  We present a theoretical understanding of the superconducting phase diagram
of the electron-doped iron pnictides. We show that, besides the Fermi surface
nesting, a peculiar motion of electrons, where the next nearest neighbor
(diagonal) hoppings between iron sites dominate over the nearest neighbor ones,
plays an important role in the enhancement of the spin fluctuation and thus
superconductivity. In the highest $T_c$ materials, the crossover between the
Fermi surface nesting and this "prioritized diagonal motion" regime occurs
smoothly with doping, while in relatively low $T_c$ materials, the two regimes
are separated and therefore results in a double dome $T_c$ phase diagram.
