Why are metalloids sometimes called semimetals?

A semimetal is a material in which there is a small bandgap between the top of the conduction band and the bottom of the valence band for the material. This makes it possible for electrons to make the jump and for the material to conduct with the addition of smaller amounts of energy than for nonmetals.

This may be why you believe are sometimes called semimetals. As it turns out, the terms are usually not used interchangeably, with the most common referred to as semimetals being arsenic, antimony, bismuth, tin, and graphite. Of these, only arsenic and antimony are metalloids.

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Good explanation, nice picture on this page http://chemistry.about.com/od/elementgroups/a/metalloids.htm