Band-gap width

Various polymorphs have been reported for SnS with band gap widths in the range 1.0-1.5 eV, depending on the preparation method. The a-SnS (herzenbergite) is the most frequently occurring phase and is a p-type semiconductor with a direct optical transition at 1.3 eV and a high absorption coefficient (> 10" cm ). The orthorhombic S-SnS phase possesses a direct gap between 1.05 and 1.09 eV. 

Orbital overlap, while important, is not the only determinant of band gap width. Another important factor is how tightly the lattice binds the electron. This is dealt with in the following model. 

Monovalent Azides. The relation of the cation s first ionization energy to ionic character and band-gap width has already been stressed. The monovalent azides TIN3, AgN, and CuNa have larger values of Ii than the alkali azides and hence less ionic character and smaller band gaps. The conduction band in each is assumed to be primarily formed of neutral cation states. The expected nature of the highest-lying valence band and of possible excitons is discussed below. Comparisons with halide compounds should be qualified by noting that the latter have different structures. 

The theoretical calculations carried out show, that due to the reasons considered, the tubes for which the relation i - j = 3q is valid, exhibit a conductivity of a metallic nature, while those with the relation i - j 3q were found to exhibit semiconductor conductivity. In accordance with the data presented in (21), the inverse proportionality of the bonding between the tube diameters and the width of the forbidden gap can be observed in Fig. 5. There are NT with a diverse range of band gap widths, including NT with a metallic conductivity, that for very low temperatures can be superconducting (23). Given certain conditions the NT can conduct current in a ballistic way and not dissipate heat while the current densities are stable and extremely high, J> 10 A/cm (24). 

Figure 5. The forbidden band gap width of SWNT as a function of diameter.

Ge-based cationic clathrates display transport properties of typical semiconductors. They exhibit a wide range of the band gap width, from 0.12 to 1.16 eV and are diamagnets [10, 11, 20, 23, 35]. Unfortunately, not a single representative... 

The number of electrons excited thermally (by heat energy) into the conduction band depends on the energy band gap width and the temperature. At a given temperature, the larger the Eg, the lower the probability that a valence electron will be promoted into an energy state within the conduction band this results in fewer conduction electrons. In other words, the larger the band gap, the lower the electrical conductivity at a given temperature. Thus, the distinction between semiconductors and insulators lies... 

In Modulation Spectroscopy, which is mosdy used to characterize semiconductor materials, the peak positions, intensities and widths of features in the absorption spectrum are monitored. The positions, particularly the band edge (which defines the band gap)> are the most useful, allowing determination of alloy concentration. 

We note finally fhaf fhe development of multilayer, band gap systems with variable width by (electro)chemical soft preparation methods provides opportunities for low-cost structures with high efficiency in solar cell devices. Examples of such devices are given elsewhere in this book. 

Interdigitated band electrode characteristics length = 29 mm, width = I m m and inter-elecirode gap width =500 pm... 

Fig. 2.2 Band structure of a semiconductor. eg denotes the energy gap (width of the forbidden band)... 

The band structure (widths of bands, energy gaps) will obviously depend on the arrangement of the atoms as well as on... 

Conductivity means that an electron moves under the influence of an applied field, which implies that field energy transferred to the electron promotes it to a higher level. Should the valence level be completely filled there are no extra higher-energy levels available in that band. Promotion to a higher level would then require sufficient energy to jump across the gap into a conduction level in the next band. The width of the band gap determines whether the solid is a conductor, a semi-conductor or an insulator. It is emphasized that in three-dimensional solids the band structure can be much more complicated than for the illustrative one-dimensional model considered above and could be further complicated by impurity levels. 

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