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Cross section of a GaAs/AlGaAs/InGaAs pHEMT
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 gaps (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 GaAs with 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 FETs, 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 characteristics. 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 phones, satellite television receivers, voltage converters, and radar equipment. They are widely used in satellite receivers, in low power amplifiers and in the defense industry.

Applications

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The applications of HEMTs include microwave and millimeter wave communications, imaging, radar, radio astronomy, and power switching. They are found in many types of equipment ranging from cellphones, power supply adapters and 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' (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.[1] 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.[2][3] These gallium nitride enhanced voltage converter applications include AC adapters, which benefit from smaller package sizes due to the power circuitry requiring smaller passive electronic components.[3]

History

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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.[4] 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,[4] by Ray Dingle, Arthur Gossard and Horst Störmer who filed a patent in April 1978.[5] Mimura filed a patent disclosure for a HEMT in August 1979, and then a patent later that year.[6] 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.[4]

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.[7] The first demonstration of an "inverted" HEMT was presented by Delagebeaudeuf and Nuyen in August 1980.[4]

One of the earliest mentions of a GaN-based HEMT is in the 1993 Applied Physics Letters article, by Khan et al.[8] 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 (Al2O3) film both as a gate dielectric and for surface passivation.[9]

Operation

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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 gaps, termed the heterojunction.[10] Some examples of previously explored heterojunction layer compositions (heterostructures) for HEMTs include AlGaN/GaN,[2] AlGaAs/GaAs, InGaAs/GaAs,[11] and Si/SiGe.[12]

Advantages

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The advantages of HEMTs over other transistor architectures, like the bipolar junction transistor and the MOSFET, are the higher operating temperatures,[10] higher breakdown strengths, and lower specific on-state resistances,[3] 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.[13]

2DEG channel creation

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The wide band element is doped with donor atoms; thus it has excess 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

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Main article: 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 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 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

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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

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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 carriers. The end result is a FET with ultra-high switching speeds and low noise. InGaAs/AlGaAs, 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

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By growth technology: pHEMT and mHEMT

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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 traps 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.[14]

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 AlInAs, with the indium concentration graded so that it can match the lattice constant of both the GaAs substrate and the 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; high indium concentration gives high gain).[citation needed]

By electrical behaviour: eHEMT and dHEMT

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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 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 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. 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

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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 billiards for quantum chaos studies, or applications in ultra stable and ultra sensitive electronic devices.[15]

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

High-electron-mobility transistor

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A high-electron-mobility transistor (HEMT) is a type of field-effect transistor (FET) that utilizes a heterojunction between two semiconductor materials with different bandgaps to form a two-dimensional electron gas (2DEG) at their interface, enabling significantly higher electron mobility compared to conventional transistors. This structure, often modulation-doped, spatially separates ionized impurities from the conducting electrons in the channel, minimizing scattering and allowing operation at very high frequencies, typically up to millimeter-wave ranges. Invented in 1979 by Takashi Mimura at Fujitsu Laboratories in Japan, the HEMT was first demonstrated using a GaAs/AlGaAs heterojunction grown via molecular beam epitaxy (MBE), with the original prototype achieving superior speed and noise performance over existing GaAs MESFETs. The HEMT's operation relies on electrons transferring from a doped wide-bandgap material (e.g., AlGaAs) to an adjacent undoped narrow-bandgap layer (e.g., GaAs), forming the high-mobility 2DEG channel that is modulated by a voltage to control current flow. Early development, detailed in Mimura's 1980 paper, highlighted the device's potential for low-noise amplification, with key milestones including its commercialization in 1983 for receivers and widespread adoption by 1987 in satellite broadcasting systems. Over the decades, HEMT technology has evolved to include materials like InP, InGaAs, and III-nitride compounds such as GaN and AlGaN, grown by techniques including MBE and metal-organic chemical vapor deposition (MOCVD), achieving frequencies exceeding 250 GHz and power densities up to 56 W/mm in advanced configurations like GaN-on-diamond. As of 2025, advancements include GaN HEMTs with breakdown voltages exceeding 650 V on 300 mm substrates and a market valued at USD 1.22 billion, driven by applications in electric vehicles and . These advancements stem from the device's fundamental advantages: exceptional velocity, reduced figures, and robustness at high temperatures and voltages. HEMTs have become indispensable in high-frequency and high-power applications, powering radio-frequency (RF) amplifiers, monolithic microwave integrated circuits (MMICs) operating from 0.3 to 10 THz, optical-fiber communication systems, , cellular base stations, and efficient power converters capable of handling over 650 V. Their role in enabling modern wireless communication and technologies earned the invention recognition as an IEEE Milestone in 2019, underscoring a profound impact on the global . Variants such as pseudomorphic HEMTs (pHEMTs) and metamorphic HEMTs (mHEMTs) further optimize performance for specific uses, including low-noise blocks in TV and high-power amplifiers in defense systems.

