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This paper presents the results of an investigation conducted to understand the formation of very low resistance nonalloyed ohmic contacts to Sn-doped nGaAs grown by molecular-beam, epitaxy (MBE). The measured specific contact resistance is in good agreement with a new thermionic field-emission model presented in this paper that accounts for a number of physical phenomena that are critical in determining the carrier transport mechanisms across the metal-semiconductor interfaces. The experimental and theoretical results are more than two orders of magnitude smaller than previous contact resistance calculations. This discrepancy predominantly occurs because previous contact theories do not account for metal wave functions and include only conduction band effects when calculating the electron wave vector in the semiconductor energy gap. A specific contact resistance as low as .cm2at T = 300 K is obtained for the nonalloyed contacts fabricated in this work.