What Is Semiconductivity?

Semiconductivity is the property by which certain materials conduct electrical current at levels intermediate between those of conductors and insulators, and do so in a manner that depends strongly on temperature, impurity content, and applied fields. Materials exhibiting this property, called semiconductors, typically have electrical resistivities in the range of 10⁻⁴ to 10⁴ ohm-centimeters at room temperature. Silicon and germanium are the most widely studied elemental semiconductors, while compound semiconductors such as gallium arsenide and indium phosphide extend the range of achievable optical and electronic properties. Semiconductivity forms the physical basis of virtually all modern electronic and optoelectronic devices.

The fundamental origin of semiconductivity lies in the quantum mechanical band structure of solids. In a semiconductor, a filled valence band is separated from an empty conduction band by an energy gap, or bandgap, typically between 0.5 and 3.5 electronvolts. At absolute zero, the semiconductor behaves as an insulator; as temperature rises, thermal energy excites electrons across the gap, generating charge carriers and increasing conductivity. This temperature dependence is one of the defining experimental signatures of the semiconducting state.

Charge Carriers and Carrier Generation

The electrical current in a semiconductor is carried by two distinct species: electrons in the conduction band and holes in the valence band. A hole is the absence of an electron in an otherwise filled band and behaves as a positively charged quasiparticle. Both carriers are generated in equal numbers by thermal excitation in an intrinsic, or undoped, semiconductor, a process described by Fermi-Dirac statistics applied to the band structure. The density of thermally generated carriers depends exponentially on the ratio of the bandgap energy to the thermal energy kT, which is why narrow-gap semiconductors such as germanium (bandgap 0.67 eV) carry substantially more current at room temperature than wide-gap materials such as silicon carbide (bandgap 3.26 eV). Carrier generation can also be induced optically, as photons with energy exceeding the bandgap promote electrons across the gap, the basis of photodetectors and solar cells. A detailed treatment of these processes is covered in resources from the Physics LibreTexts project on charge carriers in semiconductors.

Doping and Extrinsic Semiconductivity

Semiconductivity can be tuned over many orders of magnitude by introducing controlled quantities of impurity atoms, a process called doping. Adding a pentavalent impurity such as phosphorus to silicon donates a free electron to the conduction band, producing an n-type semiconductor in which electrons are the majority carriers. Adding a trivalent impurity such as boron accepts an electron from the valence band, leaving a mobile hole and producing a p-type semiconductor. Extrinsic semiconductors can achieve carrier densities many orders of magnitude above the intrinsic level, enabling the construction of p-n junctions, bipolar transistors, and field-effect transistors. Ion implantation and thermal diffusion are the primary industrial doping methods, as described in resources from the Department of Materials Science at Cambridge. Precise control of doping profiles is central to modern integrated circuit fabrication, where transistor dimensions below 5 nanometers demand atomic-level accuracy.

Applications

Semiconductivity has applications in a wide range of disciplines, including:

• Transistor design and integrated circuit fabrication for computing and communications
• Photovoltaic cells that convert sunlight to electrical energy via carrier generation
• Light-emitting diodes and laser diodes in displays and fiber-optic communications
• Radiation detectors and imaging sensors in medical and scientific instrumentation
• Power electronics for motor drives, inverters, and electric vehicle powertrains, using wide-bandgap materials such as silicon carbide and gallium nitride