Space-charge-limited thermionic current

A space-charge-limited thermionic current occurs when the negative space charge of emitted Electrons creates an electric field that obstructs further transport across the gap. Raising the emitter temperature may then increase the surface supply without producing the corresponding collector electric current. The gap geometry and collecting electric potential difference matter even when the surface is stable.

To distinguish this from an emission-limited thermionic current, vary the collecting electric potential difference at fixed emitter temperature, then repeat at another temperature. A voltage-dependent transport region followed by a collection plateau separates a gap constraint from a supply constraint. A weak collector electric current alone cannot establish a high work function. Nor does simply choosing a very large collecting electric potential difference guarantee a valid zero-field measurement: the surface barrier may then change.