This chapter focuses on semiconductors, essential materials for modern electronics and optoelectronics, emphasizing their quantum properties and band structure. It introduces the Fermi energy in systems of different dimensionality and the role of the Fermi level as the electrochemical potential of electrons. The main types of semiconductors are discussed, highlighting the importance of effective mass and charge carrier dynamics. The chapter also examines intrinsic and extrinsic materials, and concludes with band bending, a key phenomenon in semiconductor devices.