text: In classic crystalline semiconductors , the electrons can have energies only within certain bands ( i.e. ranges of levels of energy ) . Energetically , these bands are located between the energy of the ground state , corresponding to electrons tightly bound to the atomic nuclei of the material , and the free electron energy .
simplified: Like other solids , the electrons in semiconductors can have energies only within certain bands ( i.e. ranges of energy levels ) between the energy of the ground state , corresponding to electrons tightly bound to the atomic nuclei of the material , and the free electron energy , which is the energy required for an electron to escape entirely from the material .