A semiconductor refers to a material whose electrical conductivity is between that of a conductor and an insulator at room temperature. [24] The electrical conductivity of semiconductor materials can be changed by doping, and the concentration and polarity of impurities introduced into the intrinsic semiconductor will have a significant impact on the electrical conductivity of the semiconductor. In a semiconductor doped with donor impurities, the conducting carriers are mainly electrons in the conduction band, while in a semiconductor doped with acceptor impurities, it is a hole-type conductor. [25]
The discovery of semiconductor materials can be traced back to the 19th century. In 1833, British scientist Faraday was the first to discover the unique electrical conductivity phenomenon of silver sulfide semiconductor. Around 1911, the term “semiconductor” was first used by Korniecki and Weiss. In the early 20th century, although people had relatively little knowledge of semiconductors, the research on the application of semiconductor materials was quite active. In the 1950s, in order to improve the characteristics of transistors and enhance their stability, the preparation technology of semiconductor materials developed rapidly. Starting from the 1980s, the research on quantum wire materials and their semiconductor quantum devices became a hot topic in the field of materials science research, and a research craze in nanotechnology emerged internationally. After 1990, with the rapid development of communication, the second-generation semiconductor materials began to emerge. In the 21st century, the third-generation semiconductor materials such as SiC, GaN, and diamond began to show their presence. [26]
Common semiconductor materials include silicon, germanium, gallium arsenide, etc. Semiconductors can be classified into two major categories based on their chemical composition: elemental semiconductors and compound semiconductors. According to the impurities introduced, they can be classified as N-type semiconductors and P-type semiconductors. [25] The properties of semiconductors include optical properties and transport properties, etc. [21] Semiconductors are used in various fields such as integrated circuits, consumer electronics, communication systems, photovoltaic power generation, lighting, high-power power conversion, etc. For example, the diode is a device made of semiconductor materials. Basic meaning
Broadcast
There are many forms of matter, including solids, liquids, gases, plasma, etc. Materials with poor conductivity, such as coal, artificial crystals, amber, ceramics, etc., are usually called insulators. While materials with good conductivity, such as gold, silver, copper, iron, tin, aluminum, etc., are called conductors. [1] A semiconductor refers to a material whose electrical conductivity is between that of a conductor and an insulator at room temperature. It is a material with controllable electrical conductivity, ranging from insulator to conductor. From the perspective of science and technology development and economic growth, semiconductors affect people’s daily work and life until the 1930s when this material was recognized by the academic community.
Development history
Broadcast
Early discoveries
In 1833, British scientist Faraday was the first to discover that the resistance of silver sulfide semiconductor changed with temperature in a way different from ordinary metals. Generally, the resistance of metals increases with temperature, but Faraday found that the resistance of silver sulfide material decreased with the increase of temperature. This was the first discovery of the semiconductor phenomenon. [3] In 1839, French scientist Berzelius discovered the junction formed by the contact of semiconductor and electrolyte, which would generate a voltage when illuminated. This is the second characteristic of the discovered semiconductor. [3] In 1873, British scientist Smith discovered the photoconductive effect of selenium crystal materials, which increased with light exposure. This is the third characteristic of the semiconductor. [3] In 1874, German scientist Braun observed that the conductivity of certain sulfides was related to the direction of the applied electric field, that is, its conductivity has directionality. When a positive voltage is applied between its two ends, it is conductive; if the polarity of the voltage is reversed, it does not conduct. This is the rectification effect of the semiconductor, which is also its unique fourth characteristic. In the same year, Schuster also discovered the rectification effect of copper and copper oxide. [3] In 1879, American physicist Hall discovered the Hall effect. The Hall effect refers to the physical phenomenon where a lateral electric potential difference is generated in a thin film when a magnetic field acts on the charge carriers in a conductor or semiconductor. [18]
In the early stages of development
The physics revolution in the early 20th century (relativity and quantum mechanics) enabled people to understand the properties of the microscopic world (atoms and molecules), and these new theories were successfully applied to new fields (including semiconductors), the solid band theory laid a solid theoretical foundation for semiconductor technology, and the advancement of material growth technology provided a material basis for semiconductor technology (semiconductor materials require very pure substrate materials and very precise doping levels). [11] In 1906, Dunwoody invented the silicon carbide detector, thus initiating the application of semiconductors in radio. Subsequently, silicon, galena, chalcopyrite, and calomelite were all found to be suitable as detectors. [20] The term “semiconductor” was probably first used by Corniberg and Weiss in 1911. [3]
