Semiconductor materials are nominally small band gapinsulators. The defining property of a semiconductor material is that it can be compromised by doping it with impurities that alter its electronic properties in a controllable way.[1]
Because of their application in the computer and photovoltaic industry—in devices such as transistors, lasers, and solar cells—the search for new semiconductor materials and the improvement of existing materials is an important field of study in materials science.
Most commonly used semiconductor materials are crystalline inorganic solids. These materials are classified according to the periodic table groups of their constituent atoms.
Different semiconductor materials differ in their properties. Thus, in comparison with silicon, compound semiconductors have both advantages and disadvantages. For example, gallium arsenide (GaAs) has six times higher electron mobility than silicon, which allows faster operation; wider band gap, which allows operation of power devices at higher temperatures, and gives lower thermal noise to low power devices at room temperature; its direct band gap gives it more favorable optoelectronic properties than the indirect band gap of silicon; it can be alloyed to ternary and quaternary compositions, with adjustable band gap width, allowing light emission at chosen wavelengths, which makes possible matching to the wavelengths most efficiently transmitted through optical fibers. GaAs can be also grown in a semi-insulating form, which is suitable as a lattice-matching insulating substrate for GaAs devices. Conversely, silicon is robust, cheap, and easy to process, whereas GaAs is brittle and expensive, and insulation layers cannot be created by just growing an oxide layer; GaAs is therefore used only where silicon is not sufficient.[2]
By alloying multiple compounds, some semiconductor materials are tunable, e.g., in band gap or lattice constant. The result is ternary, quaternary, or even quinary compositions. Ternary compositions allow adjusting the band gap within the range of the involved binary compounds; however, in case of combination of direct and indirect band gap materials there is a ratio where indirect band gap prevails, limiting the range usable for optoelectronics; e.g. AlGaAs LEDs are limited to 660 nm by this. Lattice constants of the compounds also tend to be different, and the lattice mismatch against the substrate, dependent on the mixing ratio, causes defects in amounts dependent on the mismatch magnitude; this influences the ratio of achievable radiative/nonradiative recombinations and determines the luminous efficiency of the device. Quaternary and higher compositions allow adjusting simultaneously the band gap and the lattice constant, allowing increasing radiant efficiency at wider range of wavelengths; for example AlGaInP is used for LEDs. Materials transparent to the generated wavelength of light are advantageous, as this allows more efficient extraction of photons from the bulk of the material. That is, in such transparent materials, light production is not limited to just the surface. Index of refraction is also composition-dependent and influences the extraction efficiency of photons from the material.[3]
Types of semiconductor materials
Group IV elemental semiconductors, (C, Si, Ge, Sn)
III–V semiconductors: Crystallizing with high degree of stoichiometry, most can be obtained as both n-type and p-type. Many have high carrier mobilities and direct energy gaps, making them useful for optoelectronics. (See also: Template:III-V compounds.)
II–VI semiconductors: usually p-type, except ZnTe and ZnO which are n-type
Used in early radar detection diodes and first transistors; requires lower purity than silicon. A substrate for high-efficiency multijunction photovoltaic cells. Very similar lattice constant to gallium arsenide. High-purity crystals used for gamma spectroscopy. May grow whiskers, which impair reliability of some devices.
Excellent thermal conductivity. Superior mechanical and optical properties.
High carrier mobilities[7] and high electric breakdown field[8] at room temperature as excellent electronics characteristics.
Extremely high nanomechanical resonator quality factor.[9]
Piezoelectric. Not used on its own as a semiconductor; AlN-close GaAlN possibly usable for ultraviolet LEDs. Inefficient emission at 210 nm was achieved on AlN.
problematic to be doped to p-type, p-doping with Mg and annealing allowed first high-efficiency blue LEDs[3] and blue lasers. Very sensitive to ESD. Insensitive to ionizing radiation. GaN transistors can operate at higher voltages and higher temperatures than GaAs, used in microwave power amplifiers. When doped with e.g. manganese, becomes a magnetic semiconductor.
Used in early low to medium brightness cheap red/orange/green LEDs. Used standalone or with GaAsP. Transparent for yellow and red light, used as substrate for GaAsP red/yellow LEDs. Doped with S or Te for n-type, with Zn for p-type. Pure GaP emits green, nitrogen-doped GaP emits yellow-green, ZnO-doped GaP emits red.
second most common in use after silicon, commonly used as substrate for other III-V semiconductors, e.g. InGaAs and GaInNAs. Brittle. Lower hole mobility than Si, P-type CMOS transistors unfeasible. High impurity density, difficult to fabricate small structures. Used for near-IR LEDs, fast electronics, and high-efficiency solar cells. Very similar lattice constant to germanium, can be grown on germanium substrates.
Commonly used as substrate for epitaxial InGaAs. Superior electron velocity, used in high-power and high-frequency applications. Used in optoelectronics.
Used for infrared detectors for 1–3.8 μm, cooled or uncooled. High electron mobility. InAs dots in InGaAs matrix can serve as quantum dots. Quantum dots may be formed from a monolayer of InAs on InP or GaAs. Strong photo-Dember emitter, used as a terahertz radiation source.
Used in infrared detectors and thermal imaging sensors, high quantum efficiency, low stability, require cooling, used in military long-range thermal imager systems. AlInSb-InSb-AlInSb structure used as quantum well. Very high electron mobility, electron velocity and ballistic length. Transistors can operate below 0.5V and above 200 GHz. Terahertz frequencies maybe achievable.
Nanoparticles used as quantum dots. Intrinsic n-type, difficult to dope p-type, but can be p-type doped with nitrogen. Possible use in optoelectronics. Tested for high-efficiency solar cells.
