Semiconductor Cold Electron Emission Device

This invention relates to electron emission devices. In particular, the invention is concerned with high efficiency semiconductor cold cathodes which employ new P-N combinations of materials to form heterojunctions.

Considerable work has been done in the field of cold cathodes. Avalanche-type cold cathodes have been constructed of silicon. A discussion of this work is reported in Philips Technical Review, VOL. 43, No. 3, Jan. 1989 by G.G.P. van Gorkom and A.M.E. Hoeberechts. Unfortunately, silicon cathodes suffer from poor efficiency, e.g., .about.1%. Such cathodes reverse bias a semiconductor junction into avalanche and because the excitons, i.e., hole-electron pairs, are generated at varying locations and in a random process, the electrons reaching the surface of the cathode have a bell-shaped energy distribution of which only the tail end of the bell exceeds the work function of silicon and are emitted.

Various schemes have been proposed to use GaAs, AlAs, and alloys thereof to accelerate electrons to energies exceeding the work function of GaAlAs or cesiated GaAlAs. These approaches, unfortunately, have severe limitations. The cesiated surfaces are extremely fragile in that cesium is a highly reactive element and even minuscule exposure to virtually any contaminant will poison the surface. GaAs has a work function of 4.7 eV necessitating very high "hot" energies of electrons within the GaAs. These high energies cause the upper conduction band valleys to be populated and in so doing, the electrons become heavy and slow down thus greatly defeating the purpose of this approach. The bare GaAlAs surface is also easily poisoned by contaminants.

Diamond is thought to have a negative electron affinity surface and has been shown by M. Kordesch of Ohio University to act as a cold cathode as disclosed in Electronics Letters , 1 Aug. 91, VOL. 27, No. 16. Diamond has the added advantage that its surface, once terminated by hydrogen, is extremely stable and virtually impossible to contaminate. Unfortunately, there are no known impurities with which to reliably and controllably render diamond N-type; damage-induced N-type. activity in diamond is virtually impossible to adequately control.

In experiments by the applicant and others, AlN (aluminum nitride) has recently been shown to exhibit a negative electron affinity surface. AlN and SiC (silicon carbide) have recently been shown to form miscible alloys over the entire mole fraction compositional range and heterojunctions of AlN and SiC have also been demonstrated as have heterojunctions with AlN and AlSiCN. While N-type impurity doping of AlN is difficult because the material must be absolutely free of oxygen, it is possible. A good discussion of some recent work with SiC-AlN mixtures is contained in the "3rd International Conference on Amorphous and Crystalline and Other Group IV-IV Materials", APR 11-13, 1990, G. L. Harris, et al. Additional details regarding it are found in U.S. Pat. No. 4,382,837.

The invention includes cold electron emission semiconductor devices wherein a heterojunction is formed by at least two semiconductors, one of which is P-type material and one of which is N-type material.

By using aluminum nitride (AlN) and silicon carbide (SiC) in semiconductor compositions, a Type I heterojuntion can be formed which overcomes disadvantages of the prior art.

There are many advantages to our emission device. Unlike avalanche type devices, our cold cathode is highly efficient and the energy is monoenergetic. The electron energy of our device can be controlled and modulated unlike single composition negative electron devices such as diamond or AlN. There is no N-type doping problem, no requirement for heat as in a thermonic device , no large, non-uniform concentration of current to cause degradation and no chance of poisoning the emission capability when the device is exposed to oxygen, hydrogen or air.

FIG. 1 is a side view of elements of the invention.

FIG. 2 is an end view of a second embodiment of the invention looking toward the P-type semiconductor material.

FIG. 3 is a graph showing the bandgap energy plotted against the mole composition of a semiconductor mixture of AlN and SiC.

Referring to FIG. 1, there is shown metallic ohmic contact 11, N-type semiconductor material 12 which can be AlN or AlSiCN, P-type semiconductor material 13 which can be SiC or AlSiCN and a metallic ohmic contact 14. The contacts 11 and 14 are continuous with the N-type semiconductor material 12 and the P-type semiconductor material 13, respectively. The junction formed by semiconductor materials 12 and 13 is forwarded biased by an appropriate source shown as 15, which should be large enough to overcome the energy gap between the two semiconductor materials. This would generally exceed 2eV depending on the materials chosen, i.e., the mole fraction employed and the doping of the two semiconductor materials.

As shown in another embodiment in FIG. 2, the metallic ohmic contact 14 can take the form of a metallic grid structure deposited on the surface of the P-type material so that the density of electrons emitted from the P-type material may be modulated or totally secured thereto. In this instance the source 15 could take the form of a pulse or AC source with electron emission occurring only during the forward-biased half cycle.

FIG. 3 shows the band gap which results for various mixtures of AlN and SiC when combined to form an AlSiCN semiconductor. The AlSiCN so formed can then be impurity doped to provide an N-type material or a P-type material for use in our invention. The U.S. Pat. No. 4,382,837 discloses one method of fabricating an AlSiCN mixture and is hereby incorporated by reference into this application. Since the surface of the P-type material exhibits a negative electron affinity at mole fractions equivalent to a band gap of 5.5 eV or greater, these mole fractions have certain advantages.

In its simpliest form the invention uses AlN which has a bandgap of 6.2 eV in its wurtzitic structure and slightly less than that in its cubic form as the N-type semiconductor. Silicon Carbide which has a bandgap of 2.2 eV in its cubic form and nearly 3.3 eV in its 2H wurtzitic form is used as the P-type semiconductor material. AlN and SiC form a conventional type I heterojunction. As it relates to this invention, a type I heterojunction is known in the art as a junction in which the forbidden gap; which is defined as that energy gap not normally occupied by electrons and lying between the valence band and the conduction band; of the P-type material is completely within the forbidden gap of the N-type semiconductor material. The heterojunction offset potential between SiC and AlN is larger than any other known type I heterojunction. Unlike the GaAs/AlAs heterojunction, hot (high energy) electrons injected into the P-type SiC from AlN or into the P-type AlSiCN from AlN or a higher bandgap n-type AlSiCN alloy do not increase their mass and slow down because electron mass in the upper conduction band valley of SiC is actually less than it is in the main valley.

By virtue of the very large heterojunction offset potential between AlN and SiC conduction bands, e.g., up to 2.0 eV, and the very low electron affinity of the AlSiCN surface, e.g., between negative numbers and 1.0 eV depending on mole fraction composition, hot electrons can easily be injected from the N-type AlN into the P-type SiC or AlSiCN layers with energies sufficient to ballistically transit thin, e.g., <100 nanometers P-type SiC or AlSiCN surface layers and yet retain energy sufficient to exceed the electron affinity and work function of the surface. Since these heterojunction offset potentials can be precisely controlled by choice of mole fraction composition as depicted in FIG. 3, the energy of the electrons reaching the surface may be precisely controlled with respect to the vacuum level. This energy is nearly monoenergetic by virtue of the heterojunction offset potential, the absence of significant electron mass increase in the material, and the ballistic nature of the electrons in the P-type surface layer. The term ballistic is well known in the art to mean there is no or very little electron scattering in the thin P-type material. The monoenergetic nature of the emission is of great advantage when employed in vacuum tube amplifiers requiring low noise. For P-type mole fractions exhibiting band gaps exceeding 5.5 eV, the reqirement for ballistic action is eliminated--only minority carrier lifetime dominates efficiency.