<P> The following semiconducting systems can be tuned to some extent, and represent not a single material but a class of materials . </P> <Table> <Tr> <Th> Group </Th> <Th> Elem . </Th> <Th> Material class </Th> <Th> Formula </Th> <Th> Band gap (eV) lower </Th> <Th> upper </Th> <Th> Gap type </Th> <Th> Description </Th> </Tr> <Tr> <Td> IV </Td> <Td> </Td> <Td> Silicon - germanium </Td> <Td> Si Ge </Td> <Td> 0.67 </Td> <Td> 1.11 </Td> <Td> indirect </Td> <Td> adjustable band gap, allows construction of heterojunction structures . Certain thicknesses of superlattices have direct band gap . </Td> </Tr> <Tr> <Td> IV </Td> <Td> </Td> <Td> Silicon - tin </Td> <Td> Si Sn </Td> <Td> 1.0 </Td> <Td> 1.11 </Td> <Td> indirect </Td> <Td> Adjustable band gap . </Td> </Tr> <Tr> <Td> III - V </Td> <Td> </Td> <Td> Aluminium gallium arsenide </Td> <Td> Al Ga As </Td> <Td> 1.42 </Td> <Td> 2.16 </Td> <Td> direct / indirect </Td> <Td> 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 . 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 . </Td> </Tr> <Tr> <Td> III - V </Td> <Td> </Td> <Td> Indium gallium arsenide </Td> <Td> In Ga As </Td> <Td> 0.36 </Td> <Td> 1.43 </Td> <Td> direct </Td> <Td> 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 . </Td> </Tr> <Tr> <Td> III - V </Td> <Td> </Td> <Td> Indium gallium phosphide </Td> <Td> In Ga P </Td> <Td> 1.35 </Td> <Td> 2.26 </Td> <Td> direct / indirect </Td> <Td> used for HEMT and HBT structures and high - efficiency multijunction solar cells for e.g. satellites . Ga In P is almost lattice - matched to GaAs, with AlGaIn used for quantum wells for red lasers . </Td> </Tr> <Tr> <Td> III - V </Td> <Td> </Td> <Td> Aluminium indium arsenide </Td> <Td> Al In As </Td> <Td> 0.36 </Td> <Td> 2.16 </Td> <Td> direct / indirect </Td> <Td> 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 . </Td> </Tr> <Tr> <Td> III - V </Td> <Td> </Td> <Td> Aluminium indium antimonide </Td> <Td> Al In Sb </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> III - V </Td> <Td> </Td> <Td> Gallium arsenide nitride </Td> <Td> GaAsN </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> III - V </Td> <Td> </Td> <Td> Gallium arsenide phosphide </Td> <Td> GaAsP </Td> <Td> 1.43 </Td> <Td> 2.26 </Td> <Td> direct / indirect </Td> <Td> Used in red, orange and yellow LEDs . Often grown on GaP . Can be doped with nitrogen . </Td> </Tr> <Tr> <Td> III - V </Td> <Td> </Td> <Td> Gallium arsenide antimonide </Td> <Td> GaAsSb </Td> <Td> 0.7 </Td> <Td> 1.42 </Td> <Td> direct </Td> <Td> </Td> </Tr> <Tr> <Td> III - V </Td> <Td> </Td> <Td> Aluminium gallium nitride </Td> <Td> AlGaN </Td> <Td> 3.44 </Td> <Td> 6.28 </Td> <Td> direct </Td> <Td> Used in blue laser diodes, ultraviolet LEDs (down to 250 nm), and AlGaN / GaN HEMTs . Can be grown on sapphire . Used in heterojunctions with AlN and GaN . </Td> </Tr> <Tr> <Td> III - V </Td> <Td> </Td> <Td> Aluminium gallium phosphide </Td> <Td> AlGaP </Td> <Td> 2.26 </Td> <Td> 2.45 </Td> <Td> indirect </Td> <Td> Used in some green LEDs . </Td> </Tr> <Tr> <Td> III - V </Td> <Td> </Td> <Td> Indium gallium nitride </Td> <Td> InGaN </Td> <Td> </Td> <Td> 3.4 </Td> <Td> direct </Td> <Td> In Ga N, x usually between 0.02--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 . </Td> </Tr> <Tr> <Td> III - V </Td> <Td> </Td> <Td> Indium arsenide antimonide </Td> <Td> InAsSb </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> III - V </Td> <Td> </Td> <Td> Indium gallium antimonide </Td> <Td> InGaSb </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> III - V </Td> <Td> </Td> <Td> Aluminium gallium