Semiconductor

A semiconductor is a material whose electrical conductivity lies between that of a conductor and an insulator. Its behaviour is distinguished by the ability to control conductivity through the addition of impurities—known as doping—and by external influences such as electric fields, temperature or light. Modern electronics is fundamentally dependent on semiconductor materials, particularly silicon and gallium arsenide, which enable components such as diodes, transistors and integrated circuits. The controlled movement of charge carriers—electrons and holes—within semiconductor junctions forms the underpinning of nearly all contemporary electronic devices.

Fundamental Properties

Semiconductors in their pure, intrinsic state have low electrical conductivity because their valence bands are filled, preventing free electron movement. Conductivity can be increased through doping, in which small quantities of impurities are introduced into the crystal structure. When doped with Group V elements such as phosphorus, arsenic or antimony, a semiconductor becomes an n-type material with an excess of electrons. Doping with Group III elements, including boron or gallium, creates p-type material, characterised by an excess of holes. These free charge carriers are essential for conduction, and their behaviour is explained using concepts from quantum physics, which describes how electrons occupy energy bands within a crystal.
Temperature also affects conductivity: raising the temperature increases the number of free charge carriers in a semiconductor, whereas metals typically show reduced conductivity under the same conditions. This distinct temperature response is one of the defining features of semiconductor materials.

Junctions and Charge Behaviour

When regions of differing doping levels are brought together within a single crystal, they form a semiconductor junction. The most common example is the p–n junction, central to devices such as diodes and transistors. At this junction, electrons from the n-type region diffuse into the p-type region, while holes diffuse in the opposite direction. This recombination results in a zone depleted of mobile carriers, leaving behind fixed ions that create an electric field. This built-in field governs the directional flow of current and underpins rectification and amplification.
Semiconductors may also be configured into homojunctions involving the same base material, such as differently doped regions of germanium, or heterojunctions using two distinct semiconducting compounds. In all cases, the equilibrium between migrating electrons and holes, and the processes of carrier generation and recombination, determine device performance.
When an external potential is applied, the material deviates from thermal equilibrium. Electrons and holes respond by diffusing and drifting through the structure, a process described as ambipolar diffusion. External stimuli such as temperature gradients or photons can generate additional carriers, altering the conductivity and enabling functions such as photodetection and photovoltaic energy conversion.

Optical and Thermal Effects

Some semiconductors can release energy as light when excited electrons return to lower energy levels. This principle enables the operation of light-emitting diodes (LEDs) and quantum dots. By adjusting the semiconductor composition and the applied current, manufacturers can control the colour and intensity of emitted light.
Semiconductors with high thermal conductivity play an essential role in thermal management, particularly in high-power electronics, electric vehicles and high-brightness LEDs. These materials facilitate effective heat dissipation, improving efficiency and reliability. In addition, semiconductors with favourable thermoelectric properties can convert heat into electrical energy or provide cooling in thermoelectric generators and coolers.

Materials

The most widely used semiconductor material is silicon, valued for its abundance, stability and suitability for mass production. It is followed by gallium arsenide, common in specialised applications including microwave-frequency integrated circuits, solar cells and laser diodes. Germanium, silicon carbide, gallium nitride, and various compound semiconductors formed from elements in Groups 13–15 or 12–16 of the periodic table also exhibit valuable semiconducting properties.
Semiconductors occur in both crystalline and amorphous forms. Crystalline materials, especially monocrystalline silicon, are favoured for microelectronics and high-performance devices due to their ordered atomic structure. Amorphous semiconductors such as hydrogenated amorphous silicon and mixtures of arsenic, selenium and tellurium are used mainly in thin-film technologies. These disordered materials still display intermediate conductivity, strong temperature dependence and even negative resistance in some cases, while being more tolerant of impurities and radiation damage.
Organic semiconductors and metal–organic frameworks also exhibit semiconducting behaviour and are being developed for flexible electronics, sensors and photovoltaic applications.

Preparation and Fabrication

The fabrication of semiconductor materials for integrated circuits requires exceptional purity and crystallographic perfection. Even minute impurities or structural defects—such as dislocations, stacking faults or twinning—can impair device performance. Modern production processes grow large cylindrical ingots of monocrystalline silicon, which are then sliced into wafers used as substrates for electronic circuits. The characteristic round shape of semiconductor wafers reflects the geometry of the grown crystal ingots.
Doping must be performed with precision to define regions of p-type and n-type material within a single device. Complex components such as transistors may contain multiple such regions, forming the basis for the electrical switching and amplifying properties essential to digital electronics. Techniques such as diffusion, ion implantation and epitaxy allow manufacturers to control dopant concentration and distribution with extraordinary accuracy.

Historical Development

Early observations of semiconductor behaviour emerged during the nineteenth century, leading to the first practical application in the cat’s-whisker detector of 1904, a primitive diode used in wireless telegraphy. As quantum theory developed, it provided explanations for semiconductor phenomena at the atomic level. These advances culminated in the invention of the transistor in 1947 and the integrated circuit in 1958, which revolutionised modern electronics and launched the microelectronics era.