The doping-density dependence of scanning tunneling spectroscopy on lightly doped hydrogen-terminated Si(100) (resistivities in the range of 0.2–12 &OHgr; cm) was investigated in air with and without illumination. The observed doping-density dependence is consistent with a generation model in which the changes in the three-dimensional depletion region, induced by a scanning tunneling microscopy tip, contributes to changes in the concentration of thermally and/or photogenerated carriers in lightly doped samples. These results suggest that scanning tunneling spectroscopy can be used to image variations in dopant density in lightly doped samples. ©1998 American Institute of Physics.