1 Fundamental properties

Semiconductors are materials whose electrical behavior falls between that of metals and insulators. Their conductivity is not fixed; it can change substantially in response to temperature, illumination, electric fields, and chemical composition. This tunability is the basis for their central role in modern electronic and optoelectronic technology.

1.1 Electrical conductivity

A semiconductor conducts electricity by means of mobile charge carriers, but usually in smaller numbers than a metal. In many semiconductors, conductivity increases with temperature because heat excites more carriers into mobile states. This behavior differs from that of most metals, whose resistance generally rises with temperature.

1.2 Energy bands

The electronic properties of semiconductors are explained with band theory. Electrons in a solid occupy allowed energy ranges, or bands, separated by forbidden gaps. The arrangement of these bands determines whether the material behaves as a conductor, semiconductor, or insulator.

1.2.1 Valence band and conduction band

The valence band contains electrons that are normally bound in atomic bonds. The conduction band consists of higher-energy states in which electrons can move through the crystal more freely. In semiconductors, the separation between these bands is small enough that electrons can be promoted from one band to the other under ordinary conditions.

1.2.2 Band gap

The band gap is the energy difference between the valence band and the conduction band. A small band gap allows thermal excitation, optical absorption, or doping to influence conductivity strongly. The size of the band gap also affects color, light emission, and suitability for high-temperature or high-power operation.

1.3 Charge carriers

Electrical current in semiconductors is carried by electrons and by positively charged quasiparticles called holes. The balance and motion of these carriers determine the material’s response to external stimuli.

1.3.1 Electrons

Electrons are negative charge carriers that move in the conduction band. Their motion under an electric field produces current, and their mobility depends on crystal quality, temperature, and scattering processes. In many devices, electron transport is a primary design consideration.

1.3.2 Holes

A hole is the absence of an electron in the valence band, treated as a positive carrier. Although a hole is not a particle in the usual sense, it behaves as though it has charge and can move through the lattice as neighboring electrons fill the vacancy. Hole transport is especially important in many junction and transistor structures.

1.4 Intrinsic and extrinsic behavior

An intrinsic semiconductor is chemically pure, with carrier populations determined mainly by temperature and the material’s band gap. An extrinsic semiconductor contains controlled impurities that increase the number of electrons or holes. In practice, most semiconductor devices rely on extrinsic behavior because it offers much finer control over conductivity.

2 Types of semiconductors

Semiconductors are grouped by chemical composition, atomic structure, and processing behavior. Different classes are chosen for different electrical, optical, and mechanical requirements.

2.1 Elemental semiconductors

Elemental semiconductors are formed from a single chemical element. They are among the most widely used materials in microelectronics because they combine useful electronic properties with mature manufacturing methods.

2.1.1 Silicon

Silicon is the dominant semiconductor in integrated circuits and many solar cells. It is abundant, mechanically robust, and forms a stable native oxide that is highly valuable in device fabrication. Its combination of performance and manufacturability has made it the foundation of the semiconductor industry.

2.1.2 Germanium

Germanium was important in early transistors and remains useful in specialized applications. It offers higher carrier mobility than silicon, which can benefit certain high-speed devices. However, its smaller band gap and processing limitations restrict its use in many mainstream applications.

2.2 Compound semiconductors

Compound semiconductors contain two or more elements arranged in a stoichiometric crystal structure. They can be engineered to provide band gaps, mobilities, and optical properties that are difficult to obtain in elemental materials.

2.2.1 III-V semiconductors

III-V semiconductors combine elements from groups III and V of the periodic table, such as gallium arsenide. They are widely used in high-frequency electronics, lasers, and light-emitting devices because many of them have direct band gaps and excellent electron transport properties.

2.2.2 II-VI semiconductors

II-VI semiconductors are formed from group II and group VI elements, such as cadmium telluride and zinc selenide. Many are valued for optoelectronic and detector applications. Their properties can be tailored for light absorption, emission, or radiation sensing.

2.3 Organic semiconductors

Organic semiconductors are carbon-based materials, often consisting of conjugated molecules or polymers. They can be processed at relatively low temperatures and deposited on flexible substrates. This makes them attractive for displays, sensors, and lightweight electronic systems.

2.4 Amorphous semiconductors

Amorphous semiconductors lack the long-range periodic order of crystals. Their disordered structure can reduce carrier mobility, but it also allows inexpensive large-area deposition. Amorphous silicon is a well-known example used in thin-film electronics and some imaging and photovoltaic devices.

3 Doping and carrier control

Doping is the deliberate introduction of impurities to change the electrical properties of a semiconductor. By controlling impurity type and concentration, engineers can create regions with distinct carrier populations and form functional devices.

