Cathode End Research Articles

Contact and bulk effects in intrinsic photoconductive infrared detectors

The effects of built-in electric fields on the performance of intrinsic semiconductor quantum photodetectors are analysed, with emphasis on narrow bandgap Hg 1−xCd xTe devices with x = 0.215 operated at 77K. According to the proposed one-dimensional model, partial blocking of excess minority carriers may occur at the cathode end of the n-type photoconductor. resulting in enhanced current responsivity R i and reduced time response. In addition, the dependence of the detector voltage responsivity R ν upon the applied electric field E is discussed. It is shown that the ratio R ν/ R i corresponds to the measured dynamic differential resistance r d of the detector. The dependence of r d upon E is due to additional scattering of electrons (the high mobility majority carrier in HgCdTe) at high electric fields. Specific results of measurements on small-area HgCdTe photoconductive detectors are presented. Measured R i in excess of 3900 A ·W −1 and R νof the order of 5 × 10 5V·W −1 at a wavelength of 10 μm and at 77K arc reported.

The application of direct tissue isoelectric focusing to the study of human skeletal muscle

AbstractA method of direct tissue isoelectric focusing (DTIF) in agarose of human skeletal muscle is described. This particular method was developed to utilize the small amounts of tissue obtained by needle muscle biopsies performed for diagnostic purposes. 20 μm thick cryostat sections were adhered to the hydrophilic surfaces of small GelBond rectangles. These were then applied directly to the surface of the gels. A charge‐balanced purified agarose was used to make the gels, which contained Triton X‐100 to enhance protein solubilization. Stabilization of the pH gradient was attempted by employing a 3% w/v ampholyte which was a blend of 0.85% w/v pH 8–10.4 and 2.15 % w/v pH 3–10 Pharmalyte, by using anolyte regulation with 0.1 M aspartic acid and by focusing the gels under a CO2‐extracted nitrogen atmosphere. Equilibrium of proteins stained by Coomassie Brilliant Blue R‐250 was apparent as monitored by comigration of sections from both anode and cathode. With the exception of the cathode end, excellent resolution and reproducibility was achieved. Better cathode resolution was noted with non‐equilibirium conditions. Preliminary zymograms of lactic dehydrogenase (LDH), using a tetrazolium technique, have shown consistent patterns of multiple isoenzymes, the basic components of which were best seen using non‐equlibirium conditions.

Mechanism of pulsed surface flashover involving electron-stimulated desorption

A simple model is proposed to explain how a breakdown avalanche of secondary emission electrons can lead to surface flashover when an insulator in vacuum breaks down a few nanoseconds after high voltage is applied. The case of a plane insulator–vacuum interface perpendicular to parallel electrodes is considered. Positive surface charging is assumed to occur almost immediately upon application of the voltage, and the attendent secondary emission avalanche is assumed to be maintained at saturation throughout the prebreakdown time delay by field emission from the cathode electrode. Bombardment of the insulator by avalanche electrons desorbs a cloud of gas, which is partially ionized as it drifts through the swarm of electrons in the avalanche. The electric field at the cathode end of the insulator becomes enhanced as positive ions accumulate, which in turn increases the field emission and the rates of gas desorption and ionization. This and other regenerative processes rapidly lead to breakdown. Field enhancement at the cathode end of the insulator and increased field emission are individually considered in determining the prebreakdown time delay, with very similar results. The model predicts a time delay of the order of 10 ns at E=10 MV/m, which is in reasonable agreement with experimental observations. The proportionality we have observed between the time delay and the inverse square of the applied voltage is also predicted, as well as a dependence of the time delay on the insulator length. The model may also account for the improved performance of insulators coated with certain metal oxides.

Low frequency ion acoustic instability in the positive column of A D.C. discharge

The one-dimensional dispersion equation of the electroacoustic instability of the low pressure column is analyzed numerically. The column steady state parameters were obtained from the plasma balance equations based on a Maxwell distribution. The computed curves are compared with the experimental dispersions measured for spontaneously existing anode directed wave observed at the cathode end of the column. Although the agreement between the observed and computed dispersion curves is fairly good the current driven electroacoustic instability does not provide a universal explanation for all of these waves.

