A detour transition between thermal field emission and space charge limited regimes

Child-Langmuir law theoretically predicts the transition from thermionic electron emission to space charge limited (SCL) current density across a diode gap. In this work, we reveal a detour transition between thermal-field electron emission and SCL current regimes by tuning influential parameters. The effects of the cathode temperature T, local field enhancement factor beta, and applied electric field E on the transition behaviors are evaluated based on particle-in-cell simulations. The transmitted current density dominated by either thermionic or field emission shows good agreement with the theoretical prediction whereas the deviations from the SCL current during the transition regions are captured. A backward absorption of emitted electrons is explicitly identified in the thermionic regime when the electric field is reversed. In addition, the boundaries corresponding to the SCL regimes are demonstrated with different combinations of the tuning parameters ( T , beta , E).