Why are metalloids described as semiconductors?

Since they are neither excellent nor bad conductors, metalloids are classified as semiconductors.

Since electrons are the conductors of electricity, metals are good conductors since their valence electrons are not attached to any specific atom and can freely move throughout the solid.

Nonmetals are poor conductors of electricity because their valence electrons are bonded to the molecules and cannot freely move throughout the solid.

Metalloids are defined as metalloids (metal + Greek -oeides like) — metal-like— because they possess the characteristics of both metals and nonmetals. Their electronic structures are in the middle of those of metals and nonmetals, so they conduct electricity better than nonmetals but not as well as metals.

Because of their properties that allow for control over their conductivity, metalloids—which fall between that of metals and nonmetals—are referred to as semiconductors because they are well-suited for use in semiconductor applications.