Device Structure

Layer Composition

The high-electron-mobility transistor (HEMT) typically features a layered heterostructure designed to create a high-mobility conduction channel at the interface between materials with different bandgaps. In the foundational GaAs/AlGaAs system, the stack consists of a semi-insulating GaAs substrate, an undoped GaAs channel layer (typically 10-20 nm thick), a thin undoped AlGaAs spacer layer (2-5 nm), and a doped n-type AlGaAs barrier layer (20-50 nm thick, with doping levels around 10^17-10^18 cm^{-3}). The undoped channel layer serves as the region where electrons accumulate to form the two-dimensional electron gas (2DEG), while the doped barrier supplies carriers via modulation doping; the spacer minimizes scattering from ionized impurities. Thinner channel and barrier layers enhance carrier confinement near the interface, reducing leakage and improving mobility, though excessive thinness can introduce strain or quantum effects that degrade performance. Source and drain contacts are formed using ohmic metals such as AuGe/Ni/Au alloys annealed to the cap layer or directly to the barrier, providing low-resistance access to the 2DEG channel, while the gate electrode employs Schottky metals like Ti/Pt/Au deposited on the barrier surface to enable depletion-mode control of the channel conductivity. The semi-insulating substrate, often GaAs with resistivity >10^8 Ω·cm, isolates the active layers electrically and provides mechanical support. For higher-performance applications, the InGaAs/InAlAs system on InP substrates is common, featuring an undoped InGaAs channel (10-15 nm thick), a thin InAlAs spacer (2-3 nm), and an undoped or lightly doped InAlAs barrier (20-30 nm thick), often with an InGaAs cap for improved contacts. This configuration leverages a larger (~0.5 eV) for superior electron confinement and velocity, suitable for low-noise devices. Channel thicknesses in this range optimize confinement while accommodating strain from lattice mismatch. In , GaN/AlGaN HEMTs on substrates like or Si use an undoped GaN channel (typically 100-300 nm, though the active 2DEG forms within ~10 nm of the interface), a thin AlN interlayer (1-2 nm) for strain management, and an undoped AlGaN barrier (20-30 nm thick, Al content 20-30%). The barrier's thickness influences sheet carrier density via polarization effects, with 20-25 nm providing optimal confinement for high-voltage operation without intentional doping. Source/drain contacts use Ti/Al/Ni/Au stacks for ohmic connection to the 2DEG, and the is often recessed or uses a for enhancement-mode behavior.

