Carrier transport across metal-semiconductor barriers has been studied theoretically and experimentally to give a generalized and quantitative presentation. The thermionic and tunneling processes have been analyzed in terms of accurate quantum transmission coefficients. The effects of image-force lowering, temperature, and two-dimensional statistical variation of impurity concentration have also been incorporated in the theory. Theoretical results give a description of the current transport, due to combined effect of tunneling and thermionic emission over a temperature range from essentially absolute zero to the highest practical temperatures, and over doping densities from 10 14 cm -3 to complete degeneracy. An interesting result of the analysis is the existence of a minimum in the saturation current density Js near 10 16 cm -3; the current density rises slightly at lower dopings because of enhanced transmission coefficient for thermionic emission and increases drastically at higher dopings because of tunneling. For example for PtSiSi system at 300°K with a barrier height of 0.85 eV, Js is 80 nA/cm 2 at 10 14 cm -3, reaches a minimum of 60 nA/cm 2 at 10 16 cm -3, then rapidly increases to 10 3 A/cm 2 at 10 20 cm -3. In the high doping range the average saturation current density is considerably increased by the effect of two-dimensional impurity variation. The room-temperature transition doping for breakdown in metal-silicon systems occurs at 8×10 17 cm -3; for lower dopings the breakdown is due to avalanche multiplication, and for higher dopings it is due to tunneling of carriers from the metal Fermi level to semiconductor bands. The metal-silicon diodes were fabricated by planar technology with guard-ring structures to eliminate edge effects. Extensive experimental studies, including current-voltage, capacitance-voltage, and photoelectric measurements covering the doping range from 10 14 to 10 20 cm -3 and the temperature range from 77°K to 373°K, gave good agreement with theoretical predictions.