Observation of apparent compensation behavior in thermionic electron emission

A study of the time dependent variations of the thermionic parameters governing the emission from integrated thermionic devices is presented. The emission from these emitters is characterized by an initial peak followed by a smooth monotonic region until finally there is a precipitous drop in emission signalling the end of life. Throughout life a strong compensating behavior is observed between changes in the prefactor A and the workfunction Φ. Each cathode follows a compensation curve of the form InA = I + γΦ where I and γ are constants which depend on the operating temperature. As a result there is a characteristic or isokinetic temperature (Tc= 1/kγ) associated with these emitters which determines the emission characteristics. End of life occurs when the cathode deviates from the compensation curve such that increases in Φ are no longer compensated by a corresponding increase in A. Using the above relationship and the observed time dependence of both A and Φ it is possible to predict the useful life of these cathodes. The implication of this behavior are discussed in terms of a simple physical model.