According to electronic band theory, solids can be classified as insulators, semiconductors, semimetals, or metals. In insulators and semiconductors the filled valence band is separated from an empty conduction band by a band gap. For insulators, the magnitude of the band gap is larger (e.g. > 4 eV) than that of a semiconductor (e.g. < 4 eV). Metals have a partially filled conduction band. A semimetal is a material with a very small overlap between the bottom of the conduction band and the top of the valence band. A semimetal thus has no band gap and a negligible density of states at the Fermi level. A metal, by contrast, has an appreciable density of states at the Fermi level because the conduction band is partially filled.

Lester William Polsfuss (June 9, 1915 – August 13, 2009)—known as Les Paul—was an American jazz and country guitarist, songwriter and inventor. He was the inventor of the solid-body electric guitar which made the sound of rock and roll possible. He is credited with many recording innovations. Although he was not the first to use the technique, his early experiments with overdubbing (also known as sound on sound),delay effects such as tape delay, phasing effects and multitrack recording were among the first to attract widespread attention.

His innovative talents extended into his playing style, including licks, trills, chording sequences, fretting techniques and timing, which set him apart from his contemporaries and inspired many guitarists of the present day. He recorded with his wife Mary Ford in the 1950s, and they sold millions of records.

Among his many honors, Paul is one of a handful of artists with a permanent, stand-alone exhibit in the Rock and Roll Hall of Fame. He is prominently named by the music museum on its website as an "architect" and a "key inductee" along with Sam Phillips and Alan Freed.