Calculation of diode charateristics and proposed characterization of cathode emission capability

Abstract

It is rather common practice to characterize the emission capability of thermionic cathodes by means of the zero-field current density, j 0( T), as determined from the Schottky plot by means of extrapolation towards zero diode voltage. It will be shown that the Schottky part of the characteristics (“saturation” region) - taking into account both space charge and initial velocities - can be accurately represented by an analytic expression. However, for most practical cathodes it shows considerable deviations from experiments. This is due to the fact that the Schottky effect is overruled by other effects, such as TF emission, emission from pores, patch effects and so on. Since the relative contributions of these effects are different for different types of cathodes, their comparison on the basis of j 0 values obtained from the Schottky extrapolation is questionable. This means, for instance, that the ranking of different types of dispernser cathodes - such as the W-Os alloy matrix and the scandate cathode - is not really known. Therefore, another method for determining j 0, based on the above mentioned calculations, will be discussed. This gives rise to a proposal for characterizing the emission capability of a cathode.