The arrangement of the atoms in a solid is broken down into three classifications:
amorphous, polycrystalline, or crystalline. Silicon is the most common
semiconductor material being used today because it the most abundant and
can be found in all three arrangements.
The most common and cheapest arrangement is amorphous, which has no
recognizable long-range order. The most common in devices and the most
expensive arrangement is crystalline, where you can look at any given section
of the material and you will find the same arrangement in another section.
Polycrystalline semiconductors are in between, they have crystalline parts
that are not aligned with respect to one another.