Exciton
Exciton is considered as electron-hole pair, not a pair of electron and hole. Typically it is found in semiconductor, nowadays in quantum dots. Once electron leaves the valence band jump up into conduction band, a holes is generated at valence band. External perturbation such as heat or photon is needed to excite the electron into conduction band.
exciton generation
periodic potential
In semiconductor, electron-holes pair (or exciton) behaves differently than individual particles (electron and holes). Hence it needs to be treated as a newly entire system (quasi particle hence came the name exciton) due to Coulombic attraction between electron and holes. Nevertheless the existence of periodic potential in semiconductor, electrons interaction from neighboring electrons and surface traps give rise to the exciton energy level slightly below the band gap though classically band gap is known as forbidden region.
exciton energy levels
Recombination of electron and holes creates emission. This is the colour we see from semiconductor. However in quantum dots, this band gap hence the wavelength of emitted photon is tunable by manipulating the size of quantum dots. (more on quantum dots later entry).
Quantum mechanically, holes too have mass called effective mass. And to my suprise it can be negative as well. The fully mathematical approached is not shown here. But trust me for the moment unless you have done some advanced calculus to deal with it. But the effective mass of hole is comparable to that of electron (not always similar). The effective mass might be due to the motion of electron in dielectric medium of semiconductor (not in vacuum trivially), hole generation and electron-electron interaction.
Once exciton is formed, we can characterize the radius of exciton called exciton Bohr radius and model it using hydrogenic model of atom in crystal. But this moves on to another quantum phenomenon when Bohr exciton radius is confined. Such confinement creates quantum dots (more on quantum dots later). We get a particle in spherical box model for quantum dots.
