A Diffusion Model - Model the diffusion of electrons and holes across a potential barrier in an electronic device as follows. We have black balls (electrons) in urn A and whiteballs (holes) in urn B.
An experimental outcome selects randomly one ball from each urn. The ball from urn A is placed in urn B and that from urn B is placed in A.
Let the state of the process be the number of black balls in urn A. (By knowing the number of black balls in urn A, we know the composition of both urns.) Let denote the state of the process. Find the transition probabilities,