Editing Heterojunction (section)

== Manufacture and applications ==

Heterojunction manufacturing generally requires the use of [[molecular beam epitaxy]] (MBE)<ref name=":0">Smith, C.G (1996). "Low-dimensional quantum devices". Rep. Prog. Phys. 59 (1996) 235282, pg 244.</ref> or [[chemical vapor deposition]] (CVD) technologies in order to precisely control the deposition thickness and create a cleanly lattice-matched abrupt interface. A recent alternative under research is the mechanical stacking of layered materials into [[van der Waals heterostructures]].<ref name="GeimGrigorieva2013">{{cite journal|last1=Geim|first1=A. K.|last2=Grigorieva|first2=I. V.|title=Van der Waals heterostructures|journal=Nature|volume=499|issue=7459|year=2013|pages=419–425|issn=0028-0836|doi=10.1038/nature12385|pmid=23887427|arxiv=1307.6718|s2cid=205234832}}</ref>

Despite their expense, heterojunctions have found use in a variety of specialized applications where their unique characteristics are critical:
* ''Solar cells'': Heterojunctions are formed through the interface of a [[crystalline silicon]] substrate (band gap 1.1 eV) and [[amorphous silicon]] thin film (band gap 1.7 eV) in some solar cell architectures.<ref>{{Citation |last=Leu |first=Sylvère |title=Crystalline Silicon Solar Cells: Heterojunction Cells |date=2020 |url=http://link.springer.com/10.1007/978-3-030-46487-5_7 |work=Solar Cells and Modules |volume=301 |pages=163–195 |editor-last=Shah |editor-first=Arvind |access-date=2023-04-18 |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-030-46487-5_7 |isbn=978-3-030-46485-1 |last2=Sontag |first2=Detlef|url-access=subscription }}</ref> The heterojunction is used to separate charge carriers in a similar way to a [[p–n junction]]. The [[Heterojunction solar cell|Heterojunction with Intrinsic Thin-Layer]] (HIT) solar cell structure was first developed in 1983<ref>{{cite journal|doi=10.1143/JJAP.22.L605|title=Amorphous Si/Polycrystalline Si Stacked Solar Cell Having More Than 12% Conversion Efficiency|year=1983|last1=Okuda|first1=Koji|last2=Okamoto|first2=Hiroaki|last3=Hamakawa|first3=Yoshihiro|journal=Japanese Journal of Applied Physics|volume=22|number=9 |pages=L605–L607|bibcode=1983JaJAP..22L.605O |s2cid=121569675 }}</ref> and commercialised by [[Sanyo]]/[[Panasonic]]. HIT solar cells now hold the record for the most efficient single-junction silicon solar cell, with a conversion efficiency of 26.7%.<ref name=":0" /><ref>{{cite journal|doi=10.7567/JJAP.57.08RB20|title=High-efficiency heterojunction crystalline Si solar cells|year=2018|last1=Yamamoto|first1=Kenji|last2=Yoshikawa|first2=Kunta|last3=Uzu|first3=Hisashi|last4=Adachi|first4=Daisuke|journal=Japanese Journal of Applied Physics|volume=57|number=8S3 |pages=08RB20|bibcode=2018JaJAP..57hRB20Y |s2cid=125265042 }}</ref><ref>{{Cite web |title=HJT - Heterojunction Solar Cells |url=https://www.solarpowerpanels.net.au/hjt-heterojunction-solar-cells/ |access-date=2022-03-25 |website=Solar Power Panels |language=en-AU}}</ref>
* ''Lasers'': Using heterojunctions in [[laser]]s was first proposed<ref>{{cite journal|doi=10.1109/PROC.1963.2706|title=A proposed class of hetero-junction injection lasers|year=1963|last1=Kroemer|first1=H.|journal=Proceedings of the IEEE|volume=51|issue=12|pages=1782–1783 }}</ref> in 1963 when [[Herbert Kroemer]], a prominent scientist in this field, suggested that [[population inversion]] could be greatly enhanced by heterostructures. By incorporating a smaller [[direct band gap]] material like [[GaAs]] between two larger band gap layers like [[AlAs]], [[charge carriers in semiconductors|carriers]] can be confined so that [[lasing]] can occur at [[room temperature]] with low threshold currents. It took many years for the [[material science]] of heterostructure fabrication to catch up with Kroemer's ideas but now it is the industry standard. It was later discovered that the band gap could be controlled by taking advantage of the [[quantum size effects]] in [[quantum well]] heterostructures. Furthermore, heterostructures can be used as [[waveguide]]s to the [[step-index profile|index step]] which occurs at the interface, another major advantage to their use in semiconductor lasers. Semiconductor [[diode laser]]s used in [[CD]] and [[DVD]] players and [[fiber optic]] [[transceiver]]s are manufactured using alternating layers of various [[Semiconductor materials|III-V]] and [[Semiconductor materials|II-VI]] [[compound semiconductor]]s to form lasing heterostructures.
* ''Bipolar transistors'': When a heterojunction is used as the base-emitter junction of a [[bipolar junction transistor]], extremely high forward [[Gain (electronics)|gain]] and low reverse gain result. This translates into very good high frequency operation (values in tens to hundreds of GHz) and low [[leakage current]]s. This device is called a [[heterojunction bipolar transistor]] (HBT).
* ''Field effect transistors'': Heterojunctions are used in [[HEMT|high electron mobility transistors]] (HEMT) which can operate at significantly higher frequencies (over 500&nbsp;GHz). The proper [[doping (semiconductors)|doping]] profile and band alignment gives rise to extremely high [[electron mobility|electron mobilities]] by creating a [[2DEG|two dimensional electron gas]] within a [[intrinsic semiconductor|dopant free region]] where very little [[scattering]] can occur.
* ''Catalysis'': Using heterojuntions as photocatalyst has demonstrated that they exhibit better performance in CO<sub>2</sub> photoreduction, H<sub>2</sub> production and photodegradation of pollutants in water than single metal oxides.<ref>{{cite journal |last1=Ortiz-Quiñonez |first1=Jose-Luis |last2=Pal |first2=Umapada |title=Interface engineered metal oxide heterojunction nanostructures in photocatalytic CO2 reduction: Progress and prospects |journal=Coordination Chemistry Reviews |date=October 2024 |volume=516 |pages=215967 |doi=10.1016/j.ccr.2024.215967|doi-access=free }}</ref> The performance of the heterojunction can be further improved by incorporation of oxygen vacancies, crystal facet engineering or incorporation of carbonaceous materials.