Editing High-electron-mobility transistor

{{redirect|HEMT|the military truck|Heavy Expanded Mobility Tactical Truck}}
{{Short description|Type of field-effect transistor}}
[[File:HEMT-scheme-en.svg|350px|thumbnail|Cross section of a GaAs/AlGaAs/InGaAs pHEMT]]
[[File:HEMT-band structure scheme-en.svg|thumb|350px|[[Band diagram]] of GaAs/AlGaAs [[heterojunction]]-based HEMT, at equilibrium.]]

A '''high-electron-mobility transistor''' ('''HEMT''' or '''HEM FET'''), also known as '''heterostructure FET''' ('''HFET''') or '''modulation-doped FET''' ('''MODFET'''), is a [[field-effect transistor]] incorporating a junction between two materials with different [[band gap]]s (i.e. a [[heterojunction]]) as the channel instead of a doped region (as is generally the case for a [[MOSFET]]). A commonly used material combination is [[Gallium arsenide|GaAs]] with [[Aluminium gallium arsenide|AlGaAs]], though there is wide variation, dependent on the application of the device. Devices incorporating more [[indium]] generally show better high-frequency performance, while in recent years, [[gallium nitride]] HEMTs have attracted attention due to their high-power performance.

Like other [[FET]]s, HEMTs can be used in [[integrated circuits]] as digital on-off switches. FETs can also be used as amplifiers for large amounts of current using a small voltage as a control signal. Both of these uses are made possible by the FET’s unique [[current–voltage characteristic]]s. HEMT transistors are able to operate at higher frequencies than ordinary transistors, up to [[millimeter wave]] frequencies, and are used in high-frequency products such as [[cell phone]]s, [[satellite television]] receivers, [[voltage converter]]s, and [[radar]] equipment. They are widely used in satellite receivers, in low power amplifiers and in the defense industry.

== Applications ==
The applications of HEMTs include [[microwave]] and [[millimeter wave]] [[telecommunication|communications]], imaging, [[radar]], [[radio astronomy]], and [[Switched-mode power supply|power switching]]. They are found in many types of equipment ranging from cellphones, power supply adapters and [[Direct broadcast satellite|DBS]] receivers to [[radio astronomy]] and [[electronic warfare]] systems such as [[radar]] systems. Numerous companies worldwide develop, manufacture, and sell HEMT-based devices in the form of discrete transistors, as 'monolithic microwave integrated circuits' ([[MMIC|MMICs]]), or within power switching integrated circuits.

HEMTs are suitable for applications where high gain and low noise at high frequencies are required, as they have shown current gain to frequencies greater than 600 GHz and power gain to frequencies greater than 1THz.<ref>{{cite web|url=http://www.semiconductor-today.com/news_items/2014/OCT/NORTHROP-GRUMMAN_311014.shtml|title=Northrop Grumman sets record with terahertz IC amplifier|website=www.semiconductor-today.com}}</ref> [[Gallium nitride]] based HEMTs are used as power switching transistors for voltage converter applications due to their low on-state resistances, low switching losses, and high breakdown strength.<ref name="Chen2017">{{cite journal |last1=Chen |first1=Kevin J. |last2=Häberlen |first2=Oliver |last3=Lidow |first3=Alex |last4=Tsai |first4=Chun lin |last5=Ueda |first5=Tetsuzo |last6=Uemoto |first6=Yasuhiro |last7=Wu |first7=Yifeng |title=GaN-on-Si Power Technology: Devices and Applications |journal=IEEE Transactions on Electron Devices |year=2017 |volume=64 |issue=3 |pages=779–795 |doi=10.1109/TED.2017.2657579|bibcode=2017ITED...64..779C }}</ref><ref name = "Medjdoub2016">{{cite book |last1=Medjdoub |first1=F. |editor-first1=Farid |editor-last1=Medjdoub |title=Gallium Nitride (GaN): Physics, Devices, and Technology |year=2016 |edition=1st |publisher=CRC Press |doi=10.4324/b19387 |isbn=9781315215426 |url=https://doi.org/10.4324/b19387}}</ref> These gallium nitride enhanced voltage converter applications include [[AC adapter|AC adapters]], which benefit from smaller package sizes due to the power circuitry requiring smaller passive electronic components.<ref name="Medjdoub2016"/>

