Tunneling measurements were taken on a hydrogen terminated, 2 X 1 reconstructed, Si(100) surface, formed by an in situ passivation technique. I-V characteristics on this surface are shown to be sensitive to the electronic structure at atomic length scales. Tunneling measurements across a pn junction clearly delineate a transition of width in close agreement with that predicted by process and device simulators. In contrast, a scanning capacitance microscopy profile on the same sample exhibits a significantly larger transition width. (C) 2004 American Vacuum Society.