A biexciton is a quantum mechanical state that consists of two excitons. An exciton is a bound state of an electron and a hole (the absence of an electron) in a semiconductor or insulator. When an electron in a semiconductor absorbs energy (such as from a photon), it can be excited from the valence band to the conduction band, leaving behind a hole in the valence band. The electron and hole can then interact through electrostatic attraction, forming an exciton.
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