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102-435: Gallium arsenide is used in the manufacture of devices such as microwave frequency integrated circuits , monolithic microwave integrated circuits , infrared light-emitting diodes , laser diodes , solar cells and optical windows. GaAs is often used as a substrate material for the epitaxial growth of other III-V semiconductors, including indium gallium arsenide , aluminum gallium arsenide and others. Gallium arsenide

204-619: A band being used for Milstar . Global Navigation Satellite Systems (GNSS) including the Chinese Beidou , the American Global Positioning System (introduced in 1978) and the Russian GLONASS broadcast navigational signals in various bands between about 1.2 GHz and 1.6 GHz. Radar is a radiolocation technique in which a beam of radio waves emitted by a transmitter bounces off an object and returns to

306-514: A carcinogen , as do IARC and ECA , and it is considered a known carcinogen in animals. On the other hand, a 2013 review (funded by industry) argued against these classifications, saying that when rats or mice inhale fine GaAs powders (as in previous studies), they get cancer from the resulting lung irritation and inflammation, rather than from a primary carcinogenic effect of the GaAs itself—and that, moreover, fine GaAs powders are unlikely to be created in

408-516: A standing wave is present, they may also be used to measure the distance between the nodes , which is equal to half the wavelength. The precision of this method is limited by the determination of the nodal locations. Microwaves are non-ionizing radiation, which means that microwave photons do not contain sufficient energy to ionize molecules or break chemical bonds, or cause DNA damage, as ionizing radiation such as x-rays or ultraviolet can. The word "radiation" refers to energy radiating from

510-454: A . Microwaves travel solely by line-of-sight paths; unlike lower frequency radio waves, they do not travel as ground waves which follow the contour of the Earth, or reflect off the ionosphere ( skywaves ). Although at the low end of the band they can pass through building walls enough for useful reception, usually rights of way cleared to the first Fresnel zone are required. Therefore, on

612-431: A computer-controlled array of antennas that produces a beam that can be electronically steered in different directions. At microwave frequencies, the transmission lines which are used to carry lower frequency radio waves to and from antennas, such as coaxial cable and parallel wire lines , have excessive power losses, so when low attenuation is required, microwaves are carried by metal pipes called waveguides . Due to

714-420: A coupled electric field and magnetic field could travel through space as an electromagnetic wave , and proposed that light consisted of electromagnetic waves of short wavelength. In 1888, German physicist Heinrich Hertz was the first to demonstrate the existence of electromagnetic waves, generating radio waves using a primitive spark gap radio transmitter . Direct band gap In semiconductors ,

816-530: A frequency near 2.45 GHz (12 cm) through food, causing dielectric heating primarily by absorption of the energy in water. Microwave ovens became common kitchen appliances in Western countries in the late 1970s, following the development of less expensive cavity magnetrons . Water in the liquid state possesses many molecular interactions that broaden the absorption peak. In the vapor phase, isolated water molecules absorb at around 22 GHz, almost ten times

918-422: A harmonic generator and a mixer. The accuracy of the measurement is limited by the accuracy and stability of the reference source. Mechanical methods require a tunable resonator such as an absorption wavemeter , which has a known relation between a physical dimension and frequency. In a laboratory setting, Lecher lines can be used to directly measure the wavelength on a transmission line made of parallel wires,

1020-438: A much thinner region, and consequently can be made with a very thin active layer (often less than 1 micron thick). The absorption spectrum of an indirect band gap material usually depends more on temperature than that of a direct material, because at low temperatures there are fewer phonons, and therefore it is less likely that a photon and phonon can be simultaneously absorbed to create an indirect transition. For example, silicon

1122-519: A nearly perfect lattice; impurity density is very low and allows very small structures to be built (down to 5 nm in commercial production as of 2020). In contrast, GaAs has a very high impurity density, which makes it difficult to build integrated circuits with small structures, so the 500 nm process is a common process for GaAs. Silicon has about three times the thermal conductivity of GaAs, with less risk of local overheating in high power devices. Gallium arsenide (GaAs) transistors are used in

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1224-530: A place in the electromagnetic spectrum with frequency above ordinary radio waves , and below infrared light: In descriptions of the electromagnetic spectrum , some sources classify microwaves as radio waves, a subset of the radio wave band, while others classify microwaves and radio waves as distinct types of radiation. This is an arbitrary distinction. Bands of frequencies in the microwave spectrum are designated by letters. Unfortunately, there are several incompatible band designation systems, and even within

1326-635: A possible gate oxide for GaAs (as well as InGaAs ). The third advantage of silicon is that it possesses a higher hole mobility compared to GaAs (500 versus 400 cmVs). This high mobility allows the fabrication of higher-speed P-channel field-effect transistors , which are required for CMOS logic. Because they lack a fast CMOS structure, GaAs circuits must use logic styles which have much higher power consumption; this has made GaAs logic circuits unable to compete with silicon logic circuits. For manufacturing solar cells, silicon has relatively low absorptivity for sunlight, meaning about 100 micrometers of Si