Equivalent circuit theory of stability of some transversely inhomogeneous discharges

The equivalent circuit clement theory of the stability, etc. of the cathode end of glow discharges is tentatively extended to discharges consisting of two of three units (abnormal, normal and subnormal) in parallel, with a common cathode. Both changes in fractional cathode coverage and of current density within a unit have to be allowed for. The results are consistent with observations which show that a large part of the cathode can be stably covered by the higher current density unit in a normal–subnormal system but not in an abnormal–subnormal system. Limitations and possible extensions of the theory are discussed.

Direct current cataphoretic effects on Hg 2537-Å radiation from Hg + Ar discharges (dc fluorescent lamps)

The Hg 2537-A intensity from Hg + Ar discharges was measured as an independent function of mercury vapor pressure (as influenced by a cold spot temperature) at various constant currents and tube radii with ~4-Torr argon. As a function of mercury pressure, the intensity rises to a peak (which defines an optimum mercury cold spot temperature) and then decreases with further increase in mercury pressure due to the combination of self-absorption and electron deexcitation. The behavior of the optimum mercury cold spot temperature is dependent upon ac or dc conditions. For ac, the optimum mercury pressure is ~7 mTorr (corresponding to a Hg cold spot temperature of ~40 degrees C) and comparatively insensitive to current. By contrast, the optimum mercury cold spot temperature for the dc case is dependent upon whether the anode end or cathode end is cooled. The dc lamp with anode end cooled yields an optimum mercury cold spot temperature less than 40 degrees C and decreases with increasing current, while the optimum with cathode cooled is greater than 40 degrees C and increases with increasing current. We believe that the peak intensity always occurs at the same real mercury density (because that determines the self-absorption), but the Hg cold spot temperature required to achieve this density is affected by dc cataphoretic pumping phenomena.

Supersaturation and precipitation of carbon in austenite as a result of electrotransport

Supersaturation and precipitation of cementite and/or graphite in austenite due to electrotransport were studied. Two types of experiments were made with alloys of relatively high initial carbon concentration (1.3 wt. pct); one in the two-phase (austenite plus cementite) region at 850° and the other in the single phase (austenite) region at 927° C. In the two-phase experiments the formation of cementite and/or graphite was observed at the high-carbon end of the specimen. In the single-phase case, only a continuous layer of graphite was formed at the cathode end of the specimen. The present research suggests that electrotransport can be used to investigate processes involving supersaturation, such as precipitation from solid solution.

GaAs negative conductance junction field effect transistor

Junction FET's were fabricated from n-type epitaxial GaAs on p-type GaAs substrates, the substrate acting as the gate. These devices showed a negative differential conductance in the drain current-voltage characteristics. A notch (i.e., thinner region) in the channel gave increased negative conductance when at the cathode end of the channel but only the ordinary saturating characteristic when used at the anode end of the channel. No Gunn oscillations were detected but the negative conductance is believed to be due to intervalley transfer effects. The devices were used in bistable circuits and oscillators.

Electromigration in thin gold films

Electromigration in 525 AA Au films has been studied by transmission electron microscopy during the actual passage of current. The effects of electromigration were the growth of voids near the cathode end and the growth of hillocks near the anode end of the test strip. The hillocks were rounded in shape with heights up to 40* the average thickness of the films. An activation energy of 0.8 eV was calculated from the rate of void growth.

The Development of Random Striations in the Positive Column of a Glow Discharge

Abstract The paper represents an analysis of the characteristic properties of spontaneously excited irregu-lar striations observed in the positive column of a glow discharge. Comparisions with the properties of coherent, externally excited, growing ionization waves lead to the conclusion that irregular stria-tions are random ionization waves. Assuming that random striations originate at the cathode end of the positive column, their frequency spectrum can be approximately explained from a knowledge of the dispersion relation of ionization waves. The nonlinear properties of random striations remain to be explained

On the gamma-delta instability of the cathode dark space and negative glow

Abstract The criterion for the stability of the cathode end of a cold cathode glow discharge against Small —S perturbations is derived by an elementary method.

