N. Klein

Current runaway in insulators affected by impact ionization and recombination

When impact ionization occurs, current runaway can arise in insulators, due to a regenerative effect between the rate of ionization and the rate of current injection at the cathode. Solving a set of conduction equations analytically, the critial field for current runaway ?r was determined as a function of insulator properties, when the effect of impact ionizations is opposed by recombinations. Current transients were obtained by computer solutions for fields both smaller and larger than the critical field ?r. The current transients can be used to determine the ionization coefficient and the recombination cross section. For fields larger than ?r times to current runaway tr were found to decrease quasiexponentially with increasing field.

ac Electrical Breakdown in Thin Silicon Oxide Films

Silicon oxide capacitors were produced by vapor deposition in vacuum on glass slides. On the application of ac voltages in the frequency range 10 to 50 000 cps, single‐hole breakdown, propagating breakdown, and maximum voltage breakdown events were observed. The processes in these breakdown events and the destruction were analogous to those occurring on dc. However, the peak value of the ac maximum dielectric strength, Fam, was found to be 10%–20% larger than the dc maximum dielectric strength Fdm; this is in contrast with the much larger dc strength found for thick insulations. Good agreement was found between observations and between calculated values of Fam and Fam/Fdm. Fam of the samples tested ranged from 1.75 to 6.9 MV/cm. Above 1−10 kc/sec, Fam decreased quasilinearly with frequency due to dielectric loss. At frequencies <10 cps, Fam decreased, probably due to temperature ripple. A small decrease in Fam is also caused by large series resistors.

Thin−film breakdown counter of fission fragments

A new kind of detector of heavy fission fragments is described. The detector is a thin−film capacitor in which electrical breakdowns are nonshorting. The detection is based on the counting of breakdowns which arise when heavy fission fragments hit the capacitor. These breakdowns occur at fields distinctly lower than those due to the application of the field only. The lowering of the breakdown field is discussed considering the thermal or electronic effects for silicon dioxide detectors. The detectors offer immediate information on the magnitude of particle fluxes and may possibly serve to identify the particles.

Electroluminescence at high fields in silicon dioxide

Electroluminescence was observed in thermally grown silicon dioxide films at fields larger than 7 MV/cm. The samples had semitransparent aluminum electrodes. As breakdowns were nonshorting, observations could be extended to fields at which breakdowns occur. The light emission was found to be proportional to the leakage current and the spectrum had a cutoff for energies below 2.5 eV. The quantum efficiency, 10−5–10−4 photon/electron crossing 1000 A of insulation, was polarity independent. The luminescence does not seem to be connected with breakdown, but perhaps with trapping processes.

Current runaway in insulators affected by impact ionization and drift

Current runaway in insulators as affected by impact ionization and drift was treated rigorously by O’Dwyer. The theory did not result in explicit relations for the current runaway field Fr, and approximate relations derived later were based on rather arbitrary assumptions. To avoid these difficulties, in this report an approximate theory is developed which results in simple relations for Fr and for times to breakdown. The Fr values calculated are only a few percent smaller than those obtained with the rigorous theory. The approximate theory is presented for small and large rates of ionization, for the cases of trapping and field ionization, and for steady states and transients. To help with the identification of the breakdown mechanism on the basis of experimental observations, the properties of various breakdown models are compared and tabulated.

Electrical breakdown of aluminum oxide films flanked by metallic electrodes

Breakdown characteristics of anodically grown aluminum oxide films were determined in the range of temperature 4.2–354 K, of thickness 47–187 nm, on change of polarity and for gold and aluminum electrodes. Results could be consistently interpreted with the theory of breakdown due to impact ionization by one type of carrier, presented in the prior paper. The theory ascribes breakdown events to the complex interaction of impact ionization, electron injection, hole trapping, and drift and electron trapping. The effect of trapping was found to be of primary importance, since changes in trapping properties can greatly alter the breakdown properties of aluminum oxide. The wide range of breakdown observations from 3 to 13 MV/cm was explained with changes in trapped electron charge and possibly with change in impact ionization processes, trap to band at low fields, but band to band transitions at high fields. Anomalous breakdown characteristics demonstrate that a trapped electron charge can produce protection fro...

Fission fragment detection by thin film capacitors—I: Breakdown counter

Abstract Heavy-ion detection by solid dielectrics has been so far obtained by chemical etching. In this paper the detection of fission fragments is based on the counting of breakdowns in thin films of solid dielectrics. These films are used as dielectric materials in capacitors with one electrode usually less than 1000 A thick. Breakdowns in such capacitors are non-shorting and can be repeated up to millions of times/cm2 of the capacitor area. The property that makes these capacitors suitable for detection is that fission fragments induce breakdowns at electric fields distinctly lower than those due to the application of field alone. The characteristics of these new detecting systems are described and compared with those of the damage-track detectors.

Fission fragment detection by thin film capacitors—II

Abstract Heavy-ion detection by solid dielectrics has been so far obtained by chemical etching. In this paper the detection of fission fragments is based on the counting of breakdowns in thin films of solid dielectrics. These films are used as dielectric materials in capacitors with one electrode usually less than 1000 A thick. Breakdowns in such capacitors are non-shorting and can be repeated up to millions of times/cm2 of the capacitor area. The property that makes these capacitors suitable for detection is that fission fragments induce breakdowns at electric fields distinctly lower than those due to the application of field alone. The characteristics of these new detecting systems are described and compared with those of the damage-track detectors.

Steady‐state characteristics of a thermally switching cylindrical insulator

Normalized steady‐state temperature and current density distributions and V‐I characteristics were determined for thermally switching cylindrical insulators. The results were obtained by solving the equation of conduction of heat by numerical methods, assuming that the electrical conductivity σ obeys the relation σ3=Ae−B/T. Comparison with the analytical solution of the heat conduction equation for the case that σ2=SeaT shows that the two cases differ considerably. In the latter case, on switching, highly concentrated current filaments are formed at relatively small temperature rises. By contrast, in the first case, filaments either do not form, or are much less concentrated, though the temperature rises are relatively large.

Breakdown mechanisms of thermally grown silicon dioxide at high electric fields

Examination of breakdown observations in oxides roughly thicker than 10 nm shows that the impact ionization‐recombination (IIR) model interprets fast breakdowns on constant voltage tests by the growth of positive charge at the cathode at fields larger than a critical field Fr . Breakdown events of longer duration at fields slightly below the critical field can be observed on constant current tests. Such breakdown cannot be explained by the IIR model. It is suggested that it results from changes produced in the oxide prior to the breakdown by the generation of a large density of electron trap states of the order of 1019 cm−3. The mechanism of such breakdowns has not been identified yet. Resonant tunneling via trap states near the cathode, or degradation of the injection barrier, or transition of the oxide from an insulating to a conducting state may be processes producing breakdown in constant current tests.