Heterojunction Interface

The heterojunction interface in high-electron-mobility transistors (HEMTs) forms the critical boundary between the wider-bandgap barrier layer, typically AlGaAs, and the narrower-bandgap channel layer, such as GaAs, enabling spatial separation of charge carriers. The bandgap discontinuity (ΔEg\Delta E_g) at this interface arises from the difference in bandgaps of the two materials; for Al0.3_{0.3}Ga0.7_{0.7}As/GaAs, ΔEg≈0.37\Delta E_g \approx 0.37 eV, primarily due to the increased bandgap of AlGaAs (approximately 1.80 eV) compared to GaAs (1.42 eV). This discontinuity partitions into a conduction band offset (ΔEc\Delta E_c) and valence band offset (ΔEv\Delta E_v), with ΔEc≈0.24\Delta E_c \approx 0.24 eV dominating for typical Al compositions (x<0.41x < 0.41), calculated as ΔEc=0.79x\Delta E_c = 0.79x eV, while ΔEv≈0.13\Delta E_v \approx 0.13 eV. These offsets confine electrons to the GaAs channel, forming the basis for high-mobility transport, though precise values depend on Al mole fraction xx and growth conditions. Lattice matching at the heterojunction is essential to minimize structural defects that degrade carrier mobility. In conventional GaAs/AlGaAs HEMTs, the materials are closely lattice-matched (lattice constant difference <0.1% for x≈0.3x \approx 0.3), allowing pseudomorphic growth without significant strain or dislocations, which preserves interface sharpness and enables mobilities exceeding 106^6 cm2^2/V·s at low temperatures. In contrast, pseudomorphic variants using InGaAs channels on GaAs substrates introduce lattice mismatch (e.g., ~1.7% for In0.2_{0.2}Ga0.8_{0.8}As), inducing compressive strain in thin (~10 nm) layers that enhances ΔEc\Delta E_c but risks strain relaxation and defect formation if exceeding critical thickness. Such mismatched interfaces require precise epitaxial control to maintain coherence and avoid threading dislocations that scatter electrons. Interface traps and defects, including atomic steps and alloy disorder, significantly impact electron mobility by introducing scattering centers. Atomic steps at the AlGaAs/GaAs interface, arising from growth interruptions or substrate vicinality, cause interface roughness scattering, with step heights of 1-2 monolayers reducing mobility by up to 20% unless mitigated by optimized molecular beam epitaxy (MBE) conditions like low-temperature growth (~600°C). Alloy disorder in ternary AlGaAs, due to random Al/Ga atomic distributions, leads to potential fluctuations that scatter electrons via alloy disorder mechanism, limiting low-temperature mobilities to ~105^5 cm2^2/V·s in unoptimized structures; this is minimized through high-purity growth and lower Al content (x<0.3x < 0.3). Minimizing these defects via techniques such as migration-enhanced epitaxy (MEE) achieves interface trap densities below 1011^{11} cm−2^{-2}·eV−1^{-1}, enabling record mobilities. The undoped spacer layer, typically 2-5 nm thick AlGaAs inserted between the doped barrier and undoped channel, plays a pivotal role in reducing at the heterojunction. By spatially separating ionized donors in the AlGaAs from the two-dimensional electron gas (2DEG) in GaAs, the spacer minimizes Coulombic (remote ionized impurity) , increasing room-temperature mobility by factors of 2-3 compared to structures without it; for example, spacers of 3 nm yield mobilities >80,000 cm2^2/V·s at 300 K. This design, introduced in early modulation-doped structures, also reduces alloy disorder effects by keeping the 2DEG away from the ternary barrier, though thicker spacers (>5 nm) can lower carrier density due to reduced . Optimal spacer thickness balances confinement and reduction, as verified in MBE-grown GaAs/AlGaAs heterostructures.

Operating Principle

Formation of 2DEG

In high-electron-mobility transistors (HEMTs), the (2DEG) forms at the interface due to quantum mechanical confinement of electrons in the narrower-bandgap layer. The band discontinuity between the wide-bandgap barrier (e.g., AlGaAs) and the narrow-bandgap channel (e.g., GaAs) creates a potential step that confines electrons to a thin layer, typically on the order of 10 nm thick, at the interface. This confinement arises from the conduction ΔEc\Delta E_c, which positions the conduction band minimum of the channel below the , attracting electrons to occupy states near the interface. The potential profile experienced by these electrons is approximately triangular, resulting from the sharp band offset and the electrostatic repulsion from the accumulated charge itself. Within this triangular potential well, quantum mechanical effects quantize the electron energy levels into discrete subbands perpendicular to the interface, while electrons remain free to move in the plane parallel to it. The lowest subband (often denoted E0E_0) is typically occupied, with higher subbands separated by energies on the order of several meV, ensuring that only the ground-state subband contributes significantly to transport at low temperatures. This subband quantization is a direct consequence of solving the Schrödinger equation in the self-consistent potential, leading to a density of states that is stepwise constant for each subband. Band diagrams illustrate this process: the conduction band in the channel material bends upward near the interface due to the depletion of s and the resulting positive , forming the confining well, while the remains nearly flat and pinned relative to the donor levels in the barrier. The first experimental observation of such a 2DEG at a differentially doped GaAs-AlGaAs confirmed this confinement through Shubnikov-de Haas oscillations, demonstrating the two-dimensional nature of the electron states. Typical electron densities in the 2DEG range from 101110^{11} to 101210^{12} cm−2^{-2}, depending on the heterostructure design and doping, enabling high sheet carrier concentrations while maintaining spatial uniformity. This density corresponds to a low , often below 100 Ω\Omega/sq, which is crucial for low-loss high-frequency operation. The in the 2DEG can reach up to 10610^6 cm2^2/V⋅\cdots at low temperatures (e.g., 4 ), primarily due to the spatial separation of the conducting electrons from ionized donor impurities in the barrier layer, minimizing scattering. This high mobility was first demonstrated in modulation-doped GaAs-AlGaAs structures, where values exceeding 10510^5 cm2^2/V⋅\cdots at 77 were reported, far surpassing uniformly doped bulk materials.

