  Based on experimental data on the newly synthesized iron-based
superconductors and the relevant band structure calculations, we propose a
minimal two-band BCS-type Hamiltonian with the interband Hubbard interaction
included. We illustrate that this two-band model is able to capture the
essential features of unconventional superconductivity and spin density wave
(SDW) ordering in this family of materials. It is found that bound
electron-hole pairs can be condensed to reveal the SDW ordering for zero and
very small doping, while the superconducting ordering emerges at small finite
doping, whose pairing symmetry is qualitatively analyzed to be of nodal d-wave.
The derived analytical formulas not only give out a nearly symmetric phase
diagram for electron and hole doping, but also is likely able to account for
existing main experimental results. Moreover, we also derive two important
relations for a general two-band model and elaborate how to apply them to
determine the band width ratio and the effective interband coupling strength
from experimental data.
