The detailed structure of electron momentum distribution in relativistic diodes is studied. It is proven that in most of the diode region the momentum distribution is isotropic in fluid rest frame, with azymuthal temperature which, in general, is much larger than the cathode temperature. A diode model based on these conclusions is presented. The model allows for a continuum of electron trajectories at each point. In various limits the results are reduced to well‐known cold fluid model results.