Question

The band gap of three materials are 0, 1 and 10 eV, respectively. Explain color and conductivity differences when comparing a single crystal to powder of each material.

Answer 1

The band gap is the energy needed to promote an electron from the lower energy valence band into the higher energy conduction band. The band gap in metals is very small. Ordinary thermal energy (present at room temperature) promotes the valence electrons to the conduction band where they move freely about the metal. Therefore, metals are called "conductors" because they easily carry current. In nonmetals the band gap is very large and electrons seldom have enough energy to cross from the valence level to the conduction band. Therefore, non-metals are called "insulators" because they carry no electrical current. In semi-metals, the band gap is moderately wide, so under certain conditions, electrons are promoted into the conduction band resulting in a current. This is why semi-metals are often referred to as "semiconductors.