Various semiconductors available in practice could be: 

(i) Elemental semiconductors: Si and Ge 

(ii) Compound semiconductors: 

Examples are: 

• Inorganic: CdS, GaAs, CdSe, InP, etc. 

• Organic: anthracene, doped pthalocyanines, etc. 

• Organic polymers: polypyrrole, polyaniline, polythiophene, etc. 

Most of the currently available semiconductor devices are based on elemental semiconductors Si or Ge and compound inorganic semiconductors. However, after 1990, a few semiconductor devices using organic semiconductors and semiconducting polymers have been developed signaling the birth of a futuristic technology of polymer electronics and molecular-electronics. 

Inside the crystal each electron has a unique position and no two electrons see exactly the same pattern of surrounding charges. Because of this, each electron will have a different energy level. 

These different energy levels with continuous energy variation form what are called energy bands. The energy band which includes the energy levels of the valence electrons is called the valence band. The energy band above the valence band is called the conduction band. With no external energy, all the valence electrons will reside in the valence band.