F.-J. Niedernostheide

Observation of solitary filaments and spatially periodic patterns in a dc gas-discharge system

Abstract In a planar gas-discharge system consisting of a metal and a high ohmic semiconductor electrode we observe experimentally the generation of stationary current density filaments, a discontinuous device characteristic with hysteresis and the bifurcation from a homogeneous discharge into a spatially periodic discharge pattern.

Spatio-temporal oscillations during filament splitting in gas discharge systems

Abstract The process of generation of current density filaments has been investigated in a dc driven gas discharge systems consisting of a metal and a high ohmic semiconductor electrode and in an ac driven gas discharge system between two dielectrically covered electrodes. By means of a streak camera system we find that the generation process is accompanied by spatio-temporal filament oscillations.

Transition from pulsating to rocking localized structures in nonlinear dissipative distributive media with global inhibition

Abstract We consider localized structures generated in a certain of two component reaction-diffusion system. Four different electric systems, namely reverse-biased p-n diodes, p-n-p transistors, a gas-discharge system, and an electrical network are recapitulated and it is shown that, in spite of the existence of different underlying physical mechanisms, in all cases an activator-inhibitor system is realized. If these systems are driven by an external voltage source via a load resistor, the equation for the external circuit leads to the appearance of an integral term in the reaction terms causing a nonlocal additional inhibiting component. Numerical calculations of the equations system with the simplest reaction terms show exemplarily that there is a critical value of the load resistor in such systems: While below this critical value besides static only pulsating localized structures appear, for values larger than the critical one instead of pulsating only rocking localized structures can be stabilized. Near the critical resistor value we observe patterns composed of pulsating and rocking modes.

Travelling current density filaments in multilayered silicon devices

Abstract Spatially and temporally resolved measurements of the electroluminescence and the potential distribution reveal the existence of travelling current density filaments in silicon p-n-p-n devices. Differences and similarities of both distributions are discussed.

Pattern Formation in Nonlinear Physical Systems with Characteristic Electric Properties

Systems are considered which consist of two layers, one of them characterized by a high ohmic, approximately linear and one by an S-shaped current-voltage characteristic. We describe such systems mathematically in terms of two-component reaction-diffusion equations. Analytical and numerical investigations of these equations reveal that the systems can self-organize different stable stationary inhomogeneous nonequilibrium structures as well as spatio-temporal irregular behaviour. These results are in good qualitative agreement with the current density patterns in two experimentally investigated systems, namely a one-dimensional npnp-semiconductor structure and a one-dimensional gas discharge system.

Periodic and irregular spatio-temporal behaviour of current density filaments in Si p+−n+ −p−n− diodes

Abstract We present spatio-temporally resolved IR recombination radiation measurements in silicon p + -n + -p-n − diodes. Self-generated periodic voltage oscillations and period-doubling cascades are strongly correlated to spatial motions of a current filament. Chaotic time traces of the device voltage give clear indication of irregular spatio-temporal motions of current filaments.

Pattern Formation of the Electroluminescence in AC ZnS:Mn Devices

Spatial inhomogeneous distributions of the electroluminescence radiation density are studied in ac driven thin film ZnS:Mn electroluminescent (ACTFEL) devices. Apart from stationary microfilaments, dynamical patterns occur on a timescale ranging from tenths of a second to several seconds. In particular, moving filaments and strings, fluctuating networks of strings, autowaves, or global oscillations may arise. The spatiotemporal structures are extremely sensitive to the conditions of preparation and the driving parameters of the samples.

Analytical approach to stationary wall solutions in bistable reaction−diffusion systems

Abstract Stationary wall solutions of a two-component nonlinear reaction-diffusion system are developed by means of an analytical approach consisting mainly in replacing the nonlinearity by a piecewise linear approximation. The analytically determined solutions are in good accordance to numerically calculated solutions. The analytical description also explains qualitatively the transition to another kind of solutions, namely solitary filament structures of a special width, for approximately chosen system parameters.

Formation of Spatio-Temporal Structures in Semiconductors

We review recent theoretical and experimental advances in the formation of spatio-temporal structures in semiconductors. Charge transport in semiconductors has been demonstrated to exhibit many features of complex nonlinear spatio-temporal dynamics and self-organized pattern formation which occur in other dissipative systems as well. Recent progress has been obtained, for instance, in the theoretical description of such phenomena in terms of microscopic transport models or reaction-diffusion systems and activator-inhibitor kinetics as well as in the discovery of various patterns, e.g., in Si, GaAs, and ZnS based materials and advanced quantitative experiments. Detailed computer simulations in one and two spatial dimensions have enabled a quantitative comparison between theory and experiment. Among the particularly instructive examples are investigations of current filaments and their higher bifurcations such as travelling, rocking, breathing, or spiking filaments in layered semiconductor structures and in doped semiconductors in the regime of low temperature impurity breakdown for different contact geometries, as well as recent studies of localized two-dimensional structures in ZnS films.

Realization of a neural algorithm by means of front-propagation in a thyristor-based hybrid system

Abstract Propagating fronts are generic structures in a bistable diffusion-driven system and can be used to realize neural algorithms, as e.g., the Kohonen or the neural-gas algorithm. We present an analog–digital hybrid system based on a thyristor-like structure with several gate terminals. This structure represents the continuous part in which a propagating front, separating a region of high current density from a region of low current density, is used to control the learning process of the neural algorithm. With a system containing five neurons and five gates in a quasi one-dimensional arrangement it is demonstrated that an efficient parallel operating learning process can be realized by using the winner-take-all principle and the front propagation, i.e. exploiting the intrinsic dynamics of the semiconductor device. Finally, numerical and analytical investigations of the dependency of the front velocity and its width on the load current have been performed since these are essential parameters for improving the network performance.