B G Salamov

Analysis of prebreakdown current in a gas discharge system with a semiconducting cathode

Results are presented of an analysis of the prebreakdown current of a Townsend discharge in a planar gas discharge system with a GaAs semiconductor cathode for a gap thickness of 45 µm and gas pressures of 6.9, 8.9, 34.2 and 36.9 kPa. IR radiation excites the cathode in the system, thus controlling the current density in the gas discharge layer. It is shown that the prebreakdown current is caused by electron emission from the semiconductor surface. The present experimental investigations show that the prebreakdown current decreases anomalously with increase of the feeding voltage and that it also decreases under the illumination of a semiconductor cathode. Both these anomalies depend on gas pressure. The current density change through the cross-section of the discharge gap, i.e. the appearance of the spatio-temporal self-organization of non-linear dissipative systems, causes these observed effects. These characteristics of the system can be applied for fast IR imaging and a high speed UV light source.

Behaviour of current in a planar gas discharge system with a large-diameter semiconductor cathode

Gas breakdown in air in a planar gas discharge system was studied experimentally at various distances L between the electrodes and different diameters R of the electrode areas of the semiconductor cathodes. The cathode was irradiated on the back-side with light in a particular wavelength range that was used to control the photoconductivity of the material. The current–voltage characteristics of the gas discharge system with a large-diameter GaAs semiconductor cathode were studied in a wide range of the gas pressure p (28–550 Torr), interelectrode distances L (45–330 μm), and conductivities of the cathode. We show that for all experimental conditions the current density increases over the entire range of voltages U>Udc as the diameters R of the electrode areas increase. It is shown that, for arbitrary geometric dimensions of the semiconductor cathode (in the range of L/R under investigation), the ratio of the breakdown electric field to the gas pressure holds constant (Edc/p)min≈const. at the breakdown curve minimum. The breakdown voltage Udc is a function of both the product of the gas pressure p by the distance L and the ratio L/R (Udc = f(pL,L/R).

Spatial stabilization of Townsend and glow discharges with a semiconducting cathode

The physical processes determining the functions of an ionization system and especially the discharge stabilization by the distributed resistance of a semiconducting cathode in such a system are studied. The current - voltage (I - U) characteristics of the system with a semiconducting GaAs cathode are obtained experimentally as functions of the gap pressure P (16 - 760 Torr) and inter-electrode distance d ( to 5 mm), which are varied for the first time over very wide ranges. The experiments showed that the presence of the distributed resistance affects the passage of the discharge current so that the discharge glows uniformly throughout the cathode surface and is not divided into separate filaments. The loss of stability was primarily due to the formation of a space charge of positive ions in the discharge gap which changed the discharge from the Townsend to the glow type.

Recording the resistance inhomogeneity in high-resistivity semiconductors plates

Abstract A device for recording the resistance inhomogeneity in high-resistivity and photosensitive semiconductor plates of large diameter is described. A measurement of homogeneity is realized by recording the spatial distribution of the gas discharge glow intensity between two transparent electrodes with the plate between them. Plates of GaAs with resistivity of 10 6 –10 8 Ωcm, thickness from 0.3 to 1.5 mm and the diameter from 20 to 60 mm have been studied. A gas discharge gap was formed by dielectric separator with thickness from 20 to 80 μm. A discharge has been realized in air at a pressure from 60 to 600 Torr. The possibilities of the proposed device have been evaluated, i.e. a relative change of the resistance inhomogeneity is determined by a relative change of discharge glow intensity the resolution of which is determined by method of its recording. Semiconductor plates with the resistivity from 10 5 to 10 9 Ωcm can be used in the device proposed.

Stability and current behaviour in semiconductor gas discharge electronic devices

Breakdown and range of stable discharge glow in a homogeneous dc electric field are studied at various distances d between the electrodes and different inner diameters D (5, 9, 12, 18 and 22 mm) of GaAs semiconductor cathode areas. The current–voltage characteristics of the gas discharge system have been studied in a wide range of pressure p (16–760 Torr), interelectrode distances d (10 µm–5 mm) and conductivities of the GaAs cathode. The initiation of electrical breakdown as a result of secondary electron emission from the semiconductor cathode in low gas pressure is presented in this paper. In a planar gas discharge cell with diameters much larger than an interelectrode distance, the effects of different parameters (overvoltage, electrode separation, diameter and conductivities of the GaAs cathode, gas pressure, glow current, etc) on electrical breakdown and spatial stabilization of the current have been studied. The distributed resistance of photosensitive semiconductor cathode and the impact of the ionizing component of the discharge plasma on the control of the stable operation of a planar gas discharge system at atmospheric pressure are also investigated. Through spatially uniform irradiation of the semiconductor cathode, non-stationary states which are non-homogeneous can be generated in a system. The loss of stability is primarily due to the formation of a space charge of positive ions in the discharge gap which changes the discharge from the Townsend to the glow type.

