  The large-scale structure of the ionization field during the epoch of
reionization (EoR) can be modeled by the excursion set theory. While the growth
of ionized regions during the early stage are described by the "bubble model",
the shrinking process of neutral regions after the percolation of the ionized
region calls for an "island model". An excursion set based analytical model and
a semi-numerical code (islandFAST) have been developed. The ionizing background
and the bubbles inside the islands are also included in the treatment. With two
kinds of absorbers of ionizing photons, i.e. the large-scale under-dense
neutral islands and the small-scale over-dense clumps, the ionizing background
are self-consistently evolved in the model.