At the beginning of the 20th century, scientists conducted extensive research on semiconductor materials. The sample materials used at that time were mostly sulfides and oxides, and devices such as copper oxide rectifiers and selenium photovoltaic cells were fabricated. [44] The development of solid-state quantum theory led to the understanding of the electronic states in semiconductors, whether for light-conducting diodes, detectors, or rectifiers. At this stage, the semiconductor materials used were directly collected from nature or taken from common industrial products, without undergoing specialized purification and crystal formation processes. [20] In 1931, Lanchester and Bergman successfully developed selenium photovoltaic cells. In 1932, Germany successfully developed semiconductor infrared detectors using lead sulfide, lead selenide, and lead telluride, which were used in World War II for reconnaissance aircraft and ships. During World War II, the Allies also achieved significant progress in semiconductor research, and the United Kingdom used infrared detectors to detect German aircraft multiple times. [19]
Further development
In 1948, J. Bardeen, W.H. Brattain, and W.B. Shockley invented the transistor, which brought about a revolution in modern electronics and also promoted the vigorous development of semiconductor physics, materials, and device research. In the following decades, semiconductor microelectronics technology and semiconductor optoelectronics technology have become important technological foundations of modern society, triggering a worldwide information revolution and having a profound impact on the development of human civilization. [21]
In 1954, the proposal of the effective mass theory of semiconductors was a significant development in semiconductor theory, quantitatively describing the detailed energy band structure near the conduction band and valence band edges of semiconductors, providing theoretical methods for studying the energy levels of shallow impurities (donors and acceptors), exciton energy levels, and magnetic energy levels in semiconductors, and promoting experimental research such as cyclotron resonance, magnetic absorption, free carrier absorption, and exciton spectra at that time. [21] In 1958, integrated circuits were introduced. In 1959, the concept of pseudopotential was proposed, significantly simplifying the calculation of solid-state energy bands. Utilizing the orthogonality property of valence electron states and atomic core states, a pseudopotential was used to replace the real atomic potential, obtaining an equation satisfied by the valence electron states in a solid. Using the pseudopotential method, relatively accurate energy band structures of almost all semiconductors were obtained. In 1962, the semiconductor laser was invented. [21] In 1968, the silicon MOS (metal-oxide-semiconductor) device was invented and large-scale integrated circuits were realized through industrial production. [21]
In 1970, ultra-high vacuum surface spectroscopy analysis techniques emerged successively, initiating research on the physics of semiconductor surfaces and interfaces, including: the 7×7 surface reconstruction problem of silicon surfaces, the origin of Schottky barriers at the interface between metals and III-V compounds, the properties of CoSi/silicon and metal/silicon interfaces, and the problem of Fermi level pinning. In the early 1970s, Nishizaki Lingyu and R. Zhu, based on the idea of attempting to artificially control the potential distribution and wave function of electrons in semiconductors, first proposed the new concept of semiconductor superlattices. At the same time, Bell Laboratories in the United States invented molecular beam epitaxy technology. The ingenious combination of new ideas and new technologies resulted in the first lattice-matched component type AlyGa1-xAs/GaAs superlattice, marking the beginning of the semiconductor material development entering a new era of artificial design. In 1978, R. Dingel et al. studied the transport of two-dimensional electron gas along the direction parallel to the interface in heterojunctions and discovered the phenomenon of enhanced electron mobility. In the following years, due to process improvements, the mobility of two-dimensional electron gas was increased by nearly three orders of magnitude, leading to the emergence of high electron mobility transistors (HEMT) and creating conditions for the discovery of the quantum Hall effect. [21] In 1980, K.von Kleck from Germany discovered the integer quantum Hall effect. In 1982, Cui Qi and others discovered the fractional quantum Hall effect in Al, Ga1-y/GaAs heterojunctions with extremely high electron mobility. This was a significant discovery in semiconductor physics, and both discoveries were awarded the Nobel Prize in Physics. Due to the limiting effects of superlattices and quantum wells on electron movement, in 1984, D.A.B. Miller et al. observed the quantum confinement Stark effect where the energy of exciton absorption peaks in quantum wells changed with the electric field strength, as well as the exciton optical nonlinear effect caused by changes in exciton absorption coefficient or refractive index, providing important basis for designing new generation optical bistable devices. [21]
In 1990, L.T. Canham from the UK observed visible light photoluminescence in porous silicon at room temperature, bringing people a new glimmer of hope for all-silicon photonic integrated technology. Nanoparticles, nanoscale solids and nanofilm materials have opened up new fields in material research. These new types of functional materials, which contain a large number of surface or interface atoms, have many unique physical, chemical and mechanical properties and are hailed as the most promising materials in the 21st century. [21]
