Semiconductors are materials with electrical conductivity between conductors and insulators. They can control the flow of electrical current and are essential in modern electronics such as diodes, transistors, and integrated circuits.
Semiconductors are essential to modern electronics, enabling the reception, processing, and storage of data in numerous electronic devices. From smartphones and smartwatches to ATMs, trains, and airplanes, these tiny components are everywhere and power much of the digital world that we rely on every day.
This article offers a clear introduction to what semiconductors are, how they work, the key discoveries that led to their development, and where the industry is heading in the future.
Semiconductors have a conductivity that lies in between an insulator and a conductive material. This means that they can act both as conductors and insulators, depending on the external factors that are applied to them.
The first semiconductor effect was reported in 1833 by Michael Faraday, who noted silver sulfide’s conductivity increased with temperature - an opposite trend to that seen in metals like copper.
In 1874, Ferdinand Braun then provided the first written description of a semiconductor diode made from lead sulfide crystals. Here, he observed that current flowed freely in only one direction when probed with a thin metal wire. Though there was no application for it at the time, this discovery later became integral to early radio technology as a signal detector in crystal radios.
A major leap occurred in 1947 when John Bardeen, Walter Brattain, and William Shockley at Bell Labs used geranium to develop the first transistor, a point-contact device that could amplify and switch electrical signals. Shockley’s refinement of the design into the junction transistor paved the way for compact, reliable, and energy-efficient electronic devices. This is thought to be one of the most important inventions in history, ushering in the era of modern computing and the rise of Silicon Valley.
Semiconductors are able to manipulate electricity due to two key principles:
There are different ways that charge can be conducted through a semiconductor, the mechanism of which depends on the material that is used. The majority of semiconductors are currently made from silicon, which forms a solid state lattice that can be modified to incorporate different atoms. Charge flows through the lattice, and different atoms incorporate more of a particular type of charge carrier (either electron or hole). Other semiconducting materials such as organic molecules do not form lattices. Instead, charge flows within and between neighbouring molecules.
Understanding how semiconductors work comes down to the fundamental properties of solids.
The electrons within an atom in a solid are confined to energy bands. The valence band of an atom contains the outermost electrons, which form bonds between atoms. The electrons in the valence band have a lower relative energy, and do not usually help to conduct electricity. They are tightly bound to their atoms and cannot move freely.
The conduction band is a range of energy levels that, at room temperature, do not normally contain electrons. However, if a material is given extra energy, for example from heat or light, some electrons in the valence band become able to jump to the conduction band. When this happens, they can move freely and contribute to the material’s electrical conductivity.
The ability of a material to conduct electricity therefore depends on two main factors: the width of the gap between the conduction and valence bands (known at the energy band gap), and the number of available electrons in the conduction band.
Within conductors, the valence and conduction bands overlap which allows electrons to easily move through a material and conduct electricity.
Insulators have a very large band gap, meaning a lot of energy is required for electrons to move from the valence band to the conduction band. This is unlikely to occur at room temperature and so the materials will not conduct electricity.
Within semiconductors, the band gap lies somewhere in-between that of conductors and insulators. The gap is large enough to prevent electrons from flowing freely through the material at room temperature. When an external factor is applied, such as light or heat, some electrons in the valence band are able to gain enough energy to jump to the conduction band.
Transistors consist of three semiconductors joined together. They are used in switching and amplification devices, making it possible for current to be turned off or amplified through a device.
The evolution of semiconductors follows a concept known as Moore’s law, which states that the number of transistors on a microchip doubles approximately every two years.
Despite silicon being the most common semiconductor material, it is naturally electrically inert. Silicon has four electrons in its outer orbital shell which form covalent bonds with surrounding atoms, creating a crystalline structure. This stability means there is a lack of free moving electrons, causing silicon to behave as an insulator.
Because of this, silicon semiconductors are known as extrinsic semiconductors, meaning they are prepared by doping them with specific impurities to improve the electrical properties.These atoms typically have three or five electrons in the valence band, creating an imbalance that permits the free movement of electrons. The doping of silicon can be done in two ways:
The Fermi level describes the energy level where there is the maximum probability of finding an electron at absolute zero temperature (in which there is no transition of electron from one energy level to another). A material’s fermi level exists between the valence band and the conduction band.
In intrinsic semiconductors, where no doping has been done, the number of electrons is equal to the number of holes. Therefore, the probability of finding an electron in the conduction band is equal to the probability of finding a hole in the valence band. This results in the Fermi level being an equal distance from the valence band and conduction band.
In extrinsic semiconductors, where doping has been done, the number of electrons and the number of holes are not equal. Therefore, the probability of occupation of the different energy levels is not equal, and the Fermi level shifts either towards the conduction or valence band. In an n-type semiconductor, the addition of free electrons causes a shift towards the conduction band. In a p-type semiconductor, the increase in holes shifts the Fermi level towards the valence band.
