= Exciton binding energy
{title2=$E_B=E_{\rm cont}-E_X>0$}
The <exciton binding energy> is the positive <energy> needed to separate a bound <Electron> and <electron hole> into the relevant unbound <scattering states>. Compare the <exciton> <energy> $E_X$ with the continuum threshold $E_{\rm cont}$ at the same total <crystal momentum> and for the same <energy bands>. The negative internal <energy eigenvalue> $-E_B$ and the positive separation cost $E_B$ are different sign conventions for related quantities. An <exciton> may move freely through a perfect crystal while remaining internally bound: “free exciton” does not mean an unbound pair.
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