Modulation Doping Mechanism

The modulation doping mechanism is a foundational technique in high-electron-mobility transistors (HEMTs) that spatially separates atoms from the conducting channel to suppress impurity scattering. In typical n-type implementations, such as AlGaAs/GaAs heterostructures, donors are introduced into the wider-bandgap barrier layer (e.g., AlGaAs) while the narrower-bandgap channel layer (e.g., GaAs) remains undoped. This configuration exploits the conduction at the , driving free electrons from the ionized donors in the barrier to diffuse across the interface into the channel, where they accumulate to form the high-mobility (2DEG). The resulting separation—often enhanced by an undoped spacer layer between the doped barrier and channel—positions the electrons several nanometers away from the positively charged impurities, significantly reducing interactions that limit carrier transport in uniformly doped semiconductors. Carrier transfer occurs primarily due to the band offset, which creates a favoring electron migration from the higher-energy states in the doped barrier to the lower-energy quantum-confined states in the undoped channel. Once transferred, the electrons are confined by the potential and the from the ionized donors, establishing a triangular at the interface that supports the 2DEG. This process ensures high carrier densities (typically 10^{11} to 10^{12} cm^{-2}) with minimal direct exposure to centers, as the dopants remain in the barrier layer. Experimental demonstrations in early GaAs-based structures showed electron exceeding 50,000 cm²/V·s at low temperatures, far surpassing those in bulk-doped GaAs under similar conditions. To achieve precise control over doping profiles and further minimize , advanced variants employ delta-doping (also known as planar or sheet doping), where atoms are deposited in an atomically thin plane during (MBE) growth. This creates a sharp, two-dimensional impurity distribution with surface densities up to 10^{13} cm^{-2}, avoiding three-dimensional diffusion that could broaden the profile and increase . Delta-doping enhances uniformity in carrier supply and allows tailoring of the electric field for optimal 2DEG formation, particularly in high-performance HEMTs. The technique has been pivotal in achieving reproducible device characteristics since its introduction in GaAs structures. The primary benefit of modulation doping is a dramatic reduction in ionized impurity () scattering, estimated at approximately two orders of magnitude compared to bulk doping schemes, due to the increased average distance between electrons and impurities (often 10-20 nm via spacers). This leads to electron mobilities enhanced by factors of 10 or more at and up to 100 times at cryogenic temperatures, enabling HEMTs to operate at terahertz frequencies with low noise. Such improvements stem directly from the screened potential of remote impurities, as verified in early experiments.

Gate Control and Electrostatics

In high-electron-mobility transistors (HEMTs), the gate electrode, typically forming a Schottky contact on the barrier layer, modulates the conductivity of the (2DEG) through electrostatic control. The applied gate voltage generates an that penetrates the barrier layer, altering the in the 2DEG at the interface. For negative gate biases, this field expands a within the barrier, reducing the concentration in the 2DEG and thereby decreasing the channel conductance. The VthV_{th} defines the gate at which the 2DEG is fully depleted, marking the transition to the off-state. In standard depletion-mode HEMTs, VthV_{th} is negative, allowing conduction at zero gate-source voltage and requiring a reverse to pinch off the channel. In contrast, enhancement-mode HEMTs exhibit a positive VthV_{th}, ensuring the device remains off at zero for improved safety in power applications. Transconductance gmg_m, defined as the derivative of drain current with respect to voltage, quantifies the 's control efficiency and is given by gm∝Cgveg_m \propto C_g v_e, where CgC_g is the and vev_e is the in the channel. This relation arises because the determines the induced charge per volt, while governs current response; gmg_m typically peaks at high bias levels where approaches saturation. Modulation doping contributes to high gmg_m by separating dopants from the channel, minimizing and enhancing mobility. In nanoscale HEMTs with short gate lengths (e.g., below 100 nm), short-channel effects degrade gate control, including velocity saturation where electrons reach a maximum drift speed under high lateral fields, limiting current scaling. Additionally, ballistic transport emerges, allowing electrons to traverse the channel without significant , which alters the electrostatic potential profile and reduces the effective gate modulation compared to long-channel diffusive transport.