Hydrogen discharges operating at atmospheric pressure in a semiconductor gas discharge system

Analyses of physical processes which initiate electrical breakdown and spatial stabilization of current and control it with a photosensitive cathode in a semiconductor gas discharge system (SGDS) are carried out in a wide pressure range up to atmospheric pressure p, interelectrode distance d and diameter D of the electrode areas of the semiconductor cathode. The study compares the breakdown and stability curves of the gas discharge in the planar SGDS where the discharge gap is filled with hydrogen and air in two cases. The impact of the ionizing component of the discharge plasma on the control of the stable operation of the planar SGDS is also investigated at atmospheric pressure. The loss of stability is primarily due to modification of the semiconductor-cathode properties on the interaction with low-energy hydrogen ions and the formation of a space charge of positive ions in the discharge gap which changes the discharge from Townsend to glow type. The experimental results show that the discharge current in H2 is more stable than in air. The breakdown voltages are measured for H2 and air with parallel-plane electrodes, for pressures between 28 and 760 Torr. The effective secondary electron emission (SEE) coefficient is then determined from the breakdown voltage results and compared with the experimental results. The influence of the SEE coefficient is stated in terms of the differences between the experimental breakdown law.

Electro-optical properties of porous zeolite cathode in the gas discharge electronic devices for plasma light source applications

The stable dc air cold plasma is investigated experimentally functions of pressure p (8–760 Torr), electrode gaps d (50–250 µm), and diameters (9–22 mm) of the cathode areas in the gas discharge electronic devices (GDED) with nanoporous zeolite cathode (ZC). It is found that the current density and plasma emission (PE) intensity increase if the amplitude of the applied voltage reaches given threshold. Moreover, uniform PE inside the ZCs develops from the surface and can be generated in air up to atmospheric pressure (AP). The effect of various diameter of the ZC area on the gas breakdown is also considered. It is shown that breakdown voltage UB is reduced significantly for the larger diameters of the ZC area. Because of the very small d in our GDED, the behavior of charged particles in the electric field is described with the dc Townsend breakdown theory, depending upon the pressure range.

Current instability in a planar gas discharge system with a large-diameter semiconductor cathode

The current instability in a planar gas discharge system is studied experimentally in a wide range of the gas pressure, p (44–550 Torr), interelectrode distance, d (45–330 µm), and diameter, D (5, 9, 12, 18, 22 mm), of the electrode areas of the semiconductor cathodes. While being driven with a stationary voltage, it generates current instabilities with different amplitudes of the oscillation. Through spatially uniform irradiation of the semiconductor cathode, non-stationary states which are non-homogeneous can be generated in a system. It is shown that under the experimental conditions the discharge gap played only a passive role and was not responsible for the appearance of the current instability. At the same time, for different diameters, D, of the electrode an expanded range of current oscillations is observed. The pronounced N-type negative differential resistance is observed in the current–voltage characteristic of a planar gas discharge system with a large-diameter semiconductor cathode.

Novel multifunctional colloidal carbohydrate nanofiber electrolytes with excellent conductivity and responses to bone cancer cells.

This work presents a new approach to fabricating novel polymer nanofiber composites (NFCs) from water solution blends of PVA (hydrolyzed 89%)/ODA-MMT and Na-CMC/ODA-MMT nanocomposites as well as their folic acid (FA) incorporated modifications (NC-3-FA and NC-4-FA) through green electrospinning nanotechnology. The chemical and physical structures and surface morphology of the nanofiber composites were confirmed. Significant improvements in nanofiber morphology and size distribution of the NFC-3-FA and NFC-4-FA nanofibers with lower average means 110 and 113nm compared with those of NFC-1/NFC-2 nanofibers (270 and 323nm) were observed. The structural elements of polymer NFCs, particularly loaded partner NC-2, plays an important role in chemical and physical interfacial interactions, phase separation processing and enables the formation of nanofibers with unique morphology and excellent conductivity (NFC-3-FA 3.25×10(-9)S/cm and NFC-4-FA 8.33×10(-4)S/cm). This is attributed to the higher surface contact areas and multifunctional self-assembled supramacromolecular nanostructures of amorphous colloidal electrolytes. The anticancer activity of FA-containing nanofibers against osteocarcinoma cells were evaluated by cytotoxicity, apoptotic and necrotic analysis methods.

Formation of low resistance gas state near the surface of semiconductor electrode in pre-breakdown regime

The electrical conductivity of a thin gas discharge gap (d = 60 µm) filled with atmospheric air at 60 Torr pressure and room temperature was investigated. Current–voltage characteristics of the device when both the electrodes are metals and when one of the electrodes is a high resistivity GaAs semiconductor were measured. It is shown that in the device with the semiconductor electrode, the current is registered before the breakdown of gas. The value of pre-breakdown current depends on experimental conditions: it increases drastically if gas is subjected to breakdown before the measurements. Thus, the device has a memory effect. The value of memory effect depends on the value of the ionization current and does not depend on the polarity of the applied voltage. The effect is absent in a device with two metallic electrodes, and the effect disappears after the pumping of the gas down to 0.1 Torr. The obtained results can be explained on the basis of diffusion–drift type equilibrium of charges taking into account the charging of the semiconductor near the surface. The latter is the guarantee of the stability of residual pre-breakdown current in the gas.