This paper discusses in detail the shape of the potential barrier existing between two parallel‐plane metal electrodes separated by a thin insulating film. The emission‐limited current flow between the electrodes is determined for symmetric and asymmetric junctions. The asymmetricJ‐V(current‐voltage) characteristic is of particular interest, as it can be shown that the difference in work function of the two electrodes comprising the junction can be obtained from a perfunctory study of the characteristic. The thermionicJ‐Vcharacteristic is compared to the tunnelJ‐Vcharacteristic. For a temperature of 300°K, and for a barrier thickness less than 40 Å, the tunnelJ‐Vcharacteristic predominates. However, for barrier thickness greater than 40 Å, either the thermionic or the tunnel characteristic can predominate, depending upon the barrier height and the applied voltage. In the case of asymmetric junctions, there may be a reversal of direction of rectification with decreasing temperature. This reversal of rectification may explain the similar effect sometimes observed in tunnel junctions.