Semiconductors are materials that have properties between conductors and insulators. One key concept in understanding semiconductors is energy bands. In a solid material, the behavior of electrons is described by energy bands, which are ranges of allowed energies. The valence band is the energy band occupied by electrons in their lowest energy states, while the conduction band is the energy band above the valence band, where electrons can move freely. The energy gap between the valence band and the conduction band is known as the band gap.

Doping is another crucial aspect of semiconductor physics. Doping involves intentionally introducing impurities into a semiconductor crystal to alter its electrical properties. By adding impurities with extra electrons (n-type doping) or impurities with electron deficiencies (p-type doping), we can change the conductivity of the material. Intrinsic semiconductors, such as pure silicon or germanium, have an equal number of positive and negative charge carriers.

Semiconductors find applications in various devices. One such device is the diode, which allows electric current to flow in only one direction. Diodes consist of a p-n junction, formed by joining a p-type semiconductor region and an n-type semiconductor region. When a forward bias voltage is applied to the diode, allowing current to flow across the junction, the diode is conducting. Conversely, when a reverse bias voltage is applied, the diode acts as an insulator and does not allow current to flow.

In summary, semiconductors represent an important class of materials with energy bands, doping, and intrinsic and extrinsic behavior. By manipulating the electrical properties of semiconductors through doping, we can create semiconductor devices such as diodes, which have applications in various electronic circuits.