1428-515: A receiver, allowing the location, range, speed, and other characteristics of the object to be determined. The short wavelength of microwaves causes large reflections from objects the size of motor vehicles, ships and aircraft. Also, at these wavelengths, the high gain antennas such as parabolic antennas which are required to produce the narrow beamwidths needed to accurately locate objects are conveniently small, allowing them to be rapidly turned to scan for objects. Therefore, microwave frequencies are

1530-618: A semiconductor band gap has almost zero momentum. One important process is called radiative recombination , where an electron in the conduction band annihilates a hole in the valence band, releasing the excess energy as a photon. This is possible in a direct band gap semiconductor if the electron has a k-vector near the conduction band minimum (the hole will share the same k-vector), but not possible in an indirect band gap semiconductor, as photons cannot carry crystal momentum, and thus conservation of crystal momentum would be violated. For radiative recombination to occur in an indirect band gap material,

1632-592: A single slice of GaAs. One of the first GaAs microprocessors was developed in the early 1980s by the RCA Corporation and was considered for the Star Wars program of the United States Department of Defense . These processors were several times faster and several orders of magnitude more radiation resistant than their silicon counterparts, but were more expensive. Other GaAs processors were implemented by

1734-429: A solar cell with 33.3% efficiency based on inverted metamorphic multi-junction (IMM) technology. In IMM, the lattice-matched (same lattice parameters) materials are grown first, followed by mismatched materials. The top cell, GaInP, is grown first and lattice matched to the GaAs substrate, followed by a layer of either GaAs or GaInAs with a minimal mismatch, and the last layer has the greatest lattice mismatch. After growth,

1836-507: A solar cell, and thus a smaller (and therefore less expensive) GaAs solar cell is needed to achieve the same results. Concentrator systems have the highest efficiency of existing photovoltaics. So, technologies such as concentrator photovoltaics and methods in development to lower epitaxial growth and substrate costs could lead to a reduction in the cost of GaAs solar cells and forge a path for use in terrestrial applications. GaAs has been used to produce near-infrared laser diodes since 1962. It

1938-449: A source and not to radioactivity . The main effect of absorption of microwaves is to heat materials; the electromagnetic fields cause polar molecules to vibrate. It has not been shown conclusively that microwaves (or other non-ionizing electromagnetic radiation) have significant adverse biological effects at low levels. Some, but not all, studies suggest that long-term exposure may have a carcinogenic effect. During World War II , it

2040-558: A system the frequency ranges corresponding to some of the letters vary somewhat between different application fields. The letter system had its origin in World War 2 in a top-secret U.S. classification of bands used in radar sets; this is the origin of the oldest letter system, the IEEE radar bands. One set of microwave frequency bands designations by the Radio Society of Great Britain (RSGB),

2142-446: A tert-butyl gallium sulfide compound such as ( BuGaS) 7 . In the presence of excess arsenic, GaAs boules grow with crystallographic defects ; specifically, arsenic antisite defects (an arsenic atom at a gallium atom site within the crystal lattice). The electronic properties of these defects (interacting with others) cause the Fermi level to be pinned to near the center of

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2244-492: A vacuum under the influence of controlling electric or magnetic fields, and include the magnetron (used in microwave ovens ), klystron , traveling-wave tube (TWT), and gyrotron . These devices work in the density modulated mode, rather than the current modulated mode. This means that they work on the basis of clumps of electrons flying ballistically through them, rather than using a continuous stream of electrons. Low-power microwave sources use solid-state devices such as

2346-410: A viable and actively pursued alternative as of the 1980s was ion implantation. The second major advantage of Si is the existence of a native oxide ( silicon dioxide , SiO 2 ), which is used as an insulator . Silicon dioxide can be incorporated onto silicon circuits easily, and such layers are adherent to the underlying silicon. SiO 2 is not only a good insulator (with a band gap of 8.9 eV ), but

2448-531: Is a result of higher carrier mobilities and lower resistive device parasitics. These superior properties are compelling reasons to use GaAs circuitry in mobile phones , satellite communications, microwave point-to-point links and higher frequency radar systems. It is also used in the manufacture of Gunn diodes for the generation of microwaves . Another advantage of GaAs is that it has a direct band gap , which means that it can be used to absorb and emit light efficiently. Silicon has an indirect band gap and so

2550-426: Is also a fairly good thermal conductor, thus enabling very dense packing of transistors that need to get rid of their heat of operation, all very desirable for design and manufacturing of very large ICs . Such good mechanical characteristics also make it a suitable material for the rapidly developing field of nanoelectronics . Naturally, a GaAs surface cannot withstand the high temperatures needed for diffusion; however

2652-566: Is also more bandwidth in the microwave spectrum than in the rest of the radio spectrum; the usable bandwidth below 300 MHz is less than 300 MHz while many GHz can be used above 300 MHz. Typically, microwaves are used in remote broadcasting of news or sports events as the backhaul link to transmit a signal from a remote location to a television station from a specially equipped van. See broadcast auxiliary service (BAS), remote pickup unit (RPU), and studio/transmitter link (STL). Most satellite communications systems operate in

