Energy bands differ in the number of electrons they hold. In the 1 s and 2 s energy bands, each energy level holds up to two electrons spin up and spin down , so this band has a maximum occupancy of 2 N electrons. In the 2 p energy band, each energy level holds up to six electrons, so this band has a maximum occupancy of 6 N electrons Figure 9.
Each energy band is separated from the other by an energy gap. The electrical properties of conductors and insulators can be understood in terms of energy bands and gaps. The highest energy band that is filled is known as a valence band.
The next available band in the energy structure is known as a conduction band. In a conductor, the highest energy band that contains electrons is partially filled, whereas in an insulator, the highest energy band containing electrons is completely filled. The difference between a conductor and insulator is illustrated in Figure 9.
A conductor differs from an insulator in how its electrons respond to an applied electric field. If a significant number of electrons are set into motion by the field, the material is a conductor. In terms of the band model, electrons in the partially filled conduction band gain kinetic energy from the electric field by filling higher energy states in the conduction band. By contrast, in an insulator, electrons belong to completely filled bands.
When the field is applied, the electrons cannot make such transitions acquire kinetic energy from the electric field due to the exclusion principle. As a result, the material does not conduct electricity. Visit this simulation to learn about the origin of energy bands in crystals of atoms and how the structure of bands determines how a material conducts electricity. Explore how band structure creates a lattice of many wells. A semiconductor has a similar energy structure to an insulator except it has a relatively small energy gap between the lowest completely filled band and the next available unfilled band.
This type of material forms the basis of modern electronics. The only difference is in the size of the energy gap or band gap E g between the highest energy band that is filled the valence band and the next-higher empty band the conduction band.
In a semiconductor, this gap is small enough that a substantial number of electrons from the valence band are thermally excited into the conduction band at room temperature. These electrons are then in a nearly empty band and can respond to an applied field. As a general rule of thumb, the band gap of a semiconductor is about 1 eV. See Table 9. A band gap of greater than approximately 1 eV is considered an insulator. For comparison, the energy gap of diamond an insulator is several electron-volts.
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Crystalline Materials. Amorphous Materials. Reactions and Kinetics. Aqueous Solutions. Organic Materials. Solid Solutions. Modules Electronic Materials Concepts properties of metals and insulators, band theory of solids Drude; Bloch; Heitler and London , band gaps in metals, semiconductors, and insulators Keywords metallic bonding, free electron gas, band gap, electrical conductivity, Bloch wave, photoexcitation, charge carrier, metal, insulator, semiconductor, thermal conductivity, valence band, conduction band, antibonding orbital, bonding orbital, carrier mobility, absorption edge, thermal excitation, electron, hole, current, Paul Drude, Felix Bloch, Walter Heitler, Fritz London Chemical Substances copper Cu , beryllium Be , diamond C , silicon Si , germanium Ge , tin Sn , lead Pb Applications photovoltaics, photosensors, light-emitting diodes LEDs , temperature sensors.
Book Chapters Topics [Saylor] Need help getting started? Don't show me this again Welcome! Pauli exclusion, Hund's rule, and orbital filling; energy bands and gaps; the Fermi function; thermal promotion; metals, insulators, and semiconductors. Introduction to Semiconductors. See " Introduction to Energy Bands.
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