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Bonding Forces and Energy Bands in Solids Electrons are restricted to sets of discrete energy levels within atoms, with large gaps among them where no energy state is available for the electron to occupy. Electrons in solids also are restricted to certain energies and are not allowed at other energies. Difference in the solid, the electron has a range or band of available energies. The discrete energy levels of the isolated atom spread into bands of energies in the solid because i in the solid, the wave functions of electrons in neighboring atoms overlap, thus, it affects the potential energy term and the boundary conditions in the equation, and different energies are obtained in the solution, and ii an electron is not necessarily localized at a particular atom.
Bonds and Bands in Semiconductors - Second Edition
The influence of neighboring atoms on the energy levels of a particular atom can be treated as a small perturbation, giving rise to shifting and splitting of energy states into energy bands. Thus, an electrostatic attractive force is established, and the balance is reached when this equals the net repulsive force. Since there are no loosely bound electrons to participate in current flow NaCl is a good insulator. Metallic Bonding In metals, the outer shell is filled by no more than three electrons loosely bound and given up easily great chemical activity and high electrical conductivity.
9.11: Bonding in Semiconductors
Outer electron s contributed to the crystal as a whole solid made up of ions with closed shells immersed in a sea of free electrons, which are free to move about the crystal under the influence of an electric field. Coulomb attraction force between the ions and the electrons hold the lattice together. Covalent Bonding Exhibited by the diamond lattice semiconductors. Each atom surrounded by four nearest neighbors, each having four electrons in the outermost orbit. Each atom shares its valence electrons with its four nearest neighbors.
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Bonding forces arise from a quantum mechanical interaction between the shared electrons. Both electrons belong to each bond, are indistinguishable, and have opposite spins. No free electrons available at 0 K, however, by thermal or optical excitation, electrons can be excited out of a covalent bond and can participate in current conduction important feature of semiconductors.
Ionic character of bonding becomes more prominent as the constituent atoms move further away in the periodic table, e. Energy Bands As isolated atoms are brought together to form a solid, the electron wave functions begin to overlap.
In contrast to metals, whose electrical conductivity decreases with temperature the more intense lattice vibrations interfere with the transfer of momentum by the electron fluid , the conductivity of semiconductors increases with temperature. The presence of an impurity in a semiconductor can introduce a new band into the system. If this new band is situated within the forbidden region, it creates a new and smaller band gap that will increase the conductivity.
Bonds and Bands in Semiconductors - Second Edition | Momentum Press
The huge semiconductor industry is based on the ability to tailor the band gap to fit the desired application by introducing an appropriate impurity atom dopant into the semiconductor lattice. The dopant elements are normally atoms whose valance shells contain one electron more or less than the atoms of the host crystal. At absolute zero, all of the charge carriers reside in lower of the bands below the small band gap in a semiconductor that is, in the valence band of the illustration on the left above, or in the impurity band of the one on the right.
At higher temperatures, thermal excitation of the electrons allows an increasing fraction jump across this band gap and populate either the empty impurity band or the conduction band as shown at the right. The effect is the same in either case; the semiconductor becomes more conductive as the temperature is raised. Note that this is just the opposite to the way temperature affects the conductivity of metals.