2754-426: Is also preliminary evidence that spalling could be used to remove the substrate for reuse. An alternative path to reduce substrate cost is to use cheaper materials, although materials for this application are not currently commercially available or developed. Yet another consideration to lower GaAs solar cell costs could be concentrator photovoltaics . Concentrators use lenses or parabolic mirrors to focus light onto

2856-416: Is also used to perform rotational spectroscopy and can be combined with electrochemistry as in microwave enhanced electrochemistry . Microwave frequency can be measured by either electronic or mechanical techniques. Frequency counters or high frequency heterodyne systems can be used. Here the unknown frequency is compared with harmonics of a known lower frequency by use of a low-frequency generator,

2958-499: Is an important semiconductor material for high-cost, high-efficiency solar cells and is used for single-crystalline thin-film solar cells and for multi-junction solar cells . The first known operational use of GaAs solar cells in space was for the Venera 3 mission, launched in 1965. The GaAs solar cells, manufactured by Kvant, were chosen because of their higher performance in high temperature environments. GaAs cells were then used for

3060-400: Is epitaxial lift-off (ELO), but this method is time-consuming, somewhat dangerous (with its use of hydrofluoric acid ), and requires multiple post-processing steps. However, other methods have been proposed that use phosphide-based materials and hydrochloric acid to achieve ELO with surface passivation and minimal post- etching residues and allows for direct reuse of the GaAs substrate. There

3162-555: Is inversely proportional to the transmitted frequency. Microwaves are used in spacecraft communication, and much of the world's data, TV, and telephone communications are transmitted long distances by microwaves between ground stations and communications satellites . Microwaves are also employed in microwave ovens and in radar technology. Before the advent of fiber-optic transmission, most long-distance telephone calls were carried via networks of microwave radio relay links run by carriers such as AT&T Long Lines . Starting in

Gallium arsenide - Misplaced Pages Continue

3264-548: Is largely due to the cost of GaAs solar cells - in space applications, high performance is required and the corresponding high cost of the existing GaAs technologies is accepted. For example, GaAs-based photovoltaics show the best resistance to gamma radiation and high temperature fluctuations, which are of great importance for spacecraft. But in comparison to other solar cells, III-V solar cells are two to three orders of magnitude more expensive than other technologies such as silicon-based solar cells. The primary sources of this cost are

3366-444: Is light absorption. For the same reason as above, light with a photon energy close to the band gap can penetrate much farther before being absorbed in an indirect band gap material than a direct band gap one (at least insofar as the light absorption is due to exciting electrons across the band gap). This fact is very important for photovoltaics (solar cells). Crystalline silicon is the most common solar-cell substrate material, despite

3468-424: Is limited to a few kilometers. A spectral band structure causes absorption peaks at specific frequencies (see graph at right). Above 100 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so effective that it is in effect opaque , until the atmosphere becomes transparent again in the so-called infrared and optical window frequency ranges. In a microwave beam directed at an angle into

3570-413: Is needed to absorb most sunlight. Such a layer is relatively robust and easy to handle. In contrast, the absorptivity of GaAs is so high that only a few micrometers of thickness are needed to absorb all of the light. Consequently, GaAs thin films must be supported on a substrate material. Silicon is a pure element, avoiding the problems of stoichiometric imbalance and thermal unmixing of GaAs. Silicon has

3672-428: Is often used in alloys with other semiconductor compounds for these applications. N -type GaAs doped with silicon donor atoms (on Ga sites) and boron acceptor atoms (on As sites) responds to ionizing radiation by emitting scintillation photons. At cryogenic temperatures it is among the brightest scintillators known and is a promising candidate for detecting rare electronic excitations from interacting dark matter, due to

3774-418: Is opaque to visible light at room temperature, but transparent to red light at liquid helium temperatures, because red photons can only be absorbed in an indirect transition. A common and simple method for determining whether a band gap is direct or indirect uses absorption spectroscopy . By plotting certain powers of the absorption coefficient against photon energy, one can normally tell both what value

3876-643: Is relatively poor at emitting light. As a wide direct band gap material with resulting resistance to radiation damage, GaAs is an excellent material for outer space electronics and optical windows in high power applications. Because of its wide band gap, pure GaAs is highly resistive. Combined with a high dielectric constant , this property makes GaAs a very good substrate for integrated circuits and unlike Si provides natural isolation between devices and circuits. This has made it an ideal material for monolithic microwave integrated circuits (MMICs), where active and essential passive components can readily be produced on

3978-432: Is tabulated below: Other definitions exist. The term P band is sometimes used for UHF frequencies below the L band but is now obsolete per IEEE Std 521. When radars were first developed at K band during World War 2, it was not known that there was a nearby absorption band (due to water vapor and oxygen in the atmosphere). To avoid this problem, the original K band was split into a lower band, K u , and upper band, K

