Based on first-principles calculations and analysis of x-ray absorption measurements, an unconventional mechanism is proposed for the saturation of carriers in highly n -doped Si. The mechanism is Fermi-level pinning from a new class of defect centers containing two separated but interacting dopant atoms with no associated Si vacancies. The number of such centers increases sharply at high doping levels. A simple model provides very good agreement with the maximum carrier concentrations observed in Si. {copyright} {ital 1997} {ital The American Physical Society}