A semiconductor is a solid whose electrical conductivity falls between that of conductors and insulators and its electrical conductivity can be controlled by adding impurities. The electrical conductivity can be changed a great deal with a high level of control. Semiconductors like silicon can be made into transistors, which are small amplifiers. Transistors are used in computers, mobile phones, digital audio players and many other electronic devices. Silicon is the most important commercial semiconductor, though many others are used.
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. Semi-conductors have many different applications.