ABSTRACT
Semiconductors form the core of modern electronic and photonic devices (photonics refers to the generation, manipulation, and detection of photons). Unlike metals, the conductivity of pure (intrinsic) semiconductors increases with temperature, which cannot be explained by the Drude theory. The formal definition of a semiconductor is a material whose conductivity lies between that of a metal and an insulator. Perhaps a better definition would be a material with a bandgap greater than zero and less than some arbitrary value such as 3 eV, which is the highest energy of a visible photon. At least this definition alludes to the photonic applications of semiconductors. What makes semiconductors useful is the ability to control both the sign and the number of charge carriers by adding impurity atoms to form extrinsic semiconductors.
