The effect of diffusion on space charge currents in insulators

Abstract

The influence of the thermal diffusion on the current-voltage characteristics i(U) of insulators with ohmic electrodes is generally believed to be negligible. i(U)-curves being linear in U a r e expected at low acting voltages U. Some considerations regarding diffusion (1 to 3) give implicit i(U)-relations only. Weakly non-linear curves a r e measured generally, but also steeply rising, non-linear and diffusion-governed curves a r e reported (4) and discussed using quite different models (5, 6). The present results show a change of diffusion conditions by small temperature differences between the electrodes and a correspondingly large influence on i(U)-curves. Electron injecting (non-transparent) Ga, Al, o r In electrodes were evaporated onto cylindrical and pure ZnS crystals (Id = 10 mm, thickness L = 2/10, 1/2, and 1 mm) being cut from a large bulb in a parallel plate arrangement. The electrode temperatures could independently be stabilized and controlled at equal o r different values between 18 and 30 OC. The samples were mounted into a vacuum vessel. Fig. 1 , curve a , shows an i(U)-curve between 0 . 2 and 2.5 V in small steps, measured with both electrodes at 20 C. Curve b shows a characteristic of an equally sized sample with a constant temperature of 30 C of the positive electrode whereas the negative electrode is kept at 20 OC. If in contrast to this the negative electrode is kept at 30 C and the positive one at 20 C, curve c is found.The results a r e reproducible at identical conditions besides small deviations. This is also the case, if the measurements were made using different but equally sized samples in order to exclude any effects of formerly trapped charges. These experiments were repeated using other temperatures. There is a close correspondence of the results for crystals of different L. Between 0 . 2 and 1 eV i is proportional to L . Curve b is only observed if the positive electrode injects more charge than the negative one on thermal or any other reasons. If the larger space charge cloud is located at the positive side, a reduced current is observed folluwed 0 0 0 0