= Solution
A <Frenkel exciton> is a tightly bound electron–hole excitation localized on approximately one molecular or ionic unit. Its radius is comparable to the lattice spacing, its <exciton binding energy> can be large, and transport is described through transfer of the local excitation between units. Molecular solids such as crystalline <anthracene> provide a natural example: the optical excitation is largely associated with an individual molecule, so a broad parabolic-band hydrogen model is not the most useful starting point.
A <Wannier-Mott exciton> extends over many <unit cells>. It is described approximately by a conduction electron and a valence hole with <effective masses> and a dielectric-screened Coulomb attraction. Its binding is usually smaller, and its levels resemble a hydrogenic series. <gallium arsenide> is a suitable example because its direct-gap band-edge carriers have light <effective masses> and substantial dielectric <dielectric screening>, supporting spatially extended <excitons>. Localization length and <dielectric screening>, rather than whether a material happens to be called a semiconductor, distinguish the two limiting pictures. Real materials can lie between the limits, so neither family is a universal sharp classification.
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