== History ==
The invention of the high-electron-mobility transistor (HEMT) is usually attributed to physicist Takashi Mimura (三村 高志), while working at [[Fujitsu]] in Japan.<ref name="Mimura2002">{{cite journal |last1=Mimura |first1=Takashi  |title=The early history of the high electron mobility transistor (HEMT) |journal=IEEE Transactions on Microwave Theory and Techniques |date=March 2002 |volume=50 |issue=3 |pages=780–782 |doi=10.1109/22.989961|bibcode=2002ITMTT..50..780M }}</ref> The basis for the HEMT was the [[GaAs]] (gallium arsenide) [[MOSFET]] (metal–oxide–semiconductor field-effect transistor), which Mimura had been researching as an alternative to the standard [[silicon]] (Si) MOSFET since 1977. He conceived the HEMT in the spring of 1979, when he read about a modulated-doped heterojunction [[superlattice]] developed at [[Bell Labs]] in the United States,<ref name="Mimura2002"/> by Ray Dingle, [[Arthur Gossard]] and [[Horst Störmer]] who filed a [[patent]] in April 1978.<ref>{{cite patent | country=US | number=4163237 | title=High mobility multilayered heterojunction devices employing modulated doping | inventor=Ray Dingle, Arthur Gossard and Horst Störmer}}</ref> Mimura filed a patent disclosure for a HEMT in August 1979, and then a [[patent]] later that year.<ref>{{cite journal |last1=Mimura |first1=Takashi |title=Development of High Electron Mobility Transistor |journal=Japanese Journal of Applied Physics |date=8 December 2005 |volume=44 |issue=12R |pages=8263–8268 |doi=10.1143/JJAP.44.8263 |bibcode=2005JaJAP..44.8263M |s2cid=3112776 |url=http://pdfs.semanticscholar.org/f7bd/535554c853c7b2502acaed610e5bd7589a5a.pdf |archive-url=https://web.archive.org/web/20190308183933/http://pdfs.semanticscholar.org/f7bd/535554c853c7b2502acaed610e5bd7589a5a.pdf |url-status=dead |archive-date=8 March 2019 |issn=1347-4065}}</ref> The first demonstration of a HEMT device, the D-HEMT, was presented by Mimura and Satoshi Hiyamizu in May 1980, and then they later demonstrated the first E-HEMT in August 1980.<ref name="Mimura2002"/>

Independently, Daniel Delagebeaudeuf and Tranc Linh Nuyen, while working at [[Thomson-CSF]] in France, filed a patent for a similar type of field-effect transistor in March 1979. It also cites the Bell Labs patent as an influence.<ref>{{cite patent|country=US |number=4471366 |title=Field effect transistor with high cut-off frequency and process for forming same |inventor=Daniel Delagebeaudeuf and Tranc L. Nuyen}} ([https://patents.google.com/patent/US4471366 Google Patents])</ref> The first demonstration of an "inverted" HEMT was presented by Delagebeaudeuf and Nuyen in August 1980.<ref name="Mimura2002"/>

One of the earliest mentions of a GaN-based HEMT is in the 1993 ''Applied Physics Letters'' article, by Khan ''et al''.<ref>{{Cite journal|url=https://aip.scitation.org/doi/10.1063/1.109775|doi=10.1063/1.109775|title=High electron mobility transistor based on a GaN-AlxGa1−xN heterojunction|year=1993|last1=Asif Khan|first1=M.|last2=Bhattarai|first2=A.|last3=Kuznia|first3=J. N.|last4=Olson|first4=D. T.|journal=Applied Physics Letters|volume=63|issue=9|pages=1214–1215|bibcode=1993ApPhL..63.1214A|url-access=subscription}}</ref> Later, in 2004, P.D. Ye and B. Yang ''et al'' demonstrated a [[GaN]] (gallium nitride) [[metal–oxide–semiconductor]] HEMT (MOS-HEMT). It used [[atomic layer deposition]] (ALD) [[aluminum oxide]] (Al<sub>2</sub>O<sub>3</sub>) film both as a [[gate dielectric]] and for [[surface passivation]].<ref>{{cite journal |last1=Ye |first1=P. D. |last2=Yang |first2=B. |last3=Ng |first3=K. K. |last4=Bude |first4=J. |last5=Wilk |first5=G. D. |last6=Halder |first6=S. |last7=Hwang |first7=J. C. M. |title=GaN MOS-HEMT USING ATOMIC LAYER DEPOSITION Al2O3 AS GATE DIELECTRIC AND SURFACE PASSIVATION |journal=International Journal of High Speed Electronics and Systems |date=1 September 2004 |volume=14 |issue=3 |pages=791–796 |doi=10.1142/S0129156404002843 |issn=0129-1564}}</ref>

