and for the interconnections. For such a hybrid concept, implementing semiconducting and superconducting components, the important performance characteristics are evaluated.

Magnetic control and switching of current filaments in a semiconductor

The filamentary current flow in the avalanche breakdown regime of p-Ge at 4.2 K has been studied as a function of an applied perpendicular magnetic field. Magnetically induced switching of the current filaments between different ohmic contacts has been observed, and the principle of a new magnetic field effect transistor has been demonstrated.