Field emission from diamond and cubic boron nitride coated emitters has been investigated by field emission energy distribution (FEED) analysis. In order to study the mechanisms governing the emission process, we developed a modified FEED technique that consisted of measuring electron energy spectra as a function of the extraction voltage applied to the field emitter. This technique has led to a detailed study of band bending effects due to field penetration. Voltage drop across the emitting, wide-band-gap layers (thickness typ. 0.5 mu m) was in the order of 1% of the applied extraction voltage. The observed FEED spectra and their dependence on the extraction voltage were explained in terms of a simplified band model. In nominally undoped diamond and cubic boron nitride samples, field emission was found to originate from the conduction band minimum. Injection of electrons from the metallic back contact into the wide-band-gap material has been identified as the essential limiting factor to the emission current. Mo2C interlayers between Mo back contacts and emitting diamond layers were formed by annealing at 500 degrees C for several hours and led to more emission currents, typically enhanced by more than one order of magnitude.