4080-617: Is the range between 1 and 100 GHz (wavelengths between 30 cm and 3 mm), or between 1 and 3000 GHz (30 cm and 0.1 mm). The prefix micro- in microwave is not meant to suggest a wavelength in the micrometer range; rather, it indicates that microwaves are small (having shorter wavelengths), compared to the radio waves used in prior radio technology . The boundaries between far infrared , terahertz radiation , microwaves, and ultra-high-frequency (UHF) are fairly arbitrary and are used variously between different fields of study. In all cases, microwaves include

4182-508: Is used for monolithic radar power amplifiers (but GaN can be less susceptible to heat damage). Silicon has three major advantages over GaAs for integrated circuit manufacture. First, silicon is abundant and cheap to process in the form of silicate minerals. The economies of scale available to the silicon industry has also hindered the adoption of GaAs. In addition, a Si crystal has a very stable structure and can be grown to very large diameter boules and processed with very good yields. It

Gallium arsenide - Misplaced Pages Continue

4284-455: Is why light-emitting and laser diodes are almost always made of direct band gap materials, and not indirect band gap ones like silicon . The fact that radiative recombination is slow in indirect band gap materials also means that, under most circumstances, radiative recombinations will be a small proportion of total recombinations, with most recombinations being non-radiative, taking place at point defects or at grain boundaries . However, if

4386-602: The Big Bang , and is one of the few sources of information about conditions in the early universe. Due to the expansion and thus cooling of the Universe, the originally high-energy radiation has been shifted into the microwave region of the radio spectrum. Sufficiently sensitive radio telescopes can detect the CMBR as a faint signal that is not associated with any star, galaxy, or other object. A microwave oven passes microwave radiation at

4488-482: The Fraunhofer Institute for Solar Energy Systems achieved a 68.9% efficiency when exposing a GaAs thin film photovoltaic cell to monochromatic laser light with a wavelength of 858 nanometers. Today, multi-junction GaAs cells have the highest efficiencies of existing photovoltaic cells and trajectories show that this is likely to continue to be the case for the foreseeable future. In 2022, Rocket Lab unveiled

4590-701: The Lunokhod rovers for the same reason. In 1970, the GaAs heterostructure solar cells were developed by the team led by Zhores Alferov in the USSR , achieving much higher efficiencies. In the early 1980s, the efficiency of the best GaAs solar cells surpassed that of conventional, crystalline silicon -based solar cells. In the 1990s, GaAs solar cells took over from silicon as the cell type most commonly used for photovoltaic arrays for satellite applications. Later, dual- and triple-junction solar cells based on GaAs with germanium and indium gallium phosphide layers were developed as

4692-457: The band gap of a semiconductor can be of two basic types, a direct band gap or an indirect band gap . The minimal-energy state in the conduction band and the maximal-energy state in the valence band are each characterized by a certain crystal momentum (k-vector) in the Brillouin zone . If the k-vectors are different, the material has an "indirect gap". The band gap is called "direct" if

4794-420: The epitaxial growth costs and the substrate the cell is deposited on. GaAs solar cells are most commonly fabricated utilizing epitaxial growth techniques such as metal-organic chemical vapor deposition (MOCVD) and hydride vapor phase epitaxy (HVPE). A significant reduction in costs for these methods would require improvements in tool costs, throughput, material costs, and manufacturing efficiency. Increasing

4896-577: The field-effect transistor (at least at lower frequencies), tunnel diodes , Gunn diodes , and IMPATT diodes . Low-power sources are available as benchtop instruments, rackmount instruments, embeddable modules and in card-level formats. A maser is a solid-state device which amplifies microwaves using similar principles to the laser , which amplifies higher frequency light waves. All warm objects emit low level microwave black-body radiation , depending on their temperature , so in meteorology and remote sensing , microwave radiometers are used to measure

4998-473: The supercomputer vendors Cray Computer Corporation, Convex , and Alliant in an attempt to stay ahead of the ever-improving CMOS microprocessor. Cray eventually built one GaAs-based machine in the early 1990s, the Cray-3 , but the effort was not adequately capitalized, and the company filed for bankruptcy in 1995. Complex layered structures of gallium arsenide in combination with aluminium arsenide (AlAs) or

5100-410: The 1950s. First infrared LEDs were made in 1962. In the compound, gallium has a +3 oxidation state . Gallium arsenide single crystals can be prepared by three industrial processes: Alternative methods for producing films of GaAs include: Oxidation of GaAs occurs in air, degrading performance of the semiconductor. The surface can be passivated by depositing a cubic gallium(II) sulfide layer using

5202-438: The 2.3 GHz, 2.5 GHz, 3.5 GHz and 5.8 GHz ranges. Mobile Broadband Wireless Access (MBWA) protocols based on standards specifications such as IEEE 802.20 or ATIS/ANSI HC-SDMA (such as iBurst ) operate between 1.6 and 2.3 GHz to give mobility and in-building penetration characteristics similar to mobile phones but with vastly greater spectral efficiency. Some mobile phone networks, like GSM , use

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5304-425: The 5 GHz range. Licensed long-range (up to about 25 km) Wireless Internet Access services have been used for almost a decade in many countries in the 3.5–4.0 GHz range. The FCC recently carved out spectrum for carriers that wish to offer services in this range in the U.S. — with emphasis on 3.65 GHz. Dozens of service providers across the country are securing or have already received licenses from

