Study of a Metal-Dielectric-Metal System as a Current Source with High Internal Resistance
Abstract Generated abstract
This paper reports experimental observations of self-generated current in closed circuits containing metal, dielectric or high-resistance semiconductor, and metal structures without an applied external voltage. Measurements on salts, glass, cadmium sulfide, organic semiconductors, and related samples show that the current rises sharply above material-dependent temperatures, depends on layer thickness and resistance, and can be compensated by an external voltage of about 0.5 to 1.5 V. The author argues that the effect is unlikely to be ordinary electret decay or thermoelectric response, since its temperature, humidity, and gradient behavior do not match those mechanisms. A possible explanation is proposed in terms of oxidation-reduction reactions involving oxygen and hydrogen from the surrounding medium, supported by reduced current under high vacuum and visible changes at metal-glass contacts during storage.
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STUDY OF A METAL–DIELECTRIC–METAL SYSTEM AS A CURRENT SOURCE WITH HIGH INTERNAL RESISTANCE
S. A. ZANDMAN
(Presented by the scientific seminar of the Research Institute of Electron Introscopy)
In studying the temperature dependence of the electrical conductivity of a metal–dielectric–metal (MDM) system and a metal–high-resistance semiconductor–metal (MHSM) system, the appearance of a current in the closed circuit sample–measuring instrument was observed in the absence of an external supply voltage. Measurements were carried out on dielectrics (sodium bromide, sodium chloride), semiconducting glasses, cadmium sulfide, and certain organic semiconductors.
The sample was placed in a shielded closed volume and heated. The experimental results showed that, for all the samples studied, the magnitude of the current, beginning at a certain temperature, increases sharply. The temperature at which the intensive increase in current begins is determined by the nature of the sample under study. Moreover, the higher the specific resistance of the sample, \(10^{14} \div 10^{16}\ \Omega\cdot\text{cm}\), the more the observed phenomenon shifts into the region of higher temperatures. Thus, for glass the current increases sharply at a temperature of \(60^\circ\text{C}\), and for sodium chloride at \(150 \div 180^\circ\text{C}\) (Fig. 1). For glass samples with a specific resistance of \(10^8 \div 10^{10}\ \Omega\cdot\text{cm}\), a current can be recorded at room temperature. In this case the magnitude of the current depends on the thickness of the intermediate layer of dielectric or semiconductor and increases as the latter decreases.
Fig. 1. Dependence of current on temperature:
1) Ag—glass—Al, 2) Ag—NaCl crystal.
Complete compensation of the observed current occurs at an external voltage of \(0.5 \div 1.5\ \text{V}\) from the power source. The magnitude of the compensating voltage, which determines the EMF at the metallic electrodes, depends on the resistance of the intermediate layer and of the MDM and MHSM structures.
and almost does not change with increasing temperature (Fig. 2). The results presented in Fig. 2 were obtained on NaCl single crystals, on the opposite faces of which metallic electrodes were deposited by thermal evaporation. Analyzing the curves, it can be asserted that, with increasing temperature, the internal resistance of the samples decreases. A clear reproducibility of the results is observed in repeated temperature tests.
Fig. 2. Dependence of the source current on the compensating voltage for samples:
1 — 100° C; 2 — 140° C;
3 — 148° C; 4 — 155° C;
5 — 162° C.
The prolonged existence of the observed current at room temperature is of interest from the point of view of the nature of its occurrence. It may be assumed that we are dealing with an electret structure [1]. However, upon heating, an electret loses its internal polarization and surface charge and, beginning at a certain temperature of thermal decay, the process sharply intensifies; the current decreases, which is in direct contradiction with the results obtained. In addition, the surface charge density of an electret falls to zero with increasing humidity, whereas the observed current increases sharply in the presence of moisture. Two phenomena may possibly occur: at room temperature the current is caused by a chemical reaction, while with increasing temperature a thermo-emf arises due to a possible temperature gradient along the sample. However, experimental verification showed that even with a specially created temperature gradient of 100° C along a glass sample, the current is two orders of magnitude smaller than at the same ambient temperature.
Apparently, the appearance of current in a closed circuit of MDM and MVPM structures may be caused by an oxidation-reduction reaction due to oxygen and hydrogen of the surrounding medium [2].
This assumption is confirmed by a considerable decrease in the source current when measured in a vacuum of \(10^{-7}\) mm Hg. In addition, during prolonged storage of MVPM samples made of glass in air, the metallic electrodes lose their luster and the color of the contacting metal—glass surface changes.
It should be noted here that the smaller the degree of ionicity of the bonds in a substance [3], the greater the magnitude of the observed current. Thus, at room temperature, an MDM structure made of an NaCl crystal gives a current of \(10^{-14}\) A, whereas an MVPM based on CdS generates a current of \(10^{-10}\) A. All this indicates the complexity of the observed phenomenon and requires further investigation.
REFERENCES
- O. Ya. Myazdrikov, V. E. Manoilov. Electrets. Moscow, Gosenergoizdat, 1962.
- V. A. Orlov. Small-Sized Current Sources. Moscow, Voenizdat, 1965.
- S. S. Batsanov. Electronegativity and Effective Charges of Atoms. Moscow, “Znanie,” 1971.