  A Hubbard-type model is derived from the microscopic Schr\"odinger equation.
We found that additional terms describing direct two-electron transitions must
be added to the standard Hubbard Hamiltonian. Such a Hamiltonian generates
two-electron pairing due to on-site Coulomb repulsion. We demonstrate that the
electron pairs with opposite spin propagate across periodic structures via
direct and sequential two-electron tunneling. This mechanism can be used for a
generation of entangled electron current. Numerical calculations show stability
of the electron pairs.
