Currently a lot of electron emission experiments both in carbon nanotubes with top curvature radius <1 nm, and in emitters as short as 10(3) nm with top curvature radius of some tens nanometer are carried out. Experimental results can be explained by a tunnel emission through potential barrier in vacuum near the solid-state cathode border making use of Fowler-Nordheim law. However, in case of cathodes with radius of top curvature less than 100 nm this law is not valid. To proof this statement, the potential of an autoemission diode consisting of a tip cathode located on a flat metal base and flat anode, its dependence on the shape, height of the tip as well as voltage drop on it were calculated in this paper. It has been shown that with cathode tip height and its radius of curvature decreasing (<= 100 nm) the transparency of the potential barrier depends non-linearly on the anode voltage in the Fowler-Nordheim coordinates when the cathode tip height decreases from 1000 down to 150 nm. To obtain a measurable autoemission current, work function must not exceed the uniform field's potential drop between cathode top and its base. Deduced is the analytic formula for the electric field potential that extends approximation features for real cathodes shapes and enables more accurate electric field modeling at the surface. Crown Copyright (C) 2010 Published by Elsevier Ltd. All rights reserved.