5406-685: The 95 GHz focused beam heats the skin to a temperature of 54 °C (129 °F) at a depth of 0.4 millimetres ( 1 ⁄ 64  in). The United States Air Force and Marines are currently using this type of active denial system in fixed installations. Microwave radiation is used in electron paramagnetic resonance (EPR or ESR) spectroscopy, typically in the X-band region (~9 GHz) in conjunction typically with magnetic fields of 0.3 T. This technique provides information on unpaired electrons in chemical systems, such as free radicals or transition metal ions such as Cu(II). Microwave radiation

5508-437: The C, X, K a , or K u bands of the microwave spectrum. These frequencies allow large bandwidth while avoiding the crowded UHF frequencies and staying below the atmospheric absorption of EHF frequencies. Satellite TV either operates in the C band for the traditional large dish fixed satellite service or K u band for direct-broadcast satellite . Military communications run primarily over X or K u -band links, with K

5610-454: The FCC to operate in this band. The WIMAX service offerings that can be carried on the 3.65 GHz band will give business customers another option for connectivity. Metropolitan area network (MAN) protocols, such as WiMAX (Worldwide Interoperability for Microwave Access) are based on standards such as IEEE 802.16 , designed to operate between 2 and 11 GHz. Commercial implementations are in

5712-480: The RF power amplifiers for cell phones and wireless communicating. GaAs wafers are used in laser diodes , photodetectors , and radio frequency (RF) amplifiers for mobile phones and base stations. GaAs transistors are also integral to monolithic microwave integrated circuits (MMICs) , utilized in satellite communication and radar systems, as well as in low-noise amplifiers (LNAs) that enhance weak signals. Gallium arsenide

5814-450: The Si-SiO 2 interface can be easily engineered to have excellent electrical properties, most importantly low density of interface states. GaAs does not have a native oxide, does not easily support a stable adherent insulating layer, and does not possess the dielectric strength or surface passivating qualities of the Si-SiO 2 . Aluminum oxide (Al 2 O 3 ) has been extensively studied as

5916-425: The alloy Al x Ga 1−x As can be grown using molecular-beam epitaxy (MBE) or using metalorganic vapor-phase epitaxy (MOVPE). Because GaAs and AlAs have almost the same lattice constant , the layers have very little induced strain , which allows them to be grown almost arbitrarily thick. This allows extremely high performance and high electron mobility HEMT transistors and other quantum well devices. GaAs

6018-485: The band gap is, and whether or not it is direct. For a direct band gap, the absorption coefficient α {\displaystyle \alpha } is related to light frequency according to the following formula: where: This formula is valid only for light with photon energy larger, but not too much larger, than the band gap (more specifically, this formula assumes the bands are approximately parabolic), and ignores all other sources of absorption other than

6120-519: The band gap, so that this GaAs crystal has very low concentration of electrons and holes. This low carrier concentration is similar to an intrinsic (perfectly undoped) crystal, but much easier to achieve in practice. These crystals are called "semi-insulating", reflecting their high resistivity of 10–10 Ω·cm (which is quite high for a semiconductor, but still much lower than a true insulator like glass). Wet etching of GaAs industrially uses an oxidizing agent such as hydrogen peroxide or bromine water, and

6222-408: The band-to-band absorption in question, as well as the electrical attraction between the newly created electron and hole (see exciton ). It is also invalid in the case that the direct transition is forbidden , or in the case that many of the valence band states are empty or conduction band states are full. On the other hand, for an indirect band gap, the formula is: where: This formula involves

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6324-530: The basis of a triple-junction solar cell, which held a record efficiency of over 32% and can operate also with light as concentrated as 2,000 suns. This kind of solar cell powered the Mars Exploration Rovers Spirit and Opportunity , which explored Mars ' surface. Also many solar cars utilize GaAs in solar arrays, as did the Hubble Telescope. GaAs-based devices hold the world record for

6426-490: The body. The lens and cornea of the eye are especially vulnerable because they contain no blood vessels that can carry away heat. Exposure to microwave radiation can produce cataracts by this mechanism, because the microwave heating denatures proteins in the crystalline lens of the eye (in the same way that heat turns egg whites white and opaque). Exposure to heavy doses of microwave radiation (as from an oven that has been tampered with to allow operation even with

6528-477: The cell is mounted to a secondary handle and the GaAs substrate is removed. A main advantage of the IMM process is that the inverted growth according to lattice mismatch allows a path to higher cell efficiency. Complex designs of Al x Ga 1−x As-GaAs devices using quantum wells can be sensitive to infrared radiation ( QWIP ). GaAs diodes can be used for the detection of X-rays. Despite GaAs-based photovoltaics being

6630-411: The clear champions of efficiency for solar cells, they have relatively limited use in today's market. In both world electricity generation and world electricity generating capacity, solar electricity is growing faster than any other source of fuel (wind, hydro, biomass, and so on) for the last decade. However, GaAs solar cells have not currently been adopted for widespread solar electricity generation. This