Performance Advantages

High-electron-mobility transistors (HEMTs) exhibit superior performance compared to traditional transistors like silicon MOSFETs or GaAs MESFETs, primarily due to the high electron velocity in the (2DEG) formed at the interface through modulation doping, which minimizes ionized impurity scattering. One key advantage is the exceptionally high (f_T), which exceeds 600 GHz in advanced InP-based HEMTs with lengths around 40 nm, enabling operation at terahertz frequencies unattainable by conventional devices. This stems from the elevated carrier mobility and saturation velocity in the 2DEG, allowing faster transit times across short channel lengths. Similarly, the maximum frequency (f_max) surpasses 1 THz in sub-50 nm InP HEMTs, further highlighting their potential for ultra-high-speed applications. HEMTs also demonstrate low noise figures, below 0.5 dB at millimeter-wave frequencies such as 30 GHz in GaN-based devices, owing to reduced mechanisms in the undoped channel that preserve . This noise performance is significantly better than that of comparable GaAs FETs, which typically exceed 1 dB at similar bands. In GaN HEMTs, the breakdown voltage routinely exceeds 100 V, facilitated by the wide bandgap material and robust heterostructure that supports high without premature failure. This contrasts sharply with narrower-bandgap alternatives like devices, which break down at much lower voltages under comparable conditions. Furthermore, GaN HEMTs achieve power densities up to 10 W/mm, approximately ten times higher than the less than 1 W/mm typical of GaAs devices, due to the higher critical electric field and electron density in the 2DEG. This enables compact, high-output amplifiers with enhanced efficiency.

Historical Development

Early Invention

The development of the high-electron-mobility transistor (HEMT) was driven by the need to surpass the electron mobility limitations of gallium arsenide (GaAs) metal-semiconductor field-effect transistors (MESFETs), which constrained performance in high-frequency microwave applications such as amplifiers and receivers. At Bell Laboratories, researchers Ray Dingle, Arthur C. Gossard, and Horst L. Störmer laid the foundational concept in 1978 through a on modulation doping in multilayered devices, which spatially separated dopants from the channel to reduce scattering and enhance carrier mobility in GaAs/AlGaAs structures. This innovation, modulation doping, enabled the formation of a high-mobility (2DEG) at the interface. Building on this principle, Mimura at Laboratories conceived the HEMT in 1979 while exploring alternatives to MESFETs, applying for a patent that year and leading the fabrication of the first device, demonstrated in May 1980 using selectively doped GaAs/n-AlGaAs heterojunctions. Mimura's team reported the device's superior and in an early publication that same year.