3.1 N-type semiconductors

N-type semiconductors have an excess of electrons as majority carriers. They are typically produced by doping with atoms that contribute extra valence electrons relative to the host lattice. This shifts the electrical behavior toward electron conduction.

3.2 P-type semiconductors

P-type semiconductors contain an excess of holes as majority carriers. They are formed by introducing dopants that accept electrons, leaving mobile vacancies in the valence band. P-type regions are essential in junctions, transistors, and many optical devices.

3.3 Dopant atoms

Dopant atoms are carefully selected impurities inserted into the crystal lattice. Their valence structure determines whether they act as donors or acceptors. Common dopants must be compatible with the host material so that they can be incorporated without unduly damaging crystal quality.

3.4 Compensation and carrier concentration

When donor and acceptor impurities are both present, their effects may partially cancel, a process known as compensation. The net carrier concentration depends on the balance among dopants, temperature, and intrinsic carriers. Precise control of this balance is vital for device performance and reproducibility.

4 Semiconductor physics

The behavior of semiconductors is governed by solid-state physics, including crystal structure, carrier motion, and equilibrium statistics. These principles explain how materials respond to voltage, light, heat, and defects.

4.1 Crystal structure

Most useful semiconductors have a well-ordered crystal lattice. The regular arrangement of atoms determines the band structure, carrier mobility, and susceptibility to defects. Crystal imperfections, such as vacancies, dislocations, and grain boundaries, can influence electrical and optical performance.

4.2 Electron mobility

Electron mobility measures how quickly electrons drift under an electric field. High mobility generally supports faster device operation and lower resistive losses. Mobility is affected by scattering from phonons, impurities, and structural defects.

4.3 Recombination and generation

Recombination occurs when an electron and a hole annihilate each other as carriers return to a lower-energy state. Generation is the reverse process, creating electron-hole pairs through heat, light, or electrical excitation. These processes control carrier lifetimes and are central to diodes, detectors, and solar cells.

4.4 Diffusion and drift

Drift is carrier motion caused by an electric field, while diffusion is motion driven by concentration gradients. Both mechanisms often act simultaneously in semiconductor devices. Their interplay determines current flow across junctions and through active device regions.

4.5 Fermi level

The Fermi level is an energy reference that describes the occupancy of electronic states in a material. In semiconductors, it helps indicate whether electrons or holes dominate and how carrier populations will respond to temperature and doping.

4.5.1 Equilibrium conditions

At equilibrium, the Fermi level is uniform throughout a homogeneous semiconductor. Carrier distributions settle into a balance determined by the material’s band structure and dopant content. When different regions are brought into contact, changes in the Fermi level help drive junction formation.

4.5.2 Temperature dependence

Temperature affects the Fermi level indirectly by changing carrier populations. As temperature rises, more electrons may be thermally excited across the band gap, altering the balance between electrons and holes. This shift is important in device design and in understanding leakage currents.

5 Semiconductor devices

Semiconductor devices exploit controlled conductivity, junctions, and optical interactions to perform electrical and photonic functions. They form the basis of rectification, amplification, switching, light emission, and energy conversion.

5.1 Diodes

Diodes are two-terminal devices that preferentially allow current to flow in one direction. Their operation depends on an asymmetry in semiconductor structure, usually created by a p-n junction.

5.1.1 p-n junctions

A p-n junction is formed where p-type and n-type semiconductor regions meet. Carrier diffusion across the boundary creates a depletion region and an internal electric field. This structure produces rectifying behavior that is fundamental to many circuits.

5.1.2 Zener diodes

Zener diodes are designed to conduct in reverse bias at a controlled breakdown voltage. They are used for voltage regulation and reference circuits. Their operation relies on carefully engineered doping and junction properties.

5.2 Transistors

Transistors are semiconductor devices used for amplification and switching. They are among the most important inventions in electronics, enabling compact control of current and voltage.

5.2.1 Bipolar junction transistors

A bipolar junction transistor uses two p-n junctions arranged in either n-p-n or p-n-p form. Current in one region controls a larger current between the other two regions. These devices are valued for gain, speed, and analog performance.

5.2.2 Field-effect transistors

Field-effect transistors control current through an electric field applied to a channel. Variants such as MOSFETs dominate modern digital electronics because they scale well and consume little static power. Their operation depends strongly on surface and interface engineering.

5.3 Optoelectronic devices

Optoelectronic devices interact with light as well as electricity. Semiconductor band structure makes it possible to convert electrical energy into photons, detect light, or harvest solar radiation.

5.3.1 Light-emitting diodes

Light-emitting diodes produce light when electrons and holes recombine radiatively. The emitted color depends on the band gap of the semiconductor. LEDs are widely used in indicators, displays, illumination, and optical communication.

