Solution to an unsolved problem in diodes: Limiting current for small emission area and low emission energy

Abstract

The maximum current that can be extracted from a diode is a central question in electronic devices, especially for the generation of radiation from microwaves to x-rays. The challenge in its prediction increases significantly when the electron emission is restricted to a small area for which the classical one-dimensional Child-Langmuir law is no longer applicable. We address this unsolved problem using a model that simultaneously includes a small emission area and a small electron emission velocity. New scaling laws are presented for this difficult regime. These results are obtained from three vastly different approaches: a differential equation formulation which provides extremely high resolution, an integral equation formulation which leads to the scaling laws, and a particle-in-cell simulation which shows the temporal-spatial evolution. Comparisons of the predicted maximum current among these three approaches are performed over a large range of parameters, paying special attention to the resolution of the potential minimum in the immediate vicinity of the cathode surface. Corroborations of the scaling laws with experiments on thermionic cathodes and on photoinjectors are indicated. The model consists of a periodic array of electron sheets of finite width in a planar diode, all emitted from a cathode with the same energy. Electron motion is restricted to the direction normal to the cathode.