Question: what is the band gap in a semiconductor

title: Band gap
text: In solid - state physics , a band gap , also called an energy gap or bandgap , is an energy range in a solid where no electron states can exist . In graphs of the electronic band structure of solids , the band gap generally refers to the energy difference ( in electron volts ) between the top of the valence band and the bottom of the conduction band in insulators and semiconductors . It is the energy required to promote a valence electron bound to an atom to become a conduction electron , which is free to move within the crystal lattice and serve as a charge carrier to conduct electric current . It is closely related to the HOMO / LUMO gap in chemistry . If the valence band is completely full and the conduction band is completely empty , then electrons can not move in the solid ; however , if some electrons transfer from the valence to the conduction band , then current can flow ( see carrier generation and recombination ) . Therefore , the band gap is a major factor determining the electrical conductivity of a solid . Substances with large band gaps are generally insulators , those with smaller band gaps are semiconductors , while conductors either have very small band gaps or none , because the valence and conduction bands overlap . 

Answer: answer: In solid state physics, a band gap is an energy range in a solid where no electron states can exist. Therefore, the band gap is a major factor determining the electrical conductivity of a solid. Substances with large band gaps are generally insulators, those with smaller band gaps are semiconductors.