Steady-state electrotransport of carbon in iron

Electrotransport of carbon in γ-Fe was studied by a steady-state method. Previous experimental studies of electrotransport in solid metallic systems have been limited to the use of unsteady-state methods. The steady-state distribution in a solid system can be attained in a reasonable time provided certain conditions are fulfilled. This method has the advantage that the effective charge,Z*, can be evaluated directly from the ratio of the chemical compositions at the anode and cathode ends without involving the chemical diffusivity. C14 was introduced into Armco iron specimens by carburizing. After electrotransport for a suitable time, the steady-state concentration-distance curve was determined by the method of residual radioactivity. The effective charge,Z*, of carbon in γ-Fe under the experimental conditions employed (0.11 to 0.50 wt pct C, 927° to 1027°C, and 1100 to 2500 amp per sq cm current density) is +3.99±0.08. No dependence ofZ* on carbon content, temperature, or current density was detected. Although the valueZ* =+4 coincides with the classical valence of carbon, the mechanism of transport probably involves the electronic carriers as well as the field force on the screened carbon ion.

The role of energetic neutrals in a magnetron sputter ion pump

Radioactive tracer techniques have been used to examine the distribution of inert gas trapped on the anodes of magnetron sputter ion pumps. The results are consistent with a pumping mechanism involving energetic neutrals released from the cathode rod.The major pumping action in the magnetron pump for inert gas is shown to occur by a mechanism involving ion burial at the cathode end plates.The balance achieved between the two inert-gas pumping mechanisms influences pumping performance and the implications of this result are discussed in relation to other types of sputter ion pump.

STABLE, LONG LIFE cw EXCITATION OF HELIUM-CADMIUM LASERS BY dc CATAPHORESIS

A technique for maintaining a spatially uniform and optimum concentration of cadmium vapor in a helium—cadmium laser discharge tube is described. Cadmium vapor, continuously fed in at the anode end of the discharge tube, flows by cataphoresis to the cathode end where it is condensed on a cold surface.

Surface topology changes during electromigration in metallic thin film stripes

Electromigration damage in silver and aluminum thin film stripes was observed by scanning electron microscopy. Early formation of voids and surface growths occured in regions of maximum temperature and in the same cross-section. Once formed, void damage was comparatively stable, and failure ultimately occurred at the cathode end of the stripes. The growths in the aluminum stripe were nucleated at grain boundaries and were restricted to the boundary. The flux of atoms into one growth was approximated to be 2 X 10 8 atoms h -1. Growths in the silver stripe were not restricted by the grain size but had the appearance of surface undulations. Large protrusions appeared in coincidence with a catastrophic upswing in stripe resistance. It is felt that local failure of a thin oxide and subsequent “extrusion” may be responsible for this behavior.

Das toroidale schwachionisierte Magnetoplasma I

In a previous paper31 discrepancies between theory and experiment were found on investigating the positive column in a curved magnetic field. The approximation derived in 31 for the torus drift in a weakly ionized magnetoplasma is therefore checked here (Part I) with a refined theory which also yields the transverse electric field strength. Experimentally, both the transverse electric fields and the density profiles in the DC discharge were determined in addition to the longitudinal electric field strength. The discrepancies occurring in 31 are ascribed to the fact that the plasma concentrates at the cathode end of the magnetic field coils, this effect having a considerable influence on the form of the transverse density profile and on the stability behaviour. Part II later will show how the influence of this concentration can be eliminated and what effect in the current-carrying toroidal plasma causes a marked reduction of the charge carrier losses.