In October 2019, an international research team claimed that compared with only obtaining 3 parameters in traditional Hall measurements, the new technology can obtain up to 7 parameters at each test light intensity: including the mobility of electrons and holes; the carrier density under light, recombination lifetime, diffusion lengths of electrons, holes and bipolar types. [4]
In 2022, researchers discovered that cubic boron arsenide is one of the best semiconductors known in the scientific community, and is called the champion semiconductor. [12]
On November 5, 2024, the miniaturization of semiconductors has developed to the “nano” level, which is only 1/100,000 of the diameter of a hair. [14] In the same month, the 50-strong Forbes China Innovation Powerhouses list was released, and semiconductor-related enterprises Shanghai Super Silicon, Haiguang Information, Hanbo Semiconductor, Huawei HiSilicon, Changxin Storage, and Sinocum Company were selected. [15]
Related terms
Brief introduction Electron
In electronics, the most commonly used semiconductors are silicon and germanium. Both are 4-valent elements. The outermost electrons are least bound by the atomic nucleus and are called valence electrons. The chemical properties of a substance are determined by the number of valence electrons in the outermost shell. The electrical conductivity of semiconductors is also related to the valence electrons. Due to the close proximity of the atoms, each valence electron’s individual orbit becomes a common orbit for two valence electrons of adjacent atoms, forming a covalent bond structure in the crystal. The two electrons within a covalent bond are called bound electrons. Generally, without external excitation, there are no free electrons in silicon and germanium crystals, and only in the case of external excitation can a few electrons acquire a certain kinetic energy to break free from the covalent bond and become free electrons.
Hole
After the electrons break free from the covalent bond and become free electrons, a vacancy is left in the covalent bond, which is called a hole. Under normal circumstances, atoms are electrically neutral. When electrons break free from the covalent bond and become free electrons, the electrical neutrality of the atom is disrupted and it shows a positive charge. Under the action of an external electric field, atoms with holes can attract the valence electrons in adjacent atoms and fill this vacancy. At the same time, in the covalent bond of the adjacent atom that has lost a valence electron, another vacancy appears, which can be replenished by the valence electrons from adjacent atoms, and another vacancy appears in that atom. This continues until it seems as if the vacancies are moving. The direction of hole movement is opposite to that of valence electrons, so hole movement is equivalent to the movement of positive charges. [23]
Carrier
Therefore, when an external voltage is applied to the semiconductor, two parts of current will appear in the semiconductor: one is the electron current formed by the directional movement of free electrons; the other is the hole current formed by the substitution of vacancies by the valence electrons that are still bound by the atomic nucleus. In semiconductors, both electron conduction and hole conduction exist, which is the greatest characteristic of semiconductor conduction and also the essential difference between semiconductors and metals in terms of conduction principles. The free electrons and holes in semiconductors are called carriers. [23]
Band structure
For solids formed by the periodic arrangement of atoms, namely crystalline solids (crystals), according to the band theory, their band structure consists of a series of bands, and the adjacent bands are separated by a forbidden band. Within the energy covered by the forbidden band, there are no electron energy levels. At absolute zero temperature, the electrons in the solid occupy different energy bands from low to high, the core electrons fill all the low-energy series of energy bands, and are tightly bound to the respective atomic nuclei, so in general experimental conditions, the contribution of core electrons to the physical properties of the crystal does not need to be considered. The energy bands above the core electron bands are the valence bands, and there is a series of unoccupied energy bands above the valence bands, called the valence bands. According to the occupation of valence electrons in the valence bands, solids can be classified as metals and insulators. In the energy band structure of metals, the valence band is the unoccupied band, and the Fermi level is located in this unoccupied band; while in the energy band structure of insulators, the valence electrons fill the valence band, and the energy bands above the valence band are the valence bands, and the band gap is located just above the top of the valence band. [27]
PN Junction
In the semiconductor, due to different doping, the densities of electrons and holes in the two types of semiconductors are not the same. That is, in P-type semiconductors, there are more holes and fewer electrons, while in N-type semiconductors, there are more electrons and fewer holes. When a P-type semiconductor and an N-type semiconductor are in contact with each other, electrons in the N-type region will diffuse into the P-type region, and holes in the P-type region will diffuse into the N-type region. As a result, a positive and negative charge accumulation occurs at the junction, and a positive charge is on one side of the P region, and a negative charge is on the other side of the N region. These charges form a space charge region (electrically induced layer or depletion layer) at the junction, and this structure is called a PN junction, with a thickness typically in the micrometer range