Abstract :
The mechanisms of current through insulators in the presence of surface potential barriers and electron traps are discussed, and the simplified theory of space-charge-limited current through insulating solids is described. The steady-state equations for one-dimensional single-carrier current are derived and approximate solutions are given; theoretical current/voltage characteristics are calculated and indicate that large currents could flow and that their characteristics could be utilized in practical devices. The continuity equations for time-varying current are derived for the case when electron transit times and trapping relaxation times are both significant. Experiments are described which have demonstrated some of the predicted characteristics of space-charge-limited current; in particular a threshold voltage for current, a square-law dependence of current upon applied voltage, and rectification have been observed. The currents are of practical magnitudes; steady and reproducible current densities of several amperes per square centimetre have been drawn through insulating crystals of cadmium sulphide at room temperature with a few volts applied. Measurements with microsecond pulses have shown that there is no detectable delay by the crystal in responding to a pulse rise or fall time of the order of 0.1 microsec; there seems no fundamental reason why response times as short as 0.1 milli-microsec should not be expected. It is apparent that many new types of solid-state device may be developed by exploiting current in insulators. Some possible applications are discussed, including a dielectric triode which should combine the high input impedance of the vacuum triode with the high current output of the transistor and have a gain-bandwidth product as great as that of either device. The technical difficulties involved in developing space-charge-limited dielectric devices seem no greater than those successfully overcome in developing presently available semiconductor - - devices.
