A unified model of the oxide cathode which gives prominence to the role of localized surface states on small crystals

Abstract

A unified model of the oxide-coated cathode is suggested which attempts to combine the essential features of Nergaard's single-crystal model with those of Loosjes and Vink's pore-conduction model. The most important characteristic of the new model is that it regards the cathode as consisting of many small crystals (1 μm) whose surface properties predominate over their bulk properties. A consideration of the effects of localized impurity levels on such small crystals shows that they could produce a depletion layer which would have a depth varying from 10% to 100% of the mean size of the crystals. The band bending caused by these localized levels could also raise the work function of the crystals by amounts ranging from 0·05 to 1 v. This surface depletion layer offers an alternative hypothesis to the usual donor depletion layer, which had been assumed to be caused by electrolysis in the single-crystal model. The effects of these two types of depletion layer probably supplement each other in the decay of pulsed emission currents from the cathode. The surface depletion layer would predominate in a well-activated cathode, but field penetration effects would be important if it were poorly activated or in a high electric field. A large part of the activation process, and probably the whole of the process of the poisoning and recovery of a cathode's emission, can be accounted for simply in terms of changes in the concentrations of surface states.