Used in photoresistors and solar cells; CdS/Cu2S was the first efficient solar cell. Used in solar cells with CdTe. Common as quantum dots. Crystals can act as solid-state lasers. Electroluminescent. When doped, can act as a phosphor.
Photocatalytic. Band gap is tunable from 3 to 4 eV by alloying with magnesium oxide and cadmium oxide. Intrinsic n-type, p-type doping is difficult. Heavy aluminium, indium, or gallium doping yields transparent conductive coatings; ZnO:Al is used as window coatings transparent in visible and reflective in infrared region and as conductive films in LCD displays and solar panels as a replacement of indium tin oxide. Resistant to radiation damage. Possible use in LEDs and laser diodes. Possible use in random lasers.
Used for blue lasers and LEDs. Easy to n-type doping, p-type doping is difficult but can be done with e.g. nitrogen. Common optical material in infrared optics.
Can be grown on AlSb, GaSb, InAs, and PbSe. Used in solar cells, components of microwave generators, blue LEDs and lasers. Used in electrooptics. Together with lithium niobate used to generate terahertz radiation.
Mineral galena, first semiconductor in practical use, used in cat's whisker detectors; the detectors are slow due to high dielectric constant of PbS. Oldest material used in infrared detectors. At room temperature can detect SWIR, longer wavelengths require cooling.
Tin sulfide (SnS) is a semiconductor with direct optical band gap of 1.3 eV and absorption coefficient above 104 cm−1 for photon energies above 1.3 eV. It is a p-type semiconductor whose electrical properties can be tailored by doping and structural modification and has emerged as one of the simple, non-toxic and affordable material for thin films solar cells since a decade.
Efficient thermoelectric material near room temperature when alloyed with selenium or antimony. Narrow-gap layered semiconductor. High electrical conductivity, low thermal conductivity. Topological insulator.
N-type intrinsic semiconductor. Very high electron mobility. Used in infrared detectors, photodetectors, dynamic thin-film pressure sensors, and magnetoresistors. Recent measurements suggest that 3D Cd3As2 is actually a zero band-gap Dirac semimetal in which electrons behave relativistically as in graphene.[26]
High Seebeck coefficient, resistant to high temperatures, promising thermoelectric and thermophotovoltaic applications. Formerly used in URDOX resistors, conducting at high temperature. Resistant to radiation damage.
PbI2 is a layered direct bandgap semiconductor with bandgap of 2.4 eV in its bulk form, whereas its 2D monolayer has an indirect bandgap of ~2.5 eV, with possibilities to tune the bandgap between 1–3 eV
direct band gap for x<0.4 (corresponding to 1.42–1.95 eV); can be lattice-matched to GaAs substrate over entire composition range; tends to oxidize; n-doping with Si, Se, Te; p-doping with Zn, C, Be, Mg.[3] Can be used for infrared laser diodes. Used as a barrier layer in GaAs devices to confine electrons to GaAs (see e.g. QWIP). AlGaAs with composition close to AlAs is almost transparent to sunlight. Used in GaAs/AlGaAs solar cells.
Well-developed material. Can be lattice matched to InP substrates. Use in infrared technology and thermophotovoltaics. Indium content determines charge carrier density. For x=0.015, InGaAs perfectly lattice-matches germanium; can be used in multijunction photovoltaic cells. Used in infrared sensors, avalanche photodiodes, laser diodes, optical fiber communication detectors, and short-wavelength infrared cameras.
Used for HEMT and HBT structures and high-efficiency multijunction solar cells for e.g. satellites. Ga0.5In0.5P is almost lattice-matched to GaAs, with AlGaIn used for quantum wells for red lasers.
Buffer layer in metamorphic HEMT transistors, adjusting lattice constant between GaAs substrate and GaInAs channel. Can form layered heterostructures acting as quantum wells, in e.g. quantum cascade lasers.
Used as a buffer layer in InSb-based quantum wells and other devices grown on GaAs and GaSb substrates. Also used as the active layer in some mid-infrared LEDs and photodiodes.
InxGa1–xN, x usually between 0.02 and 0.3 (0.02 for near-UV, 0.1 for 390 nm, 0.2 for 420 nm, 0.3 for 440 nm). Can be grown epitaxially on sapphire, SiC wafers or silicon. Used in modern blue and green LEDs, InGaN quantum wells are effective emitters from green to ultraviolet. Insensitive to radiation damage, possible use in satellite solar cells. Insensitive to defects, tolerant to lattice mismatch damage. High heat capacity.
Primarily used in mid- and long-wave infraredphotodetectors due to its small bandgap, which reaches a minimum of around 0.08 eV in InAs0.4Sb0.6 at room temperature.
also InAlGaP, InGaAlP, AlInGaP; for lattice matching to GaAs substrates the In mole fraction is fixed at about 0.48, the Al/Ga ratio is adjusted to achieve band gaps between about 1.9 and 2.35 eV; direct or indirect band gaps depending on the Al/Ga/In ratios; used for waveengths between 560 and 650 nm; tends to form ordered phases during deposition, which has to be prevented[3]
Efficient solid-state x-ray and gamma-ray detector, can operate at room temperature. High electro-optic coefficient. Used in solar cells. Can be used to generate and detect terahertz radiation. Can be used as a substrate for epitaxial growth of HgCdTe.
Known as "MerCad". Extensive use in sensitive cooled infrared imaging sensors, infrared astronomy, and infrared detectors. Alloy of mercury telluride (a semimetal, zero band gap) and CdTe. High electron mobility. The only common material capable of operating in both 3–5 μm and 12–15 μm atmospheric windows. Can be grown on CdZnTe.
Used in infrared detectors, infrared imaging sensors, and infrared astronomy. Better mechanical and thermal properties than HgCdTe but more difficult to control the composition. More difficult to form complex heterostructures.
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