Editing High-electron-mobility transistor (section)

== 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>