Space charge as a source of flicker effect

The capacitive effect of space charge in a space-charge-limited or saturated diode is well known, but it has not been taken into account in forming a theory of the fluctuations. The capacitive current evoked from the space charge by the fluctuations in anode voltage consists of the motion of free electrons and is therefore itself subject to fluctuations. By including these it is found that three distinct types of fluctuation exist together. One is the shot noise, depending in the well-known way on the effect of space charge in controlling the anode current. The second is an enhancement of the shot noise by an amount dependent on the ratio between the capacitance due to space charge and the total circuit capacitance. It is like the shot noise in that the total mean-square voltage is proportional to the resistance of the anode circuit. It is, however, unlike the shot noise in that it varies with anode voltage even when the valve is saturated. The third is a flicker effect dependent on the magnitude of the electronic capacitance, and not requiring the postulation of a life-time of metastable states on the electrodes. In the course of the paper a general form of Campbell´s theorem is given, together with a discussion of the way in which the theoretical considerations must be made to match the experimental conditions. Also, new probability generating functions are put forward and utilized.