Semiconductor – Definition, Examples, Types, Uses

A semiconductor is a material whose electrical conductivity lies between that of a conductor and an insulator and, crucially, is controllable. This controllability arises from the material’s electronic structure, specifically the presence of a small energy band gap separating occupied and unoccupied electron states. By adjusting temperature, light exposure, electric fields, or chemical composition through doping, scientists and engineers regulate how electric charge flows through a semiconductor. These properties make semiconductors the foundational materials of modern electronics, enabling transistors, integrated circuits, solar cells, light-emitting diodes, sensors, and countless other technologies that underpin contemporary society.

Key Takeaways: Semiconductors

• Semiconductors conduct electricity only under specific, controllable conditions.
• Their behavior is explained using quantum mechanics and energy band theory.
• Electric current is carried by both electrons and holes.
• Doping introduces carefully selected impurities that control conductivity.
• Elemental semiconductors lie near the metalloid boundary of the periodic table.
• Semiconductors enable switching, amplification, sensing, and energy conversion.

What Is a Semiconductor?

A semiconductor is a solid material whose electrical conductivity is intermediate between that of metals and insulators (nonmetals). In metals, many electrons are free to move at all times, resulting in high conductivity. In insulators, electrons are tightly bound to atoms and cannot move easily, so electrical current does not flow. Semiconductors occupy a middle ground: under ordinary conditions, most electrons are bound, but modest amounts of energy can free some of them to carry charge.

The defining characteristic of a semiconductor is not simply intermediate conductivity, but tunability. A semiconductor behaves almost like an insulator in one situation and more like a conductor in another. This ability to control when and how current flows makes electronic switching, logic operations, and signal amplification possible.

History of Semiconductors

Observations of unusual electrical behavior in certain materials date back to the late nineteenth century. Substances such as selenium and copper oxide were found to conduct electricity better in one direction than the other, a phenomenon later called rectification. These effects were exploited in early light sensors and radio detectors, long before a full theoretical explanation existed.

The theoretical foundation for semiconductors emerged in the early twentieth century with the development of quantum mechanics and solid-state physics. Scientists learned that electrons in solids do not occupy discrete atomic energy levels, but instead fill ranges of allowed energies called bands. This insight explained why some materials conduct electricity readily, others hardly at all, and a few only under specific conditions.

A major technological turning point occurred in 1947 with the invention of the transistor. Transistors demonstrated that semiconductors could replace vacuum tubes as amplifiers and switches, offering dramatic improvements in size, reliability, and energy efficiency. This breakthrough laid the groundwork for modern electronics.

In the decades that followed, advances in purification, crystal growth, and microfabrication enabled the development of integrated circuits, allowing many transistors to be fabricated on a single piece of material. Continued scaling and refinement led to the microprocessors and memory devices that power computers, smartphones, and communication networks today.

Semiconductor research continues to evolve, addressing challenges related to miniaturization, heat management, power efficiency, optoelectronics, and quantum technologies.

Selected Timeline of Key People and Events

• 1873: Photoconductivity observed in selenium
• Early 1900s: Crystal rectifiers used in radio technology
• 1930s: Development of energy band theory
• 1947: First working transistor demonstrated
• 1958: Integrated circuit invented
• 1970s–present: Rapid scaling of semiconductor devices and rise of microelectronics

Semiconductor Materials

Semiconductor materials form a diverse class of solids unified by similar electronic behavior. They may be elemental, compound, or organic. Their properties depend strongly on atomic bonding and crystal structure.

Elemental Semiconductors and the Periodic Table

Elemental semiconductors are single-element materials whose atomic structure leads to semiconducting behavior. They lie near the boundary between metals and nonmetals on the periodic table, often called the metalloid region. Atoms in this region neither give up electrons as easily as metals nor hold them as tightly as nonmetals. Instead, they form extended covalent networks in which electrons are shared between neighboring atoms.

• Silicon
  Silicon is the most important semiconductor in modern technology. Each silicon atom forms four covalent bonds in a rigid crystal lattice that allows controlled electron motion.
• Germanium
  Germanium has a similar bonding structure but a smaller band gap, making it useful in high-speed and infrared applications.

Compound Semiconductors

Compound semiconductors consist of two or more elements, often chosen from different groups of the periodic table to tailor electronic properties.

