What Are Semiconductor Materials?

Semiconductor materials are solids with electrical conductivity intermediate between that of metals and insulators, characterized by a bandgap that separates a filled valence band from an empty conduction band at absolute zero. The bandgap typically ranges from less than 0.5 eV in narrow-gap compounds such as InSb to about 3.4 eV in gallium nitride, placing the Fermi level inside the forbidden region and producing resistivities that respond strongly to temperature, illumination, and controlled impurity doping. This tunability of electrical and optical properties through composition, doping, and structure is what makes semiconductor materials the foundation of modern electronics and photonics.

The field draws on condensed matter physics, materials science, and quantum mechanics. Silicon has dominated the electronics industry since the 1960s because of its abundant supply, thermally stable native oxide, and well-characterized doping chemistry, as captured in the NIST reference data on semiconductor properties. Compound semiconductors from groups III-V, II-VI, and IV-IV of the periodic table extend the accessible property space, offering direct bandgaps, high carrier mobilities, and wide bandgaps unavailable in elemental silicon.

Charge Carriers and Bandgap Physics

The two types of mobile charge carriers in a semiconductor are electrons in the conduction band and holes in the valence band. In intrinsic silicon at room temperature, the carrier concentration is approximately 10^10 per cubic centimeter, set by thermal excitation across the 1.12 eV bandgap. Doping with group-V impurities such as phosphorus introduces donor levels just below the conduction band edge, producing n-type material with electron concentrations orders of magnitude above the intrinsic value. Acceptor dopants such as boron create p-type material by introducing holes near the valence band. Excitons, bound electron-hole pairs held together by Coulomb attraction, play an important role in optical absorption and emission in direct-gap semiconductors, particularly in quantum-confined structures where their binding energy is enhanced. The carrier mobility measurements for silicon reported in PMC illustrate how doping concentration and crystal orientation together determine the transport properties that underpin device performance.

Compound Semiconductors and Silicon Compounds

III-V compound semiconductors such as GaAs, InP, and GaN have direct bandgaps that permit efficient radiative recombination, making them the material of choice for light-emitting diodes, laser diodes, and high-electron-mobility transistors. GaAs has six times the electron mobility of silicon, enabling microwave and millimeter-wave amplifiers used in radar and wireless communications. Silicon carbide (SiC) and silicon germanium (SiGe) occupy a middle ground: SiC's wide bandgap of approximately 3.3 eV and high thermal conductivity support high-power, high-temperature power electronics, while SiGe alloys are integrated into silicon fabrication lines to strain the lattice and boost carrier mobility in heterojunction bipolar transistors. The Chemical Reviews survey of high-k gate dielectrics captures how compound oxides such as hafnium dioxide have entered silicon complementary metal-oxide-semiconductor manufacturing as the continued scaling of silicon devices demanded replacement of silicon dioxide as the gate insulator.

High-k Dielectric Materials

As transistor gate lengths shrank below 45 nanometers, silicon dioxide gate oxides became thin enough that quantum mechanical tunneling caused unacceptable leakage current. High-dielectric-constant (high-k) materials, principally hafnium oxide and its silicate and aluminate variants, provide the capacitance needed for electrostatic control of the channel while remaining thick enough to suppress leakage. Intel introduced hafnium-based high-k dielectrics paired with metal gate electrodes at the 45 nm node in 2007, a transition that restored transistor scaling after a decade of stagnation. The acoustoelectric effect, in which traveling acoustic waves couple to and drag charge carriers, provides a separate means of probing and controlling carrier populations in piezoelectric semiconductors, connecting acoustic and electronic degrees of freedom in a single material.

Applications

Semiconductor materials have applications in a wide range of fields, including:

• Microprocessors, memory chips, and logic circuits in computing systems
• Solar cells and photovoltaic modules for energy conversion
• Light-emitting diodes and laser diodes for displays, lighting, and communications
• Power electronics for electric vehicles, industrial drives, and grid converters
• Sensors for temperature, pressure, radiation, and biological analytes
• Radio-frequency and millimeter-wave amplifiers in wireless and radar systems