Final answer:
An exciton is a coupled state of an electron and a hole attracted by Coulomb forces. Frenkel excitons have a small radius with high binding energy, found in insulators, while Wannier excitons, with lower binding energy and larger radii, are found in semiconductors.
Step-by-step explanation:
An exciton is a bound state of an electron and an electron hole which are attracted to each other by the electrostatic Coulomb force. It is an important concept in the physics of semiconductors and insulators. There are two main types of excitons: Frenkel excitons and Wannier excitons.
Frenkel excitons, often found in molecular crystals and some insulators, are characterized by a small radius where the electron and the hole are tightly bound and they reside within the same or very close neighboring unit cells. The Frenkel exciton has a larger binding energy due to the closer proximity of the electron and hole.
Wannier excitons are typically found in semiconductor materials and have a much larger radius. This means the electron and hole can be separated by many lattice sites within the crystal. Because of this, the binding energy of a Wannier exciton is lower than that of a Frenkel exciton.