• Gallium arsenide
  Gallium arsenide offers higher electron mobility than silicon and is widely used in high-frequency electronics and optoelectronics.
• Silicon carbide
  Silicon carbide operates reliably at high temperatures and voltages, making it valuable in power electronics.
• Cadmium telluride
  Cadmium telluride is commonly used in thin-film solar cells.

Organic Semiconductors

Organic semiconductors are carbon-based molecules or polymers in which charge transport occurs through delocalized molecular orbitals. They enable flexible displays, organic LEDs, and lightweight electronic devices, though typically with lower performance than inorganic semiconductors.

Fundamental Properties of Semiconductors

Semiconductors share a set of characteristic properties that distinguish them from conductors and insulators and determine how they behave in devices.

Energy Band Structure

In a crystal, allowed electron energies form bands. The valence band contains electrons bound to atoms, while the conduction band contains electrons free to move through the crystal. The band gap separating these bands is small enough in semiconductors that electrons can cross it under ordinary conditions, but large enough to prevent constant conduction.

Temperature Dependence

As temperature increases, more electrons gain sufficient energy to cross the band gap. This leads to increased conductivity with rising temperature, opposite the behavior of most metals.

Charge Carriers

Electrical current in semiconductors is carried by electrons in the conduction band and holes in the valence band. Holes represent missing electrons and behave as positively charged carriers.

Optical Sensitivity

Because electrons can be excited across the band gap by light, semiconductors respond strongly to illumination. This property enables photodetectors, cameras, and solar cells.

Types of Semiconductors

Semiconductors are classified based on purity, composition, and electronic behavior.

Intrinsic Semiconductors

An intrinsic semiconductor is a chemically pure material with no intentional impurities. Its conductivity arises only from thermally generated electron-hole pairs and is relatively low at room temperature.

Extrinsic Semiconductors

Extrinsic semiconductors contain small, carefully controlled amounts of impurity atoms called dopants. These impurities introduce additional charge carriers and dramatically increase conductivity.

n-Type Semiconductors

n-type semiconductors are doped with atoms that have more valence electrons than the host material. These dopants donate extra electrons to the conduction band, making electrons the majority carriers.

p-Type Semiconductors

p-type semiconductors are doped with atoms that have fewer valence electrons than the host material. These dopants create holes in the valence band, making holes the majority carriers.

Wide Band Gap Semiconductors

Wide band gap semiconductors have larger band gaps than silicon or germanium. They are well suited for high-power, high-frequency, and high-temperature applications.

Preparation of Semiconductor Materials

Semiconductors require exceptional purity and structural precision. Even trace impurities can strongly affect electrical behavior.

Preparation typically involves refining raw materials to extremely high purity, growing single-crystal ingots, slicing them into wafers, and introducing dopants in controlled amounts. Advanced fabrication techniques then pattern microscopic structures onto the wafer to create functional devices.

How Semiconductors Work

Understanding how semiconductors function requires linking atomic structure, quantum mechanics, and charge transport.

Energy Bands and Electron Motion

When atoms bond in a crystal, their atomic orbitals overlap, forming continuous energy bands. Whether electrons occupy the valence band or the conduction band determines whether current flows.

Charge Generation and Recombination

Electrons can be excited into the conduction band by thermal energy or light, leaving behind holes. When electrons recombine with holes, energy is released as heat or light. The balance between generation and recombination controls conductivity and light emission.

Doping and the Role of Dopants

Doping involves replacing a small fraction of atoms in the crystal lattice with different elements. A dopant is chosen based on its valence electron count relative to the host material and its ability to fit into the lattice without disrupting the structure. For example, adding a five-valence-electron atom to silicon introduces extra electrons, while adding a three-valence-electron atom creates holes. Dopants allow engineers to design materials with predictable electrical behavior.

p–n Junctions

When p-type and n-type regions are in contact, a depletion region forms where mobile carriers are scarce. This junction allows current to flow preferentially in one direction and forms the basis of diodes, transistors, and integrated circuits.

Energy Transfer: Photons and Phonons

When charge carriers move and recombine in a semiconductor, energy transfer involves not only electrical current but also light and heat. These energy-transfer processes include photons and phonons, which are central to understanding light emission, heat generation, and efficiency limits in semiconductor devices.