6732-802: The crystal momentum of electrons and holes is the same in both the conduction band and the valence band ; an electron can directly emit a photon. In an "indirect" gap, a photon cannot be emitted because the electron must pass through an intermediate state and transfer momentum to the crystal lattice. Examples of direct bandgap materials include hydrogenated amorphous silicon and some III–V materials such as InAs and GaAs . Indirect bandgap materials include crystalline silicon and Ge . Some III–V materials are indirect bandgap as well, for example AlSb . Interactions among electrons , holes , phonons , photons , and other particles are required to satisfy conservation of energy and crystal momentum (i.e., conservation of total k-vector). A photon with an energy near

6834-407: The deposition rate could reduce costs, but this cost reduction would be limited by the fixed times in other parts of the process such as cooling and heating. The substrate used to grow these solar cells is usually germanium or gallium arsenide which are notably expensive materials. One of the main pathways to reduce substrate costs is to reuse the substrate. An early method proposed to accomplish this

6936-508: The door open) can produce heat damage in other tissues as well, up to and including serious burns that may not be immediately evident because of the tendency for microwaves to heat deeper tissues with higher moisture content. Microwaves were first generated in the 1890s in some of the earliest radio wave experiments by physicists who thought of them as a form of "invisible light". James Clerk Maxwell in his 1873 theory of electromagnetism , now called Maxwell's equations , had predicted that

7038-512: The early 1950s, frequency-division multiplexing was used to send up to 5,400 telephone channels on each microwave radio channel, with as many as ten radio channels combined into one antenna for the hop to the next site, up to 70 km away. Wireless LAN protocols , such as Bluetooth and the IEEE 802.11 specifications used for Wi-Fi, also use microwaves in the 2.4 GHz ISM band , although 802.11a uses ISM band and U-NII frequencies in

7140-819: The earth's surface as ground waves , or reflect from the ionosphere , so terrestrial microwave communication links are limited by the visual horizon to about 40 miles (64 km). At the high end of the band, they are absorbed by gases in the atmosphere, limiting practical communication distances to around a kilometer. Microwaves are widely used in modern technology, for example in point-to-point communication links, wireless networks , microwave radio relay networks, radar , satellite and spacecraft communication , medical diathermy and cancer treatment, remote sensing , radio astronomy , particle accelerators , spectroscopy , industrial heating, collision avoidance systems , garage door openers and keyless entry systems , and for cooking food in microwave ovens . Microwaves occupy

7242-528: The entire super high frequency (SHF) band (3 to 30 GHz, or 10 to 1 cm) at minimum. A broader definition includes UHF and extremely high frequency (EHF) ( millimeter wave ; 30 to 300 GHz) bands as well. Frequencies in the microwave range are often referred to by their IEEE radar band designations: S , C , X , K u , K , or K a band , or by similar NATO or EU designations. Microwaves travel by line-of-sight ; unlike lower frequency radio waves , they do not diffract around hills, follow

7344-435: The excited electrons are prevented from reaching these recombination places, they have no choice but to eventually fall back into the valence band by radiative recombination. This can be done by creating a dislocation loop in the material. At the edge of the loop, the planes above and beneath the "dislocation disk" are pulled apart, creating a negative pressure, which raises the energy of the conduction band substantially, with

7446-400: The fact that it is indirect-gap and therefore does not absorb light very well. As such, they are typically hundreds of microns thick; thinner wafers would allow much of the light (particularly in longer wavelengths) to simply pass through. By comparison, thin-film solar cells are made of direct band gap materials (such as amorphous silicon, CdTe , CIGS or CZTS ), which absorb the light in

7548-407: The following six essential factors: For this purpose an optical fiber tip of an optical fiber temperature sensor is equipped with a gallium arsenide crystal. Starting at a light wavelength of 850 nm GaAs becomes optically translucent. Since the spectral position of the band gap is temperature dependent, it shifts about 0.4 nm/K. The measurement device contains a light source and a device for

7650-414: The frequency can then be calculated. A similar technique is to use a slotted waveguide or slotted coaxial line to directly measure the wavelength. These devices consist of a probe introduced into the line through a longitudinal slot so that the probe is free to travel up and down the line. Slotted lines are primarily intended for measurement of the voltage standing wave ratio on the line. However, provided

7752-406: The frequency of the microwave oven. Microwave heating is used in industrial processes for drying and curing products. Many semiconductor processing techniques use microwaves to generate plasma for such purposes as reactive ion etching and plasma-enhanced chemical vapor deposition (PECVD). Microwaves are used in stellarators and tokamak experimental fusion reactors to help break down

7854-458: The gas into a plasma and heat it to very high temperatures. The frequency is tuned to the cyclotron resonance of the electrons in the magnetic field, anywhere between 2–200 GHz, hence it is often referred to as Electron Cyclotron Resonance Heating (ECRH). The upcoming ITER thermonuclear reactor will use up to 20 MW of 170 GHz microwaves. Microwaves can be used to transmit power over long distances, and post- World War 2 research

