Absorption edge, fundamental temperature

The gap Eq is not the minimum gap Eg which determines the absorption edge and temperature dependence of carrier concentration, but the average gap corresponding to the separation of the bonding and antibonding states in the covalent bond picture. It has fundamental importance for many basic properties of the solid and appears in the theories of the relation between the chemical and optical properties of crystals (Phillips (1970)). 

These aspects of the optical spectra of solids are illustrated in the upper portion of Figure 1, which displays the reflectance curve (R) at room temperature for a typical semiconductor, GaAs. The fundamental absorption edge around 1.4 eV produces only a weak shoulder. Some structure is apparent in the two features around 3 eV and the large, broad peak near 5 eV. However, the dominant aspect of the line shape is the slowly varyii background. The derivative namre of Modulation Spectroscopy suppresses the uninteresting background effects in favor of sharp, deriva-... 

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