When an electron in the conduction band recombines with a hole in the valence band, the energy difference between the two states must be released. In some semiconductors, this energy is emitted directly as a photon, a quantum of electromagnetic radiation. This process produces light in devices such as light-emitting diodes and laser diodes. The wavelength (and color) of the emitted light is determined primarily by the band gap energy of the semiconductor.

In indirect band gap semiconductors, electron–hole recombination cannot occur by photon emission alone because the electron and hole differ in momentum as well as energy. To conserve momentum, the recombination process must involve a phonon, a quantum of lattice vibration. Phonon-assisted recombination releases most of the energy as heat rather than light, which explains why materials like silicon are inefficient light emitters.

Phonons also play a broader role in semiconductor behavior. Interactions between charge carriers and lattice vibrations limit carrier mobility and contribute to electrical resistance. Energy transferred to phonons appears as heat, making thermal management a fundamental consideration in semiconductor device design and operation.

Direct vs Indirect Band Gap Semiconductors

In direct band gap semiconductors, the minimum energy of the conduction band aligns with the maximum energy of the valence band in momentum space. This allows electrons to recombine with holes and emit light efficiently.

In indirect band gap semiconductors, the conduction band minimum and valence band maximum occur at different momenta. Recombination requires an additional interaction, making light emission inefficient. This distinction explains why materials like Gallium arsenide are used in LEDs and lasers, while Silicon is not.

Semiconductor Devices Built From p–n Junctions

p–n junctions are the building blocks of many semiconductor devices.

• Diodes allow current to flow in one direction.
• Bipolar junction transistors amplify or switch electrical signals.
• Field-effect transistors control current using electric fields.
• Solar cells and photodiodes convert light into electrical signals.

Many important electronic components are built using p–n junctions and related structures.

Why Purity Matters: Defects and Real Crystals

Real crystals are not perfect. Defects such as missing atoms, dislocations, and unintended impurities trap charge carriers or enhance recombination. Because semiconductor devices rely on precise carrier control, even one unintended impurity atom among billions can affect performance. This sensitivity explains why semiconductor fabrication is complex and costly.

Carrier Mobility and Electrical Conductivity

Electrical conductivity depends not only on carrier concentration but also on mobility, a measure of how easily carriers move through the lattice. Mobility depends on lattice vibrations, impurities, and defects. Materials with high mobility allow faster device operation.

Comparison With Conductors and Insulators

Conductors have overlapping energy bands and abundant free electrons. Insulators have large band gaps that prevent carrier generation. Semiconductors fall between these extremes, with moderate band gaps that enable controlled conduction.

Applications of Semiconductors

Semiconductors have applications across many industries because they allow precise electrical control.

• Electronics and Computing: Logic circuits, processors, memory, and integrated circuits rely on semiconductor switching behavior.
• Energy Conversion and Power Electronics: Solar cells convert light into electricity, while power semiconductors regulate voltage and current in electrical systems.
• Lighting and Displays: Light-emitting diodes and laser diodes provide efficient, controllable light.
• Communications: Semiconductors enable radio transmitters, receivers, fiber-optic systems, and satellite electronics.
• Medicine and Scientific Instrumentation: Semiconductor sensors and detectors are popular in imaging, diagnostics, and analytical instruments.

Common Misconceptions About Semiconductors

• Semiconductors conduct halfway between metals and insulators.
  While their conductivity is intermediate, the key distinction is that their conductivity is controllable. It can vary by many orders of magnitude depending on temperature, light, and doping.
• Holes are physical particles like electrons.
  A hole is not a real particle. It represents the absence of an electron in the valence band and behaves like a positively charged carrier because other electrons move to fill it.
• Doping just adds free electrons to the material.
  Doping introduces impurity atoms that either donate electrons (n-type) or accept them, creating holes (p-type). It alters the band structure and carrier concentrations, not just by adding excess electrons.
• Semiconductors are always solid and crystalline.
  Most semiconductors are crystalline solids, but there are also amorphous semiconductors (like a-Si), polycrystalline films, organic semiconductors, and some that exist in liquid or flexible forms.
• Silicon is useful for LEDs because it emits light.
  Silicon has an indirect band gap, meaning most electron-hole recombination results in heat, not light. Direct band gap materials like gallium arsenide yield efficient light emission.
• A p–n junction always conducts electricity.
  A p–n junction conducts only under forward bias. Under reverse bias, it blocks current flow until breakdown. This unidirectional behavior is essential to diodes and rectifiers.

