Regarding a semiconductor where there is a conduction band & valance band.
From quantum mechanics the energy of electrons in a box is quantified: it can have only discrete values. In solid state materials, where their are many electrons, these discrete energy states are broadened because of the interactions between the electrons. One speaks of energy bands. Any material possesses a band structure that depends on the nature of its constituting atoms, the arrangement of these atoms, the symmetries involved. To change energy an electron must move from one band to another. Photons, for example, can transfer their energy to electrons.
Depending on the nature of the material the various available electrons on the outside skirt of the atoms are filling the various bands in the order of increasing energy. Two situations do occur: 1) the highest energy band is only partly filled, 2) the highest energy electrons do fill the upper energy band. The electrons cannot gain energy in this band that is called the valence band.
In case 1) the highest energy electrons can gain energy inside the upper band in a continuous way: they are free to move. This situation is found in metals and the band is called the conduction band.
In case 2) the highest energy electrons need to undergo a discrete energy jump to the next higher energy band called again conduction band. This energy jump is called the energy band gap. Its value is a signature for materials (see Robert's answer above). If the band gap is less than, say, 3 eV a small amount of energy can promote electrons that are then free to move in the material, one speaks of semi-conductors (silicon, gallium arsenide...). The gap energy is comparable to the energy of the photons in the visible light that can promote electrons: this effect is at the very basis of the photovoltaic generation of electricity.
For larger band gaps one deals with insulators because it is difficult to promote free electrons and the conduction dramatically drops down. This is the case for window glass for which the gap is of the order of 10 eV while the maximum energy of the photons in visible light is less than 5eV: the photons cannot excite electrons, the glass is transparent to light.
The attached file shows the nature of a band gap in a simple way.
From quantum mechanics the energy of electrons in a box is quantified: it can have only discrete values. In solid state materials, where their are many electrons, these discrete energy states are broadened because of the interactions between the electrons. One speaks of energy bands. Any material possesses a band structure that depends on the nature of its constituting atoms, the arrangement of these atoms, the symmetries involved. To change energy an electron must move from one band to another. Photons, for example, can transfer their energy to electrons.
Depending on the nature of the material the various available electrons on the outside skirt of the atoms are filling the various bands in the order of increasing energy. Two situations do occur: 1) the highest energy band is only partly filled, 2) the highest energy electrons do fill the upper energy band. The electrons cannot gain energy in this band that is called the valence band.
In case 1) the highest energy electrons can gain energy inside the upper band in a continuous way: they are free to move. This situation is found in metals and the band is called the conduction band.
In case 2) the highest energy electrons need to undergo a discrete energy jump to the next higher energy band called again conduction band. This energy jump is called the energy band gap. Its value is a signature for materials (see Robert's answer above). If the band gap is less than, say, 3 eV a small amount of energy can promote electrons that are then free to move in the material, one speaks of semi-conductors (silicon, gallium arsenide...). The gap energy is comparable to the energy of the photons in the visible light that can promote electrons: this effect is at the very basis of the photovoltaic generation of electricity.
For larger band gaps one deals with insulators because it is difficult to promote free electrons and the conduction dramatically drops down. This is the case for window glass for which the gap is of the order of 10 eV while the maximum energy of the photons in visible light is less than 5eV: the photons cannot excite electrons, the glass is transparent to light.
The attached file shows the nature of a band gap in a simple way.
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