7956-456: The high cost and maintenance requirements of waveguide runs, in many microwave antennas the output stage of the transmitter or the RF front end of the receiver is located at the antenna. The term microwave also has a more technical meaning in electromagnetics and circuit theory . Apparatus and techniques may be described qualitatively as "microwave" when the wavelengths of signals are roughly

8058-452: The highest-efficiency single-junction solar cell at 29.1% (as of 2019). This high efficiency is attributed to the extreme high quality GaAs epitaxial growth, surface passivation by the AlGaAs, and the promotion of photon recycling by the thin film design. GaAs-based photovoltaics are also responsible for the highest efficiency (as of 2022) of conversion of light to electricity, as researchers from

8160-572: The low-microwave/high-UHF frequencies around 1.8 and 1.9 GHz in the Americas and elsewhere, respectively. DVB-SH and S-DMB use 1.452 to 1.492 GHz, while proprietary/incompatible satellite radio in the U.S. uses around 2.3 GHz for DARS . Microwave radio is used in point-to-point telecommunications transmissions because, due to their short wavelength, highly directional antennas are smaller and therefore more practical than they would be at longer wavelengths (lower frequencies). There

8262-759: The main frequencies used in radar. Microwave radar is widely used for applications such as air traffic control , weather forecasting, navigation of ships, and speed limit enforcement . Long-distance radars use the lower microwave frequencies since at the upper end of the band atmospheric absorption limits the range, but millimeter waves are used for short-range radar such as collision avoidance systems . Microwaves emitted by astronomical radio sources ; planets, stars, galaxies , and nebulas are studied in radio astronomy with large dish antennas called radio telescopes . In addition to receiving naturally occurring microwave radiation, radio telescopes have been used in active radar experiments to bounce microwaves off planets in

8364-545: The main logic families used were: Some electronic properties of gallium arsenide are superior to those of silicon . It has a higher saturated electron velocity and higher electron mobility , allowing gallium arsenide transistors to function at frequencies in excess of 250 GHz. GaAs devices are relatively insensitive to overheating, owing to their wider energy band gap, and they also tend to create less noise (disturbance in an electrical signal) in electronic circuits than silicon devices, especially at high frequencies. This

8466-515: The origin of the Universe . Microwave technology is extensively used for point-to-point telecommunications (i.e., non-broadcast uses). Microwaves are especially suitable for this use since they are more easily focused into narrower beams than radio waves, allowing frequency reuse ; their comparatively higher frequencies allow broad bandwidth and high data transmission rates , and antenna sizes are smaller than at lower frequencies because antenna size

8568-559: The other hand, if a plot of h ν {\displaystyle h\nu } versus α 1 / 2 {\displaystyle \alpha ^{1/2}} forms a straight line, it can normally be inferred that there is an indirect band gap, measurable by extrapolating the straight line to the α = 0 {\displaystyle \alpha =0} axis (assuming E p ≈ 0 {\displaystyle E_{\text{p}}\approx 0} ). In some materials with an indirect gap,

8670-425: The printed circuit inverted F antenna (PIFA) used in cell phones. Their short wavelength also allows narrow beams of microwaves to be produced by conveniently small high gain antennas from a half meter to 5 meters in diameter. Therefore, beams of microwaves are used for point-to-point communication links, and for radar . An advantage of narrow beams is that they do not interfere with nearby equipment using

8772-474: The process must also involve the absorption or emission of a phonon , where the phonon momentum equals the difference between the electron and hole momentum. It can also, instead, involve a crystallographic defect , which performs essentially the same role. The involvement of the phonon makes this process much less likely to occur in a given span of time, which is why radiative recombination is far slower in indirect band gap materials than direct band gap ones. This

8874-410: The production or use of GaAs. Microwave Microwave is a form of electromagnetic radiation with wavelengths shorter than other radio waves but longer than infrared waves. Its wavelength ranges from about one meter to one millimeter, corresponding to frequencies between 300 MHz and 300 GHz, broadly construed. A more common definition in radio-frequency engineering

8976-401: The result that the electrons cannot pass this edge. Provided that the area directly above the dislocation loop is defect-free (no non-radiative recombination possible), the electrons will fall back into the valence shell by radiative recombination, thus emitting light. This is the principle on which "DELEDs" (Dislocation Engineered LEDs) are based. The exact reverse of radiative recombination

9078-431: The same approximations mentioned above. Therefore, if a plot of h ν {\displaystyle h\nu } versus α 2 {\displaystyle \alpha ^{2}} forms a straight line, it can normally be inferred that there is a direct band gap, measurable by extrapolating the straight line to the α = 0 {\displaystyle \alpha =0} axis. On

9180-639: The same as the dimensions of the circuit, so that lumped-element circuit theory is inaccurate, and instead distributed circuit elements and transmission-line theory are more useful methods for design and analysis. As a consequence, practical microwave circuits tend to move away from the discrete resistors , capacitors , and inductors used with lower-frequency radio waves . Open-wire and coaxial transmission lines used at lower frequencies are replaced by waveguides and stripline , and lumped-element tuned circuits are replaced by cavity resonators or resonant stubs . In turn, at even higher frequencies, where

