Absence of space-charge-limited current from field emission due to non-FN law

With the emergence of non-FN based field emitters, the establish transition from source-limited field emission (FE) at low voltage to space-charge-limited current (SCLC) at high voltage is no longer valid. In this paper, we predict a critical pre-factor field scaling exponent k
c
=3/2, where for emitters with kc
, unconventional transition behaviours could occur, which includes: (i) exclusively FE process with absent of SCLC; and (ii) three-stage FE-to-SCLC-to-FE transition, depending on gap spacing D. While for emitters with k>k
c
, classical two-stage FE-to-SCLC transition is expected. Several existing non-FN based FE models are used as examples in this paper to illustrate the unconventional transition behaviours.