Transition from the regime of thermionic emission to the space-charge limited current regime under strong Shottky effects

Abstract

We have examined the transition from the thermionic to the space-charge limited current (SCLC) regime under strong Schottky effects as the electric field increases. We have used the methodology of Lau et al. for their study of the transition from the field emission (FE) to the SCLC regime. We have obtained current–voltage, I–V, characteristics, exhibiting both an exponential and a power-law part signifying the presence of the transition. We have found that this transition does not occur at all temperatures but there is a range of temperatures and work functions for which it takes place. Furthermore, the diode width plays a crucial role. As the field is increased, the emitting material may enter first the FE regime before it transitions to the SCLC regime. The condition that the emitter remains within the thermal regime before it transitions to the SCLC regime places restrictions on the operating temperature T and work function W of the emitter. However, we observe the thermionic to the SCLC transition for all thermionic diodes with work functions 1 eV ≤ W ≤ 2 eV (which includes all modern thermionic cathodes), but the diode widths must be in a specific (but extended) range which includes the nm range.