Semiconductor: materials, physics, and applications

Electronic band structure

The distinctive properties of semiconductors arise from their electronic band structure. Electrons occupy a valence band and may be excited into a conduction band; the energy gap between these bands is the band gap. At nonzero temperature some electrons are thermally promoted across the gap, producing mobile carriers. The atomic arrangement and the positions of the atomic nuclei determine these bands and the size of the band gap. Materials with a small band gap behave more like conductors at room temperature, while large-gap materials behave like insulators.

Band gaps can be direct or indirect. In a direct-gap semiconductor, electrons can recombine with holes by emitting photons more efficiently; this property makes certain compound semiconductors well suited to light-emitting diodes and lasers. In indirect-gap materials, radiative recombination is less likely and other mechanisms dominate.

Doping and carrier types

Intrinsic (pure) semiconductors can be intentionally modified by adding small concentrations of other elements in a process called doping. Donor impurities provide extra electrons and create n-type material, while acceptor impurities create holes and produce p-type material. Carefully placed p-type and n-type regions form p–n junctions, which control the direction and magnitude of current flow and serve as the foundation for diodes, photodiodes and many sensor structures.

Carrier mobility (how quickly electrons or holes move under an electric field) and lifetime (how long carriers persist before recombining) are key parameters that determine device speed and efficiency. These depend on scattering by atoms, defects and other carriers, and can be improved by material purification and optimized processing.

Common materials

Elemental semiconductors such as silicon and germanium were the earliest used in electronic devices. Silicon dominates modern microelectronics because of its natural abundance, a robust native oxide that facilitates reliable insulating layers, and mature fabrication techniques. Compound semiconductors, for example gallium arsenide and related III–V materials, are important where higher electron velocity, direct band gaps for light emission, or other specialized properties are required. Wide-bandgap materials, such as silicon carbide and gallium nitride, are increasingly used for high-power and high-temperature applications.

Devices and functions

Semiconductor devices convert and control electrical signals. The basic components include p–n diodes, bipolar junction transistors and field-effect transistors (including the MOSFET). Transistors act as switches or amplifiers and are the building blocks of integrated circuits. Photovoltaic cells convert light into electrical energy, while photodetectors and light-emitting devices couple electrical and optical domains. Sensors exploit changes in conductivity, voltage or charge in response to physical stimuli such as light, temperature, pressure or chemical exposure.

Fabrication and processing

Manufacturing semiconductor devices involves crystal growth, doping, oxidation, thin-film deposition, lithography and etching to form microscopic structures on wafers. Oxide layers and patterned metal contacts define transistors and interconnections. Advances in lithography, material purity and process control have enabled progressive miniaturization and the integration of millions to billions of devices on a single chip.

History and development

1. Early 20th-century studies of solid-state physics established the basic principles of semiconductors.
2. Mid-20th-century inventions of point-contact and junction diodes and the transistor made active semiconductor devices practical.
3. The integrated circuit and subsequent developments in fabrication and design enabled the rapid growth of consumer and industrial electronics.

Applications and trends

Semiconductors power computing, communications, sensing, lighting and energy conversion. They are central to microprocessors, memory chips, mobile devices, solar panels and LED lighting. Emerging trends include materials research (two-dimensional crystals, new compounds), device concepts (beyond-CMOS logic, quantum devices), and sustainability concerns such as energy-efficient processing and supply-chain resilience. Wide-bandgap devices expand the capability for efficient power conversion in electric vehicles and renewable energy systems.

Practical considerations and further reading

• Integrated devices rely on engineered interfaces such as p–n junctions and insulating oxides formed on silicon.
• Some semiconductors are optimized for light emission (LEDs, lasers), others for high-speed logic or robust power handling.
• Materials availability, thermal management and device scaling are active engineering challenges.

For concise introductions and more technical material see resources on basic semiconductor concepts, electrical conduction and carrier dynamics, comparisons with conductors, materials such as copper and silver, insulating materials like rubber and plastic, formal definitions of insulators, the advantages of compound semiconductors, experimental and practical treatments of n-type and p-type doping, introductions to the transistor, and atomic-scale explanations for electronic behavior at the atomic level.