  Superconductivity in iron pnictides is studied by using a two-orbital Hubbard
model in the large U limit. The Coulomb repulsion induces an orbital-dependent
pairing between charge carriers. The pairing is found mainly from the
scattering within the same Fermi pocket. The inter-pocket pair scatterings
determine the symmetry of the superconductivity, which is extended s-wave at
small Hund's coupling, and d-wave at large Hund's coupling and large U. The
former is consistent with recent experiments of ARPES and Andreev reflection
spectroscope.
