Application of the image-force model to the theory of contact rectification and of rectifier breakdown

Abstract

The assumptions underlying the current diffusion theory of rectification are critically examined. By replacing the familiar step-like potential distribution at the rectifying junction by one with a more gradual rise in potential, the theory is extended to cover the range of high inverse voltages, where existing theories fail. On the application of an inverse voltage the height of the barrier is reduced and the leakage current increases. The subsequent rise in temperature in turn causes a further increase in current. With sufficiently high inverse voltages this process can eventually lead to the onset of intrinsic conduction or to thermal instability long before the barrier has been reduced completely. By way of example, the theory is applied to the selenium plate rectifier. It is found possible to choose a consistent set of parameters giving an inverse current/voltage characteristic in good agreement with experiment. Hysteresis as well as a number of certain phenomena observed on breakdown can be explained on the assumption of weak spots in the barrier. The theory predicts the onset of thermal instability with reasonable accuracy. As this instability is one of the limiting factors of rectifier operation it is shown that the choice of materials which make up useful rectifying junctions is restricted to those combining a reasonably large barrier height with a fairly low electronic conductivity of the semi-conductor.