indium phosphide </Td> <Td> AlGaInP </Td> <Td> </Td> <Td> </Td> <Td> direct / indirect </Td> <Td> 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--650 nm; tends to form ordered phases during deposition, which has to be prevented </Td> </Tr> <Tr> <Td> III - V </Td> <Td> </Td> <Td> Aluminium gallium arsenide phosphide </Td> <Td> AlGaAsP </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> III - V </Td> <Td> </Td> <Td> Indium gallium arsenide phosphide </Td> <Td> InGaAsP </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> III - V </Td> <Td> </Td> <Td> Indium gallium arsenide antimonide </Td> <Td> InGaAsSb </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> Use in thermophotovoltaics . </Td> </Tr> <Tr> <Td> III - V </Td> <Td> </Td> <Td> Indium arsenide antimonide phosphide </Td> <Td> InAsSbP </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> Use in thermophotovoltaics . </Td> </Tr> <Tr> <Td> III - V </Td> <Td> </Td> <Td> Aluminium indium arsenide phosphide </Td> <Td> AlInAsP </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> III - V </Td> <Td> </Td> <Td> Aluminium gallium arsenide nitride </Td> <Td> AlGaAsN </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> III - V </Td> <Td> </Td> <Td> Indium gallium arsenide nitride </Td> <Td> InGaAsN </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> III - V </Td> <Td> </Td> <Td> Indium aluminium arsenide nitride </Td> <Td> InAlAsN </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> III - V </Td> <Td> </Td> <Td> Gallium arsenide antimonide nitride </Td> <Td> GaAsSbN </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> III - V </Td> <Td> 5 </Td> <Td> Gallium indium nitride arsenide antimonide </Td> <Td> GaInNAsSb </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> III - V </Td> <Td> 5 </Td> <Td> Gallium indium arsenide antimonide phosphide </Td> <Td> GaInAsSbP </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> Can be grown on InAs, GaSb, and other substrates . Can be lattice matched by varying composition . Possibly usable for mid-infrared LEDs . </Td> </Tr> <Tr> <Td> II - VI </Td> <Td> </Td> <Td> Cadmium zinc telluride, CZT </Td> <Td> CdZnTe </Td> <Td> 1.4 </Td> <Td> 2.2 </Td> <Td> direct </Td> <Td> 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 . </Td> </Tr> <Tr> <Td> II - VI </Td> <Td> </Td> <Td> Mercury cadmium telluride </Td> <Td> HgCdTe </Td> <Td> 0 </Td> <Td> 1.5 </Td> <Td> </Td> <Td> 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 . </Td> </Tr> <Tr> <Td> II - VI </Td> <Td> </Td> <Td> Mercury zinc telluride </Td> <Td> HgZnTe </Td> <Td> 0 </Td> <Td> 2.25 </Td> <Td> </Td> <Td> 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 . </Td> </Tr> <Tr> <Td> II - VI </Td> <Td> </Td> <Td> Mercury zinc selenide </Td> <Td> HgZnSe </Td> <Td> </Td> <Td> </Td> <Td> </Td> <Td> </Td> </Tr> <Tr> <Td> other </Td> <Td> </Td> <Td> Copper indium gallium selenide, CIGS </Td> <Td> Cu (In, Ga) Se </Td> <Td> </Td> <Td> 1.7 </Td> <Td> direct </Td> <Td> CuIn Ga Se . Polycrystalline . Used in thin film solar cells . </Td> </Tr> </Table> <Tr> <Th> Group </Th> <Th> Elem . </Th> <Th> Material class </Th> <Th> Formula </Th> <Th> Band gap (eV) lower </Th> <Th> upper </Th> <Th> Gap type </Th> <Th> Description </Th> </Tr> <Tr> <Td> IV </Td> <Td> </Td> <Td> Silicon - germanium </Td> <Td> Si Ge </Td> <Td> 0.67 </Td> <Td> 1.11 </Td> <Td> indirect </Td> <Td> adjustable band gap, allows construction of heterojunction structures . Certain thicknesses of superlattices have direct band gap . </Td> </Tr>

Which one of the following is not a semiconductor