  Metal-semiconductor contacts are a pillar of modern semiconductor technology.
Historically, their microscopic understanding has been hampered by the
inability of traditional analytical and numerical methods to fully capture the
complex physics governing their operating principles. Here we introduce an
atomistic approach based on density functional theory and non-equilibrium
Green's function, which includes all the relevant ingredients required to model
realistic metal-semiconductor interfaces and allows for a direct comparison
between theory and experiments via I-V bias curves simulations. We apply this
method to characterize an Ag/Si interface relevant for photovoltaic
applications and study the rectifying-to-Ohmic transition as function of the
semiconductor doping.We also demonstrate that the standard "Activation Energy"
method for the analysis of I-V bias data might be inaccurate for non-ideal
interfaces as it neglects electron tunneling, and that finite-size atomistic
models have problems in describing these interfaces in the presence of doping,
due to a poor representation of space-charge effects. Conversely, the present
method deals effectively with both issues, thus representing a valid
alternative to conventional procedures for the accurate characterization of
metal-semiconductor interfaces.