9282-406: The same frequency, allowing frequency reuse by nearby transmitters. Parabolic ("dish") antennas are the most widely used directive antennas at microwave frequencies, but horn antennas , slot antennas and lens antennas are also used. Flat microstrip antennas are being increasingly used in consumer devices. Another directive antenna practical at microwave frequencies is the phased array ,

9384-477: The same strategy has been described in a patent relating to processing scrap components containing GaAs where the Ga is complexed with a hydroxamic acid ("HA"), for example: This reaction produces arsenic acid . GaAs can be used for various transistor types: The HBT can be used in integrated injection logic (IL). The earliest GaAs logic gate used Buffered FET Logic (BFL). From c.  1975 to 1995

9486-898: The sky, a small amount of the power will be randomly scattered as the beam passes through the troposphere . A sensitive receiver beyond the horizon with a high gain antenna focused on that area of the troposphere can pick up the signal. This technique has been used at frequencies between 0.45 and 5 GHz in tropospheric scatter (troposcatter) communication systems to communicate beyond the horizon, at distances up to 300 km. The short wavelengths of microwaves allow omnidirectional antennas for portable devices to be made very small, from 1 to 20 centimeters long, so microwave frequencies are widely used for wireless devices such as cell phones , cordless phones , and wireless LANs (Wi-Fi) access for laptops , and Bluetooth earphones. Antennas used include short whip antennas , rubber ducky antennas , sleeve dipoles , patch antennas , and increasingly

9588-631: The solar system, to determine the distance to the Moon or map the invisible surface of Venus through cloud cover. A recently completed microwave radio telescope is the Atacama Large Millimeter Array , located at more than 5,000 meters (16,597 ft) altitude in Chile, which observes the universe in the millimeter and submillimeter wavelength ranges. The world's largest ground-based astronomy project to date, it consists of more than 66 dishes and

9690-524: The spectral detection of the band gap. With the changing of the band gap, (0.4 nm/K) an algorithm calculates the temperature (all 250 ms). GaAs may have applications in spintronics as it can be used instead of platinum in spin-charge converters and may be more tunable. The environment, health and safety aspects of gallium arsenide sources (such as trimethylgallium and arsine ) and industrial hygiene monitoring studies of metalorganic precursors have been reported. California lists gallium arsenide as

9792-433: The surface of the Earth, microwave communication links are limited by the visual horizon to about 30–40 miles (48–64 km). Microwaves are absorbed by moisture in the atmosphere, and the attenuation increases with frequency, becoming a significant factor ( rain fade ) at the high end of the band. Beginning at about 40 GHz, atmospheric gases also begin to absorb microwaves, so above this frequency microwave transmission

9894-464: The temperature of objects or terrain. The sun and other astronomical radio sources such as Cassiopeia A emit low level microwave radiation which carries information about their makeup, which is studied by radio astronomers using receivers called radio telescopes . The cosmic microwave background radiation (CMBR), for example, is a weak microwave noise filling empty space which is a major source of information on cosmology 's Big Bang theory of

9996-410: The wavelength of the electromagnetic waves becomes small in comparison to the size of the structures used to process them, microwave techniques become inadequate, and the methods of optics are used. High-power microwave sources use specialized vacuum tubes to generate microwaves. These devices operate on different principles from low-frequency vacuum tubes, using the ballistic motion of electrons in

10098-416: Was built in an international collaboration by Europe, North America, East Asia and Chile. A major recent focus of microwave radio astronomy has been mapping the cosmic microwave background radiation (CMBR) discovered in 1964 by radio astronomers Arno Penzias and Robert Wilson . This faint background radiation, which fills the universe and is almost the same in all directions, is "relic radiation" from

10200-443: Was done to examine possibilities. NASA worked in the 1970s and early 1980s to research the possibilities of using solar power satellite (SPS) systems with large solar arrays that would beam power down to the Earth's surface via microwaves. Less-than-lethal weaponry exists that uses millimeter waves to heat a thin layer of human skin to an intolerable temperature so as to make the targeted person move away. A two-second burst of

10302-446: Was first synthesized and studied by Victor Goldschmidt and his co-partner Donder Vwishuna in 1926 by passing arsenic vapors mixed with hydrogen over gallium(III) oxide at 600 °C. The semiconductor properties of GaAs and other III-V compounds were patented by Heinrich Welker at Siemens-Schuckert in 1951 and described in a 1952 publication. Commercial production of its monocrystals commenced in 1954, and more studies followed in

10404-493: Was observed that individuals in the radiation path of radar installations experienced clicks and buzzing sounds in response to microwave radiation. Research by NASA in the 1970s has shown this to be caused by thermal expansion in parts of the inner ear. In 1955, Dr. James Lovelock was able to reanimate rats chilled to 0 and 1 °C (32 and 34 °F) using microwave diathermy. When injury from exposure to microwaves occurs, it usually results from dielectric heating induced in

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