Semiconductors are materials whose electrical conductivity lies between that of good conductors such as copper and electrical insulators such as glass, and whose conductivity can be controlled by material composition, temperature, electric fields, light and other physical conditions. Silicon is the most important semiconductor for modern electronics, although germanium, gallium arsenide, gallium nitride, silicon carbide and other compound or elemental semiconductors are used for particular applications.
The fundamental reason semiconductors are useful is that their electrons occupy energy states governed by a band structure containing a valence band and a conduction band separated by an energy gap. At ordinary conditions, some electrons can acquire enough energy to move into the conduction band, where they contribute to electrical current, while the vacancies they leave behind behave as positively charged mobile carriers called holes. This controllable population of electrons and holes allows engineers to construct components whose electrical behaviour can be deliberately manipulated rather than merely conducting or blocking electricity.
Pure semiconductor material is called intrinsic semiconductor material and has relatively limited conductivity at ordinary temperatures. Its electrical behaviour can be changed dramatically through doping, which means deliberately introducing very small concentrations of impurity atoms into the crystal. In silicon, atoms such as phosphorus or arsenic contribute an extra valence electron and create n-type material, in which electrons are the majority carriers, while atoms such as boron have one fewer valence electron and create p-type material, in which holes are the majority carriers.
The dopant concentration is carefully controlled because even tiny changes can substantially alter electrical properties. Doping does not simply make silicon conduct electricity like metal; instead, it creates regions with predictable carrier concentrations and energy levels that can be combined to form junctions, transistors, diodes and many other semiconductor devices.
The p-n junction is one of the fundamental structures that explains semiconductor operation. When p-type and n-type materials are joined, electrons from the n-type region initially diffuse towards the p-type region while holes move in the opposite direction. Their recombination leaves behind charged dopant ions that cannot move, creating a depleted region known as the depletion region. This region contains an internal electric field and creates a built-in potential barrier that influences the movement of charge carriers.
Applying an external voltage can alter this barrier. Forward bias reduces the effective barrier and permits substantial current to flow, while reverse bias increases the barrier and greatly limits ordinary current. This controllable junction behaviour forms the basis of semiconductor diodes and also plays a central role in the operation of transistor structures.
Semiconductor behaviour is governed by quantum mechanics as well as classical electrical principles. In a crystalline material, atoms form an ordered lattice and the allowed electron energies form bands rather than isolated atomic energy levels. The band gap determines how readily electrons can be promoted into conducting states. In metals, available energy states allow abundant conduction, whereas in insulators the relevant energy gap is usually large enough to prevent significant conduction under ordinary conditions.
Semiconductors have intermediate band gaps that make their carrier populations controllable. Temperature increases can generate more electron-hole pairs, generally increasing intrinsic semiconductor conductivity, which is different from the typical behaviour of metals where increasing temperature tends to increase resistance. This sensitivity to physical conditions is useful in electronic sensors as well as in conventional switching and computing devices.
Electric fields provide another mechanism for controlling semiconductor carriers. In a field-effect transistor, for example, a voltage applied to a control electrode changes the electrical conditions in a semiconductor channel and thereby controls whether current can flow strongly between other terminals. The ability to control conductivity using voltage is central to digital logic, memory and amplification.
Semiconductor devices can also respond to light because photons with sufficient energy can create electron-hole pairs, forming the basis of photodiodes, image sensors and photovoltaic cells. Conversely, electrical energy can cause semiconductors to emit light in devices such as light-emitting diodes and laser diodes. Semiconductor physics therefore provides mechanisms for converting and controlling electrical, optical and sometimes thermal information.
Modern semiconductor devices use carefully engineered regions with different concentrations and types of dopants. Ion implantation and diffusion are among the processes used to introduce dopants into selected areas during fabrication, while thermal treatments activate dopants and repair crystal damage. Semiconductor manufacturers also manipulate thin insulating and conducting layers, interfaces and geometries at extremely small scales.
The electrical behaviour of a finished transistor depends on factors such as carrier mobility, channel dimensions, oxide thickness, threshold voltage, junction characteristics, leakage mechanisms and parasitic capacitance and resistance. At advanced technology nodes, quantum mechanical effects, variability, heat generation and manufacturing imperfections become increasingly significant. Semiconductor engineering is therefore not simply the selection of a material with moderate conductivity; it is the precise physical engineering of charge transport and electric fields within microscopic structures.
Semiconductors enable both analogue and digital electronics. In analogue circuits, semiconductor devices can amplify signals, regulate voltage, detect light, generate oscillations and perform continuous signal processing. In digital circuits, transistors are arranged into logic gates and larger structures that represent binary states and perform Boolean operations. Memory cells use semiconductor devices to store information, while sensors exploit changes in electrical characteristics caused by physical quantities such as light, temperature, pressure or magnetic fields. Power semiconductors control substantial currents and voltages in applications such as electric vehicles, power supplies and renewable-energy systems. The same underlying semiconductor principles therefore support devices ranging from tiny logic transistors in processors to large power switches capable of controlling industrial electrical systems.
Semiconductor technology developed from early investigations of electrical conduction and solid-state physics into practical devices such as point-contact and junction transistors, and subsequently into integrated circuits containing enormous numbers of transistors. Silicon became dominant because it combines useful electronic properties with an abundant raw material base and, crucially, forms a high-quality native oxide that is valuable for MOS transistor fabrication.
Modern semiconductor manufacturing uses extremely pure crystalline substrates, controlled doping, thin-film deposition, lithography, etching, cleaning and metallisation to create multilayer structures. The resulting devices exploit the controllability of charge carriers rather than simply relying on the inherent conductivity of the raw material. Understanding semiconductors therefore requires combining concepts from atomic physics, solid-state physics, materials science and electrical engineering, because the practical operation of modern chips emerges from precisely controlled interactions among electrons, holes, electric fields, materials and microscopic structures.