Key Milestones and Researchers

Following the initial invention of the high-electron-mobility transistor (HEMT) by Takashi Mimura and colleagues at Laboratories in 1980, subsequent developments in the 1980s focused on refining AlGaAs/GaAs structures for practical applications. Independently, Daniel Delagebeaudeuf and Nguyen T. Linh at demonstrated a functional metal-(n)AlGaAs/GaAs field-effect transistor (TEGFET), an early HEMT variant, in 1980, achieving improved charge control and high-frequency performance through engineering. Their work laid the groundwork for integrating HEMTs into monolithic integrated circuits (MMICs), with the first HEMT-based MMICs reported by 1983, enabling compact, low-noise amplifiers operating up to 20 GHz. In the 1990s, advancements in channel materials significantly enhanced HEMT performance, particularly through the introduction of pseudomorphic InGaAs channels on GaAs substrates by researchers at and other institutions. These pseudomorphic high-electron-mobility transistors (pHEMTs) incorporated strained InGaAs layers to increase and saturation velocity, pushing cutoff frequencies beyond 100 GHz in the early 1990s while maintaining low noise figures suitable for millimeter-wave applications. Key contributions also came from researchers including Dimitri Pavlidis at the , who advanced metamorphic HEMT (mHEMT) growth techniques using compositionally graded buffers to enable lattice-mismatched InGaAs channels on GaAs, reducing defects and enabling scalable high-frequency devices with performance rivaling InP-based HEMTs. The first mHEMT was demonstrated by G. Wang et al. in 1994. Parallel efforts in the late shifted toward wide-bandgap materials, with Umesh Mishra and his team at the , developing AlGaN/GaN HEMTs that leveraged high breakdown voltages and power densities for microwave applications. Their 1997 demonstration of AlGaN/GaN structures achieved power densities >3 W/mm at 18 GHz, addressing limitations in GaAs-based devices for high-power scenarios. Commercial adoption accelerated in the , with GaAs-based HEMTs integrated into low-noise amplifiers for satellite communications, enabling reliable signal reception in direct broadcast systems and transponders operating in Ku- and Ka-bands. By the , GaN HEMTs entered military radar systems, where their superior power handling—exceeding 10 W/mm—supported compact, high-efficiency active electronically scanned arrays (AESAs) for airborne and shipborne platforms, marking a transition to production-scale deployment in defense electronics.

Recent Developments (2000s–2025)

In the 2010s and 2020s, HEMT technology continued to advance with InP-based devices achieving record cutoff frequencies exceeding 1 THz, such as a 1.3 THz f_T reported in 2021 for sub-millimeter wave applications in communications. GaN HEMTs on substrates emerged for cost-effective high-power applications, enabling widespread adoption in base stations by 2020, with power densities up to 10 W/mm at Ka-band. As of 2025, metamorphic and N-polar GaN HEMTs have pushed power efficiencies over 50% in mm-wave amplifiers for and automotive systems.

Fabrication Techniques

Epitaxial Growth Methods

High-electron-mobility transistors (HEMTs) rely on precisely engineered heterostructures, where epitaxial growth methods are essential for depositing layered materials with controlled composition and minimal defects. These techniques enable the formation of high-quality interfaces that support the (2DEG) critical to HEMT performance. Among the primary methods, (MBE) and metal-organic chemical vapor deposition (MOCVD) dominate, each offering distinct advantages in precision and scalability. Molecular beam epitaxy (MBE) involves the evaporation of elemental sources in an environment (typically 10^{-10} ), allowing atomic-layer precision in depositing III-V semiconductors like GaAs and AlGaAs for early HEMT structures. This method was pivotal in the initial demonstration of HEMTs, enabling sharp heterojunctions with interface abruptness on the order of one monolayer. MBE's shuttered growth control facilitates in situ monitoring via reflection high-energy electron diffraction (RHEED), ensuring low defect densities below 10^8 cm^{-2} in optimized GaAs-based layers. Its slow growth rate, often 0.1-1 Å/s, supports complex doping profiles essential for modulation doping in HEMTs. In contrast, metal-organic chemical vapor deposition (MOCVD) employs precursor gases decomposed at elevated temperatures (around 700-1000°C) under atmospheric or low pressure, making it highly scalable for large-area substrates, particularly for GaN-based HEMTs used in . MOCVD achieves growth rates of approximately 1 μm/hr for GaN layers, enabling commercial production on substrates up to 200 mm in while maintaining interface roughness below 1 nm through optimized precursor flows and substrate preparation. High-quality AlGaN/GaN heterostructures grown by MOCVD exhibit dislocation densities as low as 10^8 cm^{-2}, comparable to MBE for many applications, though it requires careful management of parasitic reactions to minimize carbon incorporation. Lattice mismatch between epilayers and substrates poses a challenge in HEMT fabrication, addressed through metamorphic growth techniques that incorporate compositionally graded buffers to accommodate strain. In metamorphic HEMTs, such as InGaAs channels on GaAs substrates, linearly graded InGaAs buffer layers (e.g., from GaAs to In_{0.3}Ga_{0.7}As) relax the lattice over 1-2 μm thickness, reducing threading densities to below 10^8 cm^{-2} and preserving 2DEG mobility. This approach, applicable in both MBE and MOCVD, enables integration on cost-effective substrates without significant performance degradation.