Frequently Asked Questions

Why are semiconductors essential for modern electronics?
They allow precise switching, amplification, and control of electrical current.

Are semiconductors metals or nonmetals?
They are distinct from both and often lie near the metalloid boundary of the periodic table.

What types of engineers work with semiconductors?
Electrical engineers, materials engineers, chemical engineers, applied physicists, and semiconductor process engineers.

Can semiconductors emit light?
Yes. Electron-hole recombination can release energy as photons.

Glossary

Band gap: The energy difference between the valence band (filled with electrons) and the conduction band (where electrons can move freely). Determines whether a material is a conductor, insulator, or semiconductor.

Carrier mobility: A measure of how quickly electrons or holes can move through a material when subjected to an electric field.

Charge carrier: A particle (electron or hole) that carries electric charge through a material.

Conduction band: The range of energy levels in a solid where electrons are free to move and conduct electric current.

Crystal lattice: The ordered, repeating atomic structure in a solid material like silicon. Lattice defects can affect conductivity and carrier motion.

Direct band gap: A band structure in which electrons can recombine with holes directly, emitting photons efficiently (good for LEDs and lasers).

Dopant: An impurity atom introduced into a semiconductor to create either extra electrons (n-type) or holes (p-type). Common dopants include phosphorus, boron, and antimony.

Doping: The process of intentionally adding impurity atoms to a semiconductor to modify its electrical properties by increasing the number of electrons or holes.

Electron: A negatively charged subatomic particle. In semiconductors, electrons can move to the conduction band and act as mobile charge carriers.

Extrinsic semiconductor: A doped semiconductor with conductivity using either electrons or holes introduced by dopants.

Hole: A conceptual positive charge representing the absence of an electron in the valence band. Holes act as mobile positive charge carriers in semiconductors.

Indirect band gap: A band structure in which electron-hole recombination requires the involvement of a phonon, making photon emission inefficient (e.g., silicon).

Intrinsic semiconductor: A pure semiconductor material with no intentional doping. Electrical conduction arises from thermally generated electron-hole pairs.

n-type semiconductor: A semiconductor doped with donor atoms that supply extra electrons to the conduction band, making electrons the majority charge carriers.

p–n junction: The boundary formed between a p-type and an n-type semiconductor. It allows current to flow in one direction and is the basis of many electronic devices.

p-type semiconductor: A semiconductor doped with acceptor atoms that create holes in the valence band, making holes the majority charge carriers.

Phonons: Quanta of lattice vibration (mechanical energy). In indirect band gap semiconductors, phonons assist in electron-hole recombination by conserving momentum.

Photons: Quanta of electromagnetic radiation (light). Emitted during electron-hole recombination in direct band gap semiconductors.

Recombination: The process by which an electron falls from the conduction band into a hole in the valence band, releasing energy as light or heat.

Semiconductor: A material with electrical conductivity between that of a conductor and an insulator, with conductivity that is controllable by temperature, light, electric fields, or doping.

Valence band: The range of energy levels full of electrons that are bound to atoms and not free to conduct electricity.

References

• Allen, J. W. (1960). “Gallium Arsenide as a Semi-insulator”. Nature. 187 (4735): 403–405. doi:10.1038/187403b0
• Kittel, Charles (1995). Introduction to Solid State Physics (7th ed.). Wiley. ISBN 0-471-11181-3.
• Sze, Simon M. (1981). Physics of Semiconductor Devices (2nd ed.). John Wiley and Sons (WIE). ISBN 978-0-471-05661-4.
• Voelkel, Reinhard (2012). “Wafer-scale micro-optics fabrication”. Advanced Optical Technologies. 1 (3): 135–150. doi:10.1515/aot-2012-0013
• Yu, Peter Y.; Cardona, Manuel (2004). Fundamentals of Semiconductors: Physics and Materials Properties. Springer. ISBN 978-3-540-41323-3.