GRADED ELECTRON-AFFINITY ELECTRON SOURCE

We describe a new electron source using electric field and low electro n affinity semiconductor materials to bring charge to potential energy levels near the vacuum level while still in the solid. The basic idea involves moving some of the potential barrier from the surface to the bulk, and distributing the barrier over a thin layer below the surfac e. In so doing, the emission physics is changed fundamentally from a q uantum mechanical tunneling process largely controlled by surface prop erties to a classical transport process largely controlled by the band structure of a wide bandgap semiconductor. The composition of the thi n layer below the surface would be graded such that the conduction ban d minimum changes from an energy close to the substrate Fermi level to an energy significantly closer to the vacuum level. Electrons from th e substrate would be drawn into the graded composition layer with an e lectric field produced by a pointed emitter structure and extraction g ate similar to that used in field emitter arrays. Relative to a conven tional field emitter array, the new source is expected to require lowe r extraction fields and exhibit improved emission uniformity, stabilit y, and noise. The I-V characteristics are not Fowler-Nordheim and may include a saturation effect at high current densities that could be en gineered to improve reliability and uniformity. The group III nitrides can be grown with a continuous range of composition that provides the range of electron affinity needed to produce the proposed electronic structure. (C) 1996 American Vacuum Society.