Device Processing

The fabrication of high-electron-mobility transistors (HEMTs) begins with epitaxial wafers featuring structures that form the (2DEG). Subsequent device processing involves patterning and metallization to create functional source, drain, and gate electrodes, along with isolation and passivation layers to ensure reliable operation. Ohmic contacts for the source and drain are typically formed using alloyed AuGe/Ni metallization stacks, which provide low-resistance interfaces to the channel layer. These contacts are deposited via electron-beam and then annealed at temperatures around 400–450°C to promote germanium diffusion and n-type doping, achieving contact resistances below 0.1 Ω·mm, often as low as 0.05–0.07 Ω·mm in pseudomorphic HEMTs. This alloying process enhances carrier injection efficiency while minimizing parasitic resistances, critical for high-frequency performance. Gate electrodes are defined using (EBL) to achieve sub-100 nm footprints, essential for minimizing and maximizing cutoff frequencies in high-speed devices. A tri-layer resist stack, such as CSAR/LOR/PMMA, enables single-exposure patterning of T-shaped gates with foot lengths down to 50 nm and head widths of 250–500 nm, followed by metallization with stacks like Ti/Pt/Au and lift-off. This precise ensures sharp gate edges and high aspect ratios, supporting gate lengths as small as 70 nm for applications exceeding 300 GHz. Passivation layers, commonly silicon nitride (SiN_x) deposited by plasma-enhanced chemical vapor deposition (PECVD), are applied to encapsulate the device and mitigate surface traps that cause current collapse and leakage. A typical 120 nm SiN_x layer reduces surface trap density and suppresses trapping effects by passivating dangling bonds on the AlGaN barrier, improving dynamic on-resistance by orders of magnitude. Device isolation is achieved through mesa etching, using chlorine-based to define active regions and prevent lateral current spreading, with high etch selectivity preserving the underlying heterostructure. Yield optimization in HEMT processing relies on defect mapping and uniformity testing across wafers to address variations from epitaxial defects or lithographic misalignment. Techniques such as photoluminescence mapping for Al composition and uniformity assessments achieve within-wafer variations below 0.2 V, enabling yields exceeding 98% for sub-100 nm gates and supporting scalable production on 200 mm substrates. These steps ensure consistent electrical performance, with breakdown voltages uniformly above 900 V in power devices.

Types of HEMTs

pHEMT and mHEMT

Pseudomorphic high-electron-mobility transistors (pHEMTs) incorporate a strained InGaAs channel grown pseudomorphically on a GaAs substrate, exploiting a lattice mismatch of approximately 2% to induce compressive strain in the channel. This strain lowers the effective and reduces intervalley , resulting in an electron mobility enhancement of 20-30% compared to unstrained channels. To maintain coherent strain and prevent relaxation into misfit dislocations, the channel thickness is restricted to less than 10 nm, typically around 100 for optimal quantum confinement and transport properties. Metamorphic high-electron-mobility transistors (mHEMTs), on the other hand, utilize a compositionally graded InAlAs buffer layer on a GaAs substrate to gradually transition the toward that of InP-like materials, accommodating higher fractions in the InGaAs channel (up to 50-60%) without relying on pseudomorphic strain. The graded buffer effectively traps threading dislocations at its interfaces, minimizing defect propagation to the active channel region and allowing for thicker channels (beyond 10 nm) with improved structural quality and reduced buffer-related trapping effects. This approach enables the realization of InP-equivalent heterostructures on cost-effective, larger-diameter GaAs wafers, supporting applications requiring performance beyond 100 GHz. While pHEMTs benefit from the pronounced strain effects for superior short-channel velocity saturation, they face reliability challenges from potential strain-driven defect generation under high or . In comparison, mHEMTs offer enhanced device integration and on GaAs platforms but demand more intricate epitaxial grading to control composition uniformity and . Regarding performance, pHEMTs typically achieve frequencies (fT) around 300 GHz, whereas mHEMTs enable InP-based-like designs with fT exceeding 500 GHz in optimized structures. These variants are primarily realized through precise epitaxial growth techniques, such as , which control layer composition and strain profiles.

