We have investigated the contact resistivity of Co/Si contact systems with the implantation of B+ ions in the doping range from 3 X 10(14) to 1 X 10(16) cm(-2). The concentrations of electrically activated B atoms are limited by the solubility in Si substrates for the doses above 5 X 10(15) cm(-2) at annealing temperatures of 900-1100 degrees C and the measured contact resistivities are also ruled by those concentrations. The contact resistivities in the high impurity concentration are theoretically calculated based on a tight-binding model in consideration of the formation of impurity bands and band edge tails. The calculated results are in good agreement with the measured values, and a minimum contact resistivity is obtained to be 1.3 X 10(-7) Omega cm(2) for samples with a dose of 1 X 10(16) cm(-2).