A novel planar cold‐cathode structure has been developed based on a GaAs&sngbnd;(AlGa)As heterojunction and a negative electron affinity GaAs surface. As an important feature of the device, the lateral confinement of the current flow to the desired area of the emitting surface is obtained by the selective diffusion of zinc. Under pulsed conditions, emission efficiencies as high as 4% and emission current densities as high as 7 A/cm2have been achieved. The release of impurities from the anode as a result of electron‐stimulated desorption has been found to be a major factor affecting the cathode life under dc operation. The influence of this factor can be greatly minimized by different techniques such as using low anode voltages and magnetic field deflection.