P-N junctions

Now consider what happens if two such systems are brought together and become a single system. Initially, a non-equilibrium situation exists and electrons from one material transfer to the other material. Eventually this process is balanced by another that has the opposite effect. Labeling the systems with the subscripts 1 and 2, we expect the transfer probability from material 1 into material 2 to be proportional to the density of filled states in 1 and the density of vacant states in 2 at a given energy E