= 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.
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