Edge Effect on the Current-Temperature Characteristic of Finite-Area Thermionic Cathodes

We perform a computational study, based on the molecular-dynamics method, of the shape of Miram curves obtained from microscale planar diodes. We discuss the smooth transition from the source-limited regime to the space-charge-limited regime due to the finite size of the emitter (i.e., the ``knee'' in the Miram curve). In our model we find that the smoothing occurs mostly due to the increased emission at the external edges of the emitting area, and that the knee becomes softer when the size of the emitting area decreases. We relate this to recent work that has described how a heterogeneous work function similarly affects the Miram curve.