== Operation ==
Field effect transistors whose operation relies on the formation of a two-dimensional electron gas ([[2DEG]]) are known as HEMTs. In HEMTS electric current flows between a drain and source element via the 2DEG, which is located at the interface between two layers of differing [[band gap|band gaps]], termed the [[heterojunction]].<ref name="Meneghini2021">{{cite journal |last1=Meneghini |first1=Matteo |last2=De Santi |first2=Carlo |last3=Abid |first3=Idriss |last4=Buffolo |first4=Matteo |last5=Cioni |first5=Marcello |last6=Khadar |first6=Riyaz Abdul |last7=Nela |first7=Luca |last8=Zagni |first8=Nicolò |last9=Chini |first9=Alessandro |last10=Medjdoub |first10=Farid |last11=Meneghesso |first11=Gaudenzio |last12=Verzellesi |first12=Giovanni |last13=Zanoni |first13=Enrico |last14=Matioli |first14=Elison |title=GaN-based power devices: Physics, reliability, and perspectives |journal=Journal of Applied Physics |year=2021 |volume=130 |issue=18 |pages=181101 |doi=10.1063/5.0061354 |doi-access=free |bibcode=2021JAP...130r1101M |hdl=11380/1255364 |hdl-access=free }}</ref> Some examples of previously explored heterojunction layer compositions (heterostructures) for HEMTs include AlGaN/GaN,<ref name="Chen2017"/> AlGaAs/GaAs, InGaAs/GaAs,<ref name="Pattnaik2021">{{cite conference |last1=Pattnaik |first1=Geeta |last2=Mohapatra |first2=Meryleen |editor1-last=Sabut |editor1-first=Sukanta Kumar |editor2-last=Ray |editor2-first=Arun Kumar |editor3-last=Pati |editor3-first=Bibudhendu |editor4-last=Acharya |editor4-first=U. Rajendra |title=Design of AlGaAs/InGaAs/GaAs-Based PHEMT for High Frequency Application |year=2021 |publisher=Springer Singapore |pages=329–337 |isbn=978-981-33-4866-0}}</ref> and Si/SiGe.<ref name="Kasamatsu2004">{{cite journal |last1=Kasamatsu |first1=A |last2=Kasai |first2=K |last3=Hikosaka |first3=K |last4=Matsui |first4=T |last5=Mimura |first5=T |title=60nm gate-length Si/SiGe HEMT |journal=Applied Surface Science |year=2004 |volume=224 |issue=1 |pages=382–385 |doi=10.1016/j.apsusc.2003.08.064 |bibcode=2004ApSS..224..382K |url=https://www.sciencedirect.com/science/article/pii/S016943320301119X|url-access=subscription }}</ref> 

=== Advantages ===
The advantages of HEMTs over other transistor architectures, like the [[bipolar junction transistor]] and the [[MOSFET]], are the higher operating temperatures,<ref name="Meneghini2021"/> higher [[Breakdown voltage|breakdown strengths]], and lower specific on-state resistances,<ref name="Medjdoub2016"/> all in the case of GaN-based HEMTs compared to Si-based MOSFETs. Furthermore, InP-based HEMTs exhibit low noise performance and higher switching speeds.<ref name="Ajayan2021">{{cite journal |last1=Ajayan |first1=J. |last2=Nirmal |first2=D. |last3=Mathew |first3=Ribu |last4=Kurian |first4=Dheena |last5=Mohankumar |first5=P. |last6=Arivazhagan |first6=L. |last7=Ajitha |first7=D. |title=A critical review of design and fabrication challenges in InP HEMTs for future terahertz frequency applications |journal=Materials Science in Semiconductor Processing |year=2021 |volume=128 |pages=105753 |doi=10.1016/j.mssp.2021.105753 |url=https://www.sciencedirect.com/science/article/pii/S1369800121000949|url-access=subscription }}</ref>

=== 2DEG channel creation ===
The wide band element is doped with donor atoms; thus it has excess [[electron|electrons]] in its conduction band. These electrons will diffuse to the adjacent narrow band material’s conduction band due to the availability of states with lower energy. The movement of electrons will cause a change in potential and thus an electric field between the materials. The electric field will push electrons back to the wide band element’s conduction band. The diffusion process continues until electron diffusion and electron drift balance each other, creating a junction at equilibrium similar to a [[p–n junction]]. Note that the undoped narrow band gap material now has excess majority charge carriers. The fact that the charge carriers are majority carriers yields high switching speeds, and the fact that the low band gap semiconductor is undoped means that there are no donor atoms to cause scattering and thus yields high mobility.

