Why are metalloids described as semiconductors?
Metalloids are semiconductors because they are neither good nor poor conductors.
The valence electrons of metals are not bound to any particular atom. The electrons are free to move throughout the whole solid. Since the electrons are the conductors of electricity, metals are good conductors.
The valence electrons of nonmetals are bound to the molecules themselves. The electrons are not free to move throughout the solid. Thus, nonmetals are poor conductors of electricity.
Metalloids have the properties of both metals and nonmetals. Their electronic structures are intermediate between those of metals and nonmetals. Thus, they conduct electricity better than nonmetals but not as well as metals. They are metalloids (metal + Greek -oeides like) — metal-like.