Electrical ohmic contacts can be simultaneously formed on silicon carbide (SiC) semiconductors having donor and acceptor impurities (n- and p-type doping, respectively). This implies that such contacts can be formed on SiC layers in one process step during the fabrication of the semiconductor device. This also means that the multiple process steps for fabricating contacts onto n- and p-type surfaces, which is characteristic of the prior art, will be greatly reduced, thereby reducing time and cost, and increasing yield (more process steps and complexity increases chances for lower yields). Another significance of this invention is that this scheme can serve as a non-discriminatory, universal ohmic contact to both n- and p-type SiC, without compromising the reliability of the specific contact resistivity when operated at temperatures in excess of 600 °C.

The supporting theory is based on the use of a combination of material work functions as the basis for non-discriminatory charge transport across the metallurgical junction of the metal and the semiconductor. Generally, in the absence of Fermi level pinning that is largely attributed to surface states, ohmic contact formation is favored when a metal or metal compound having a work function that is lower than the work function of the n-doped (donor-type) semiconductor is deposited on such semiconductor. However, when such metal or compound is deposited on the p-doped (acceptor-type) semiconductor with a greater work function, a rectification occurs. Conversely, a metal or metal conductor with a work function greater than the work function of a p-doped semiconductor favors an ohmic contact formation, but would rectify on the n-doped layer.

Based on the theoretical description of metal-semiconductor charge transport on which this invention is based, it is desired to form ohmic contact simultaneously on both n-type and p-type SiC. By a careful selection of the appropriate combination of metal carbides/silicides with work functions greater than the work function of p-type SiC, and another combination of metal carbides/silicides with work functions less than the work function of n-type SiC, and then mixing both combinations together, simultaneous ohmic contacts on both n- and ptype surfaces would be favored. Metal carbides mixed with silicides were selected for the proof of concept using a mixture of tungsten and nickel, which when annealed (heated) on SiC, would form metal compounds of tungsten carbide, tungsten silicide, and nickel silicide.

This work was done by Robert S. Okojie of Glenn Research Center. NASA Glenn Research Center seeks to transfer mission technology to benefit U.S. industry. NASA invites inquiries on licensing or collaborating on this technology for commercial applications. For more information, please contact NASA Glenn Research Center’s Technology Transfer Office at This email address is being protected from spambots. You need JavaScript enabled to view it. or visit us on the web at https://technology.grc.nasa.gov/ . Refer to LEW-18538-1.

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NASA Tech Briefs Magazine

This article first appeared in the September, 2014 issue of NASA Tech Briefs Magazine.

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