5.3.2 Photodiodes

Photodiodes convert incident light into electrical current. When photons generate electron-hole pairs, the device’s internal fields separate the carriers and produce a measurable signal. They are used in imaging, sensing, and fiber-optic receivers.

5.3.3 Solar cells

Solar cells transform sunlight into electrical power through the photovoltaic effect. Semiconductor junctions separate photo-generated carriers and drive current through an external circuit. Material choice influences efficiency, spectral response, and manufacturing cost.

5.4 Integrated circuits

Integrated circuits combine many semiconductor components on a single chip. They may include transistors, diodes, resistors, capacitors, and interconnect networks. This integration allows high performance, compact size, and low cost per function.

6 Materials and fabrication

Semiconductor performance depends not only on material composition but also on manufacturing quality. Fabrication processes aim to create highly controlled structures with minimal defects and precise dimensions.

6.1 Crystal growth

Crystal growth produces semiconductor ingots or epitaxial layers with controlled purity and structure. Techniques are chosen to achieve low defect density and specific electrical characteristics. Large, uniform crystals are essential for reliable device manufacture.

6.2 Wafer production

Wafers are thin slices cut from crystal ingots and polished to a highly flat surface. They serve as the substrate for device fabrication. Wafer quality affects yield, uniformity, and the electrical properties of final products.

6.3 Lithography

Lithography defines microscopic patterns on a wafer. It allows manufacturers to create transistors, wires, and other structures with precise alignment. As device dimensions shrink, lithographic resolution and process control become increasingly important.

6.4 Deposition and etching

Deposition adds thin films to a wafer, while etching removes selected material to form patterns. These steps are used repeatedly to build multilayer structures. Control over thickness, composition, and sidewall shape is critical to device behavior.

6.5 Packaging and interconnects

Packaging protects semiconductor chips and provides electrical connection to external circuits. Interconnects link internal device regions and connect chips to circuit boards. Thermal management, signal integrity, and mechanical reliability are major concerns in this stage.

7 Applications

Semiconductors are used across nearly all sectors of electronics and energy technology. Their versatility arises from the ability to tailor electrical and optical behavior for specific tasks.

7.1 Computing and data processing

Semiconductor devices enable microprocessors, memory chips, and logic circuits. Their switching speed and small size make digital computation practical at large scale. Continued progress in this area has been driven by dense integration and low-power transistor design.

7.2 Communications

Semiconductors support radio-frequency circuits, optical transmitters, receivers, and signal-processing hardware. They are essential in mobile devices, networking equipment, and satellite systems. High-frequency compound semiconductors are particularly important where speed and efficiency matter.

7.3 Power electronics

Power semiconductor devices regulate and convert electrical energy in applications such as motor drives, power supplies, and grid interfaces. They are designed to handle high voltages, large currents, and thermal stress. Wide-bandgap materials have expanded performance in this field.

7.4 Sensing and instrumentation

Semiconductors are used in sensors for light, temperature, pressure, motion, and chemical composition. Their electrical response can be engineered for high sensitivity and rapid readout. Instrumentation often relies on semiconductor detectors for accurate measurement.

7.5 Renewable energy systems

Semiconductors play a major role in photovoltaic generation and in power conversion for renewable energy installations. They are also used in control electronics for energy storage and grid integration. Material efficiency and long-term stability are central considerations in these systems.

8 Research and emerging topics

Research on semiconductors continues to expand into smaller devices, new materials, and novel quantum and nanoscale effects. These developments aim to improve performance, reduce power use, and open new functions.

8.1 Nanostructured semiconductors

Nanostructured semiconductors include quantum wells, nanowires, and quantum dots. At these scales, confinement effects can alter optical and electrical behavior in useful ways. Such structures are studied for advanced lasers, detectors, and low-dimensional electronics.

8.2 Wide-bandgap materials

Wide-bandgap semiconductors have larger energy gaps than conventional silicon. They can operate at higher temperatures, voltages, and frequencies, making them attractive for power and radio-frequency devices. Silicon carbide and gallium nitride are notable examples.

8.3 Two-dimensional semiconductors

Two-dimensional semiconductors are materials only a few atoms thick. Their extreme thinness can improve electrostatic control in transistors and enable flexible or transparent devices. They are also of interest for tunneling, sensing, and layered heterostructures.

8.4 Quantum devices

Quantum devices use quantum-mechanical effects such as tunneling, discrete energy levels, and coherent control. Semiconductor platforms are widely studied for quantum dots, single-electron systems, and certain quantum-information components. Their development depends on precise materials engineering and isolation from noise.

Manufacturing trends include tighter process control, more advanced lithography, improved packaging, and greater use of heterogeneous integration. Industry development also emphasizes energy efficiency, yield improvement, and specialized chips for distinct workloads. As devices become more complex, integration across materials and chiplets has become increasingly important.