Space charge in a vacuum diode: From macroscopic to microscopic gaps

The space charge effect is important in free-electron devices and sometimes plays a key role. A vacuum-compatible micromanipulation platform was devised to study this effect in a diode structure while changing the width of the gap between the emitter and the collector in situ in the range of hundreds of micrometers to tens of micrometers. The current-voltage characteristics were found to upshift with a decrease in the interelectrode distance; the space charge-limited current increased by approximately two orders of magnitude when the interelectrode distance decreased from 550 to 50 mu m. The simplicity of the parallel-plate structure enabled analysis based on one-dimensional emission and transport with the measurement results showing a good fit to the model by Longo combined with the Child-Langmuir theory.