Explanation
The energy band of the electrons in the chemical bond (such as covalent bond) in a semiconductor. The valence band is composed of all the quantum states of the bonding electrons. At ideal thermodynamics zero, the valence band is filled with bonding electrons. At a certain temperature or under external excitation, the electrons in the valence band can be excited to the energy level in the conduction band or forbidden band, leaving vacancy of electrons. The vacancy of these electrons can be described by imaginary particle "holes". The hole is positively charged and can be regarded as a particle with effective mass.
Classification
Information materials -> Photosensitive and display materials -> Physical basis