 Exciton (source code)

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= Exciton
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An <exciton> is a <bound state> of an <electron> and an <electron hole>. Their opposite <electric charges> attract through the <Coulomb potential>, producing a neutral <quasiparticle> with internal relative motion and motion of its <center of mass>. In a <semiconductor>, promoting an <electron> from the <valence band> into the <conduction band> leaves an <electron hole>; the pair can occupy either an <exciton> <bound state> or an unbound <scattering state>. Merely creating the two <charge carriers> does not establish binding. A neutral <exciton> can transport excitation <energy> without transporting net <electric charge>. Coupling this material excitation strongly to a <photon> can form a <polariton>.

The striking optical consequence is that an <exciton optical transition> can absorb a <photon> with less <energy> than the threshold for creating unbound <electrons> and <electron holes>. The difference is the <exciton binding energy>, not an extra one-electron <energy band> inside the <band gap>. The <Wannier-Mott exciton> supplies a useful large-radius model; the <Frenkel exciton> describes the opposite, tightly localized limit.<Excitons in Semiconductor Quantum Dots by Matjaž Humar>[§3, printed pp. 4–6: bound pairs, effective masses and localized contrast]{ref}

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