A thumb-sized chip can contain billions of transistors, which are small units that are used for conducting and switching electrical current. The fabrication process of traditional silicon semiconductor is, therefore, often long and complex. It combines physics, chemistry, electronics, metallurgy, and more.
Before the fabrication process begins, silicon must be prepared into a usable form. Mined silicon contains impurities which must be removed to some extent.Semiconductors require silicon that is over 99.9% pure, a much higher grade than other applications. The Siemens Process is used to purify metallurgical grade silicon to an ultra-high purity. Here, impurities such as iron and aluminium are removed by reacting powdered MG-Si with anhydrous HCl to form SiHCl3. The impurities simultaneously react to form their halides (FeCl3, AlCl3) which can then be separated via distillation. The SiHCl3 is then reacted with hydrogen at a very high temperature for a long period of time to obtain the final silicon rods that are suitable for semiconductor production.
The pure silicon is then shaped into cylindrical rods and doped with boron or phosphorus to improve its electrical properties. It is then cut into thin, flat wafers that can be used in the fabrication process.
The silicon wafer is polished so that it has a smooth, reflective finish. This makes it suitable for a patterned layer to be printed on top.
The silicon wafer is set in a thermal processing system, and exposed to oxygen at a high temperature. This creates a layer of silicon oxide. A silicon nitride layer is then formed by introducing silane and ammonia gases
A UV-sensitive layer is applied to the wafer using a coating device. The coating is commonly known as ‘photoresist’, and there are two types that may be used: positive resist and negative resist. Positive resist becomes more soluble when exposed to UV light, and creates an exact copy of a pattern on a semiconductor wafer. Negative resist is the opposite, and hardens when exposed to UV light. As a result, it leaves a mask that is the inverse of the pattern used.Positive resist is more common within the fabrication industry thanks to its higher resolution, though negative resist is faster and has a lower operating cost.
A lithography machine is used to project a desired pattern across a wafer multiple time. Deep UV (using optical lenses) or extreme UV (using mirrors) light is focused onto the resist later, which causes chemical change to occur in the coating.
Patterns are engraved onto the wafer using an etching machine. This selectively removes photoresist from the wafer to reveal the desired pattern. The wafer is then baked and developed.
The etching and lithography stages are repeated several times to layer different patterns on top of one another. This creates the layers of transistors, which are connected via copper wiring and enable signals to travel across the chip.
More doping may be done at this stage to tune the electrical conducting properties of the pattern on the wafer.
A single silicon wafer is used to create many semiconductor chips. Diamonds are used to extract individual chips from the wafers, with a typical semiconductor being around 300 mm in size. Before packaging, a probe station is used to test individual dies on a wafer to verify electrical performance and identify defects. A probe station enables micron-level positioning of microscopic probs onto contact pads without damaging the wafer, for high-accuracy measurements.
Afterwards, the chips are positioned onto a substrate with metal foils directing input and output signals to other system components.
Organic semiconductors differ from traditional inorganic semiconductors in that their structure is primarily composed of carbon and hydrogen atoms. They contain π-conjugated systems in which bonding orbitals along the carbon backbone are overlapping, leading to electron delocalization within each molecule. This delocalization enables the movement of free electrons, making conductivity possible.
The energy levels that these electrons occupy are described as the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), which is similar to the valence and conduction bands in organic semiconductors.
Additionally, bonding orbitals can overlap between neighbouring molecules, creating intermolecular electronic coupling. In this case, delocalized electrons can hop between molecules, further facilitating charge transport.
Generally, organic semiconductors are more affordable and easier to process compared to inorganic semiconductors. Small molecule organic semiconductors can be made into well-defined, high quality films and layered devices without needing intense heat or ultra-clean conditions. The chemistry behind organic semiconductors means that their applications are slightly different. Most prominently, organic semiconductors are being explored for applications in flexible and printable electronics such as light-emitting diodes (LEDs), solar cells, and sensors.
Semiconductors are one of the most traded goods in the world, and sales are continuously growing. US companies invest around one fifth of their annual revenue into semiconductor research and development, and the huge surge in the use of generative AI is causing demand for computational power, and therefore semiconductors, to skyrocket. Current projections suggest that the semiconductor industry will exceed $1 trillion before the end of the decade.
Currently, the future of semiconductor research can be categorized into two distinct routes; better semiconductors for more complex applications, and improved sustainability within the manufacturing industry.
For example, semiconductors are at the centre of the emerging quantum computing industry and are required to create qubits, the basic units of quantum information. Qubits can exist in multiple states at once making it possible for some computational problems to be solved much faster, and to perform complex calculations that classical computers cannot. This research is still in its early stages, and more research is needed to maintain stability within the qubits and scale up quantum systems.
Additionally, demand for semiconductors is ever increasing and there is currently a big trend to build resilience to volatility within the supply chain. As such, researchers are looking for better, more cost effective and environmentally friendly methods of producing semiconductors.