Complex sentence: In an unconfined (bulk) semiconductor, an electron-hole pair is typically bound within a characteristic length, which is called the exciton Bohr radius and is estimated by replacing the positively charged atomic core with the hole in the Bohr formula.
Split into simpler sentences: In an unconfined (bulk) semiconductor, an electron-hole pair is typically bound within a characteristic length, called the exciton Bohr radius. This is estimated by replacing the positively charged atomic core with the hole in the Bohr formula.