  In this paper we study analytically a one-dimensional model for a
semiconductor-metal junction. We study the formation of Tamm states and how
they evolve when the semi-infinite semiconductor and metal are coupled
together. The non-linear current, as a function of the bias voltage, is studied
using the non-equilibrium Green's function method and the density matrix of the
interface is given. The electronic occupation of the sites defining the
interface has strong non-linearities as function of the bias voltage due to
strong resonances present in the Green's functions of the junction sites. The
surface Green's function is computed analytically by solving a quadratic matrix
equation, which does not require adding a small imaginary constant to the
energy. The wave function for the surface states is given.