Studies of Cataphoresis and Electrophoresis—Some New Effects

A study is made of the rate of transport of Hg in a uniform positive column in a mixture of a rare gas and Hg vapor. In one case, the phenomenon of retrograde cataphoresis is encountered, that of Xe+Hg at currents above 1 A, in which the Hg is carried to the anode end of the tube. In all other cases, the Hg is transported to the cathode end. A pulsating discharge is described in which the color changes from the red of Ne to the blue-white of Hg and back again within a period of 1–2 sec. The boundary between the two colors travels along as a wave motion. This phenomenon is related to the properties of the tube walls and the nature of the ions striking them. When an ac discharge is passed through a cataphoretic mixture, the trace gas of lower ionizing potential is urged from the ends toward the center of the tube, e.g., Hg in Ar, Ne and Kr, but from the center to the ends in He+Hg. With a trace of Ar in He, the Ar is urged toward the center of the lamp. With Xe+Hg, the Hg may go either to the center or to the ends, depending upon conditions. The phenomenon is believed to be due to traveling striations which start out from the anode and progress only part way toward the cathode. Such a striated discharge pumps Hg faster than a steady discharge in the case of Ar, Kr, and Ne, but apparently slower in the case of He. Attempts to demonstrate electrophoretic effects in a straight sealed-off tube failed. However, the positive pressure effects are quite striking in a tube with an external return path.

Determination of Serum Haptoglobin Levels by Cellulose Acetate Electrophoresis

Basic conditions for the application of cellulose acetate electrophoresis to the quantitation of serum haptoglobin levels were investigated and a convenient method was established. This method consisted of addition of hemoglobin into serum to a final concentration of 300mg/dl, and applying a 4μl aliquot of the serum on a strip of cellulose acetate mambrane at 1/3 from the cathode end. Electrophoresis was carried out at 0.5mA/cm and at 110-140V for 1 hour in a 0.05M phosphate buffer, pH 7.0. After drying the membrane, it was stained with o-dianisidine for 10 minutes and washed with distilled water for an additional 10-minute period. Densitometric scanning was carried out at 430mμ. When the concentration of added hemoglobin was less than 300mg/dl, the densitometric reading of free hemoglobin was found to be almost always 1.25 times that of haptoglobin-hemoglobin complex.It was, therefore, necessary to correct densitometric data by the following equation in order to get haptoglobin levels;Hp(mg/dl)=300×1.25(Hp-Hb)/methemalb.+1.25(Hp-Hb)+free Hb

Use of Cataphoresis to Determine Discharge Parameters

The balance between diffusion and ion migration in dc discharges in certain mixtures of metal vapors and rare gases produces a very sharp line of demarcation between the region containing only the rare gas and the region containing both the rare gas and metal vapor. Although the formation of such a sharp boundary depends in a complex way upon the elastic and inelastic cross sections for electron-atom collisions, it can be used in a simple way for the determination of certain discharge parameters. If a cold spot with a well-defined location is provided near the cathode end of the positive column, the distance from the cold spot to the line of demarcation and the potential drop across the tube are, at a given current and rare gas pressure, accurately reproducible functions of the cold-spot temperature. If these functions are determined, and if the neutral-atom diffusion coefficient and positive ion mobility are known for the metal vapor in the rare gas, a simple theory yields values for electron concentration, electron mobility, and the ratio of electron to ion current as a function of pressure of the metal vapor. Detailed application has been made to a hot-cathode neon-mercury discharge over a range of mercury pressure from 2.5 to 25μ and for a neon pressure of 3 Torr. For a 500-mA discharge n− is about 3.5×1011 cm−3, μ− ranges from 1.4 to 2.5×106 cm2/V sec, and μ−/μ+, from 800 to 1800.

Positive-Ion Trapping in Electron Beams

Residual gas within the envelope of an electron tube forms positive ions by electron collision. Ordinarily these ions are quickly lost to the cathode; however, if they can be held within the beam space, the spreading due to the space charge of the beam might be considerably reduced. A "trapping" electrode, with an appropriate positive potential, placed at the cathode end of the drift tube was believed sufficient to affect trapping. Since this simple method was first proposed, attempts to make the scheme work yielded inexplicable results. A detailed study of the phenomenon reveals that the trapping process is more complex than was originally believed. A qualitative theory is advanced which is consistent with experiment and leads to suggestions for an improved performance. By a proper design of the trapping electrode, transmission through a drift space was improved from twelve per cent to eighty per cent.