In the case of GaAs HEMTs, they make use of high mobility electrons generated using the heterojunction of a highly doped wide-bandgap n-type donor-supply layer (AlGaAs in our example) and a non-doped narrow-bandgap channel layer with no dopant impurities (GaAs in this case). The electrons generated in the thin n-type AlGaAs layer drop completely into the GaAs layer to form a depleted AlGaAs layer, because the heterojunction created by different band-gap materials forms a [[quantum well]] (a steep canyon) in the conduction band on the GaAs side where the electrons can move quickly without colliding with any impurities because the GaAs layer is undoped, and from which they cannot escape. The effect of this is the creation of a very thin layer of highly mobile conducting electrons with very high concentration, giving the channel very low [[resistivity]] (or to put it another way, "high electron mobility").

=== Electrostatic mechanism ===
{{main|Heterojunction}}

Since GaAs has higher [[electron affinity]], free electrons in the AlGaAs layer are transferred to the undoped GaAs layer where they form a two dimensional high mobility electron gas within 100 [[ångström]] (10 [[nanometre|nm]]) of the interface. The n-type AlGaAs layer of the HEMT is depleted completely through two depletion mechanisms:
* Trapping of free electrons by surface states causes the surface depletion.
* Transfer of electrons into the undoped GaAs layer brings about the interface depletion.

The [[Fermi level]] of the gate metal is matched to the pinning point, which is 1.2 [[Electron volt|eV]] below the conduction band. With the reduced AlGaAs layer thickness, the electrons supplied by donors in the AlGaAs layer are insufficient to pin the layer. As a result, band bending is moving upward and the two-dimensional electrons gas does not appear. When a positive voltage greater than the threshold voltage is applied to the gate, electrons accumulate at the interface and form a two-dimensional electron gas.

=== Modulation doping in HEMTs ===
An important aspect of HEMTs is that the band discontinuities across the conduction and valence bands can be modified separately. This allows the type of carriers in and out of the device to be controlled. As HEMTs require electrons to be the main carriers, a graded doping can be applied in one of the materials, thus making the conduction band discontinuity smaller and keeping the valence band discontinuity the same. This diffusion of carriers leads to the accumulation of electrons along the boundary of the two regions inside the narrow band gap material. The accumulation of electrons leads to a very high current in these devices. The term "[[modulation doping]]" refers to the fact that the dopants are spatially in a different region from the current carrying electrons. This technique was invented by [[Horst Störmer]] at [[Bell Labs]].

== Manufacture ==
MODFETs can be manufactured by [[epitaxial growth]] of a strained [[SiGe]] layer. In the strained layer, the [[germanium]] content increases linearly to around 40-50%. This concentration of germanium allows the formation of a [[quantum well]] structure with a high [[conduction band]] offset and a high density of very mobile [[charge carrier]]s. The end result is a FET with ultra-high switching speeds and low noise. [[InGaAs]]/[[AlGaAs]], [[Aluminium gallium nitride|AlGaN]]/[[InGaN]], and other compounds are also used in place of SiGe. InP and GaN are starting to replace SiGe as the base material in MODFETs because of their better noise and power ratios.

== Versions of HEMTs ==

=== By growth technology: pHEMT and mHEMT ===
Ideally, the two different materials used for a heterojunction would have the same [[lattice constant]] (spacing between the atoms). In practice, the lattice constants are typically slightly different (e.g. AlGaAs on GaAs), resulting in crystal defects. As an analogy, imagine pushing together two plastic combs with a slightly different spacing. At regular intervals, you'll see two teeth clump together. In semiconductors, these discontinuities form [[deep-level trap]]s and greatly reduce device performance.

