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Valence band

 

 
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Valence band
 
 
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In solid
Solid

A solid object is in the states of matter characterized by resistance to deformation and changes of volume. In other words, it has high values both of Young's modulus and of shear modulus; this contrasts e.g....
s, the valence band is the highest range of electron
Electron

The electron is a subatomic particle that carries a negative electric charge. It has elementary particle and is believed to be a point particle....
 energies
Energy

In physics, energy is a scalar physical quantity that describes the amount of Work_ that can be performed by a force. Energy is an attribute of objects and systems that is subject to a conservation law....
 where electrons are normally present at absolute zero
Absolute zero

Absolute zero is a temperature marked by a 0 entropy configuration. It is the coldest temperature theoretically possible, and cannot be reached, by artificial or natural means....
.

In semiconductor
Semiconductor

A semiconductor is a material that has electrical conductivity between those of a Electrical conductor and an electrical insulation; it can vary over that wide range either permanently or dynamically....
s and insulator
Electrical insulation

An insulator, also called a dielectric, is a material that resists the flow of electric current. An insulating material has atoms with tightly bonded valence electrons....
s, there is a band gap
Band gap

In solid state physics and related applied fields, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states exist....
 above the valence band, followed by a conduction band
Conduction band

In the physics field of semiconductors and Electrical insulations, the conduction band is the range of electron energy, higher than that of the valence band, sufficient to make the electrons free to accelerate under the influence of an applied electric field and thus constitute an electric current....
 above that. In metal
Metal

In chemistry, a metal is a chemical element whose atoms readily lose electrons to form positive ions , and form metallic bonds between other metal atoms and ionic bonds between nonmetal atoms....
s, the conduction band has no energy gap separating it from the valence band (basically, this is correct only for semimetals. All solids have forbidden energy levels between the energy bands). The rest of this article refers to the valence band in semiconductors and insulators.

Semiconductors and insulators owe their low conductivity
Electrical conductivity

Electrical conductivity or specific conductance is a measure of a material's ability to electrical conduction an electric current. When an electrical potential difference is placed across a conductor, its movable charges flow, giving rise to an electric current....
 to the properties of the valence band in those materials.






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In solid
Solid

A solid object is in the states of matter characterized by resistance to deformation and changes of volume. In other words, it has high values both of Young's modulus and of shear modulus; this contrasts e.g....
s, the valence band is the highest range of electron
Electron

The electron is a subatomic particle that carries a negative electric charge. It has elementary particle and is believed to be a point particle....
 energies
Energy

In physics, energy is a scalar physical quantity that describes the amount of Work_ that can be performed by a force. Energy is an attribute of objects and systems that is subject to a conservation law....
 where electrons are normally present at absolute zero
Absolute zero

Absolute zero is a temperature marked by a 0 entropy configuration. It is the coldest temperature theoretically possible, and cannot be reached, by artificial or natural means....
.

In semiconductor
Semiconductor

A semiconductor is a material that has electrical conductivity between those of a Electrical conductor and an electrical insulation; it can vary over that wide range either permanently or dynamically....
s and insulator
Electrical insulation

An insulator, also called a dielectric, is a material that resists the flow of electric current. An insulating material has atoms with tightly bonded valence electrons....
s, there is a band gap
Band gap

In solid state physics and related applied fields, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states exist....
 above the valence band, followed by a conduction band
Conduction band

In the physics field of semiconductors and Electrical insulations, the conduction band is the range of electron energy, higher than that of the valence band, sufficient to make the electrons free to accelerate under the influence of an applied electric field and thus constitute an electric current....
 above that. In metal
Metal

In chemistry, a metal is a chemical element whose atoms readily lose electrons to form positive ions , and form metallic bonds between other metal atoms and ionic bonds between nonmetal atoms....
s, the conduction band has no energy gap separating it from the valence band (basically, this is correct only for semimetals. All solids have forbidden energy levels between the energy bands). The rest of this article refers to the valence band in semiconductors and insulators.

Semiconductors and insulators owe their low conductivity
Electrical conductivity

Electrical conductivity or specific conductance is a measure of a material's ability to electrical conduction an electric current. When an electrical potential difference is placed across a conductor, its movable charges flow, giving rise to an electric current....
 to the properties of the valence band in those materials. It just so happens that the number of electrons is precisely equal to the number of states available up to the top of the valence band. There are no available states in the band gap. This means that when an electric field
Electric field

In physics, the space surrounding an electric charge or in the presence of a time-varying magnetic field has a property called an electric field ....
 is applied, the electrons cannot increase their energy (i.e., accelerate) because there are no states available to the electrons where they would be moving faster than they are already going.

There is some conductivity in insulators, however. This is due to thermal excitation—some of the electrons get enough energy to jump the band gap in one go. Once they are in the conduction band, they can conduct electricity, as can the hole
Electron hole

An electron hole is the conceptual and mathematical opposite of an electron, useful in the study of physics and chemistry. The concept describes the lack of an electron....
 they left behind in the valence band. The hole is an empty state that allows electrons in the valence band some degree of freedom.

It is a common misconception to refer to electrons in insulators as "bound"—as if they were somehow attached to the nucleus and could not move. Electrons in insulators are free to move. They are also delocalized, having no well-defined position within the sample.

See also

  • Semiconductor
    Semiconductor

    A semiconductor is a material that has electrical conductivity between those of a Electrical conductor and an electrical insulation; it can vary over that wide range either permanently or dynamically....
     for a full explanation of the band structure of materials.
  • Electrical conduction
    Electrical conduction

    Electrical conduction is the movement of electric charge particles through a transmission medium . The movement of charge constitutes an Current ....
     for more information about conduction in solids, and another description of band structure.
  • Conduction band
    Conduction band

    In the physics field of semiconductors and Electrical insulations, the conduction band is the range of electron energy, higher than that of the valence band, sufficient to make the electrons free to accelerate under the influence of an applied electric field and thus constitute an electric current....
  • Band Theory
    Electronic band structure

    In solid-state physics, the electronic band structure of a solid describes ranges of energy that an electron is "forbidden" or "allowed" to have....
  • Fermi sea
    Fermi liquid

    Fermi liquid is a generic term for a quantum mechanics liquid of fermions that arises under certain physical conditions when the temperature is sufficiently low....


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