Semiconductors valence and conduction bands

Electron Level Position. One essential condition of spectral sensitization by electron transfer is that the LUMO of the dye be positioned above the bottom of the conduction band, eg, > —3.23 eV in AgBr or > —4.25 eV in ZnO (108). To provide the desired frontier level position respectively to the valence and conduction bands of the semiconductor, it is necessary to use a polymethine with suitable electron-donor abiHty (Pq. Increasing the parameter (Pq leads to the frontier level shift up, and vice versa. Chain lengthening is known to be accompanied by a decrease of LUMO energy and hence by a decrease of sensitization properties. As a result, it is necessary to use dyes with high electron-donor abiHty for sensitization in the near-ir. The desired value of (Pq can be provided by end groups with the needed topological index Oq or suitable substituents (112). 

A hst of some impurity semiconductors is given in Table 5. Because impurity atoms introduce new localized energy levels for electrons that are intermediate between the valence and conduction bands, impurities strongly influence the properties of semiconductors. If the new energy levels are unoccupied and He close to the top of the valence band, electrons are easily excited out of the filled band into the new acceptor levels, leaving electron holes... 

Fig. 1. The energy levels in a semiconductor. Shown are the valence and conduction bands and the forbidden gap in between where represents an occupied level, ie, electrons are present O, an unoccupied level and -3- an energy level arising from a chemical defect D and occurring within the forbidden gap. The electrons in each band are somewhat independent, (a) A cold semiconductor in pitch darkness where the valence band levels are filled and conduction band levels are empty, (b) The same semiconductor exposed to intense light or some other form of excitation showing the quasi-Fermi level for each band. The energy levels are occupied up to the available voltage for that band. There is a population inversion between conduction and valence bands which can lead to optical gain and possible lasing. Conversely, the chemical potential difference between the quasi-Fermi levels can be connected as the output voltage of a solar cell. Fquilihrium is reestabUshed by stepwise recombination at the defect levels D within the forbidden gap.

The largest band gap between valence and conduction band is in an insulator. There is no energy gap between the valence and conduction bands in a metal. Metalloids are semiconductors they have a small band gap between the valence and conduction bands. 

Recall from Figure 6.2 that the gap between the valence and conduction bands called the band gap. Eg, can be used to classify materials as conductors, insulators, or semiconductors also recall that for semiconductors the value of Eg is typically on the order of 1-2 eV. The magnitude of the band gap is characteristic of the lattice alone and varies widely for different crystals. In semiconductors, the valence and conduction bands do not overlap as in metals, but there are enough electrons in the valence band that can be promoted to the conduction band at a certain temperature to allow for limited electrical conduction. For example, in silicon, the energies of the valence electrons that bind the crystal together lie in the valence band. All four... 

Whereas in good-conducting doped or polymeric dyes ft-or -type conductivity can be explained without difficulty by analogy with inorganic semiconductors, the p- and -type photoconductivity in insulating (intrinsic) dye films cannot be explained in this manner. It is necessary to take into consideration the existence of defect states (lattice defects, dislocations, impurities etc.) distributed at different depths in the forbidden zone between valence and conduction band these defect states are able to trap electrons and holes, respectively, with different probability 10,11,88),... 

The energy difference between the valence and conduction bands in semiconductors. It is related to the absorption edge of the semiconductor. 

Bulk crystalline or amorphous solid-state materials whose conductivity is intermediate between metals and insulators and whose resistance decreases with increasing temperature. The valance band of an undoped semiconductor is completely filled, whereas its conduction band is empty. The energy difference between the valence and conduction bands (the band-gap) defines a semiconductor (see Fig. 95). 

Fig. 6.122. The band picture of a semiconductor with an interatomic spacing of dsc, showing the small energy gap between the valence and conduction band.

What the addition of electron acceptors and donors means in the band picture can be easily understood from Figs. 6.128 and 6.129. The electron acceptors and donors enter the lattice of the semiconductor and introduce electron-energy levels between the valence and conduction bands. Thus, with an n-type of semiconductor (Fig. 6.128), only a small part of the electrons in the conduction band arise by thermal excitation from the valence band the rest come from the ionization of electron donors. The hole concentration, however, depends only upon the number of valence electrons that are excited into the conduction band. The hole concentration can therefore be made small. 

For conductors the valence band is completely filled and the conduction band is only partially filled (Fig. 2a). This means that there are many carriers. A semiconductor possesses a completely filled valence band and completely empty conduction band at 0° K (Fig. 2 b) but at higher temperatures the number of conduction electrons is controlled by the distribution of electrons between the valence and conduction band. This in turn is controlled by the width of the energy gap and the density of states curve, i. e. the number of allowed states for an energy lying within the range E + 6E (Fig. 2 c). 

Figure 3.44 In an insulator the band gap energy (BGE) is very large, while it is small in semiconductors (SC). In metals, M, the valence and conduction bands overlap...

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