Potentiostatic electrochemical, ellipsometric, and etch‐rate measurements have been made on boron‐ and phosphorus‐doped {100} and {111} silicon wafers in order to study the well‐known etch‐stop phenomenon observed when using orientation‐dependent etching solutions. The current‐voltage curves indicate that as the doping density increases, the separation of the open‐circuit potential and the passivation potential decreases sharply near a doping density of 1019 cm−3. These data indicate that a prepassive monolayer forms at open‐circuit conditions promoted by the high density of dopant impurities. A model for this etch‐stop phenomenon is proposed.