A HEMT where this rule is violated is called a '''pHEMT''' or '''pseudomorphic''' HEMT. This is achieved by using an extremely thin layer of one of the materials – so thin that the crystal lattice simply stretches to fit the other material. This technique allows the construction of transistors with larger [[bandgap]] differences than otherwise possible, giving them better performance.<ref name=Cooke2006>{{cite web|url=http://www.semiconductor-today.com/features/Semiconductor%20Today%20-%20Transcending%20frequency%20and%20integration%20limits.pdf|title=Indium Phosphide: Transcending frequency and integration limits. Semiconductor TODAY Compounds&AdvancedSilicon • Vol. 1 • Issue 3 • September 2006}}</ref>

Another way to use materials of different lattice constants is to place a buffer layer between them. This is done in the '''mHEMT''' or '''metamorphic''' HEMT, an advancement of the pHEMT. The buffer layer is made of [[Aluminium indium arsenide|AlInAs]], with the indium concentration graded so that it can match the lattice constant of both the GaAs substrate and the [[Indium gallium arsenide|GaInAs]] channel. This brings the advantage that practically any Indium concentration in the channel can be realized, so the devices can be optimized for different applications (low indium concentration provides low [[Noise (electronic)|noise]]; high indium concentration gives high [[Gain (electronics)|gain]]).{{cn|date=January 2016}}

=== By electrical behaviour: eHEMT and dHEMT ===
HEMTs made of semiconductor hetero-interfaces lacking interfacial net polarization charge, such as AlGaAs/GaAs, require positive gate voltage or appropriate donor-doping in the AlGaAs barrier to attract the electrons towards the gate, which forms the 2D electron gas and enables conduction of electron currents. This behaviour is similar to that of commonly used field-effect transistors in the enhancement mode, and such a device is called enhancement HEMT, or '''eHEMT'''.

When a HEMT is built from [[AlGaN]]/[[GaN]], higher power density and breakdown voltage can be achieved. Nitrides also have different crystal structure with lower symmetry, namely the [[wurtzite crystal structure|wurtzite]] one, which has built-in electrical polarisation. Since this polarization differs between the [[GaN]] ''channel'' layer and [[AlGaN]] ''barrier'' layer, a sheet of uncompensated charge in the order of 0.01-0.03 C/m<math>^2</math> is formed. Due to the crystal orientation typically used for epitaxial growth ("gallium-faced") and the device geometry favorable for fabrication (gate on top), this charge sheet is positive, causing the 2D electron gas to be formed even if there is no doping. Such a transistor is normally on, and will turn off only if the gate is negatively biased - thus this kind of HEMT is known as ''depletion HEMT'', or '''dHEMT'''. By sufficient doping of the barrier with acceptors (e.g. [[magnesium|Mg]]), the built-in charge can be compensated to restore the more customary '''eHEMT''' operation, however high-density p-doping of nitrides is technologically challenging due to dopant diffusion into the channel.

===Induced HEMT===
In contrast to a modulation-doped HEMT, an induced high electron mobility transistor provides the flexibility to tune different electron densities with a top gate, since the charge carriers are "induced" to the [[2DEG]] plane rather than created by dopants. The absence of a doped layer enhances the electron mobility significantly when compared to their modulation-doped counterparts. This level of cleanliness provides opportunities to perform research into the field of [[Quantum billiard ball|quantum billiards]] for [[quantum chaos]] studies, or applications in ultra stable and ultra sensitive electronic devices.<ref>{{Cite journal |last1=Hu |first1=Zhixiang |last2=Zhou |first2=Licheng |last3=Li |first3=Long |last4=Ying |first4=Binzhou |last5=Zhao |first5=Yunong |last6=Wang |first6=Peng |last7=Li |first7=Huayao |last8=Zhang |first8=Yang |last9=Liu |first9=Huan |date=18 April 2023 |title=Quantum Dots-Sensitized High Electron Mobility Transistor (HEMT) for Sensitive NO2 Detection |journal=Chemosensors |language=en |volume=11 |issue=4 |pages=252 |doi=10.3390/chemosensors11040252 |doi-access=free |issn=2227-9040}}</ref>

== See also ==

* [[Junction field-effect transistor]] (JFET)

==References==
{{Reflist}}

==External links==
* [http://www.iue.tuwien.ac.at/phd/smirnov/node71.html Modulation-doped FET]

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[[Category:Transistor types]]
[[Category:Microwave technology]]
[[Category:Terahertz technology]]
[[Category:Field-effect transistors]]
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[[Category:MOSFETs]]
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