Current-voltage characteristics of tunnel contacts to n(+)-Si(100) surfaces are modified through covalently bonded molecules. Scanning tunneling microscopy is used to characterize tunnel contacts to clean, hydrogen terminated and styrene terminated surfaces. The clean surface has pinned energy bands by silicon pi* surface states resulting in an absence of recti. cation. Passivation with hydrogen removes band pinning and allows minority and majority carrier transport and a 1:10 forward/reverse current recti. cation ratio (RRF/R). Styrene also removes band pinning however, it introduces a surface dipole that limits the reverse bias to minority carrier transport and an opposite RRF/R of 10:1 is observed. Crown Copyright (C) 2009 Published by Elsevier B. V. All rights reserved.