Editing High-electron-mobility transistor (section)

== 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]].