eHEMT and dHEMT

High-electron-mobility transistors (HEMTs) operate in either depletion mode (dHEMT) or enhancement mode (eHEMT), distinguished primarily by the state of the () channel at zero -source voltage (V_g = 0 V), which stems from control mechanisms that modulate carrier density through electrostatic effects. Depletion-mode HEMTs, or dHEMTs, are normally-on devices where the is present at V_g = 0 V, enabling conduction without applied bias; this results in a negative (V_th), typically -2 to -4 V for GaN-based structures. These characteristics make dHEMTs particularly suitable for radio-frequency (RF) applications, such as amplifiers, where the inherent channel conductivity supports high-frequency operation with minimal drive complexity. In contrast, enhancement-mode HEMTs, or eHEMTs, are normally-off devices in which the 2DEG is depleted at V_g = 0 V, requiring a positive gate voltage to form the channel and turn the device on; this positive V_th enhances safety in power circuits. Achieving this normally-off behavior typically involves techniques like thinning the barrier layer to reduce polarization-induced charge or fluorine ion implantation under the gate to compensate for the 2DEG, thereby shifting V_th positively. Recent advancements include p-GaN gate structures for reliable enhancement-mode operation in III-nitride HEMTs. eHEMTs are favored in logic circuits and power switching applications, where the normally-off state prevents unintended conduction during or faults, improving reliability and simplifying driver designs. From a circuit perspective, dHEMTs excel in analog RF amplifiers due to their low on-resistance and high gain at zero , while eHEMTs are essential for digital and integration, enabling direct-coupled logic without additional depletion-load circuitry. However, eHEMTs often exhibit challenges such as reduced (g_m), approximately 20% lower than comparable dHEMTs, owing to the modified barrier structures that limit maximum channel , though this trade-off is critical for their role in integrated systems.

Induced HEMT

In induced high-electron-mobility transistors (HEMTs), the two-dimensional electron gas (2DEG) is not permanently present but is dynamically formed solely by the application of a gate bias in an undoped heterostructure. Unlike standard HEMTs that rely on modulation doping or polarization effects for a built-in carrier channel, the induced variant uses electrostatic gate control to bend the conduction band at the heterojunction, accumulating electrons from the undoped channel layer only under positive gate voltage. This results in a normally-off device where the channel density, typically ranging from 0.75 × 10^{11} to 3.34 × 10^{11} cm^{-2}, is precisely tunable by the gate potential, enabling high electron mobilities up to 2.93 × 10^6 cm^2 V^{-1} s^{-1} due to the elimination of dopant-related scattering. The key advantages of this design include ultra-low power dissipation and minimal off-state leakage current, as the absence of a permanent 2DEG prevents unintended conduction and reduces static power loss. These properties are particularly beneficial for low-voltage , with the high-purity channel supporting low-disorder . GaAs/AlGaAs heterostructures serve as a foundational material system for induced HEMTs, offering mature epitaxial growth and demonstrated high-mobility performance in surface-gate configurations. However, induced HEMTs exhibit lower maximum drive currents than doped variants, constrained by the limited carrier accumulation possible without doping, which restricts their suitability for high-current scenarios. Emerging prominently since the with advances in undoped epitaxial techniques, these devices face ongoing challenges in , including uniform large-area fabrication and integration into complex circuits, limiting commercial viability despite promising prototypes.

Applications

High-Frequency Electronics

High-electron-mobility transistors (HEMTs) are pivotal in high-frequency electronics due to their superior and low noise characteristics, enabling operation in (RF) and systems for applications such as communication and sensing. These devices leverage a (2DEG) to achieve high cutoff frequencies and minimal signal degradation, making them ideal for amplifying weak signals in demanding environments. In particular, InP-based HEMTs excel in millimeter-wave regimes, supporting bandwidth-intensive technologies beyond . In low-noise amplifiers (LNAs), HEMTs provide exceptional performance for receivers, where preserving is critical. For instance, a W-band (75–110 GHz) InP HEMT LNA has demonstrated an average of 1.9 dB across 80–100 GHz, with gains exceeding 15 dB, enabling sensitive detection in deep-space communications. Such low noise figures, often below 2 dB at 100 GHz, stem from the high in InP lattices, outperforming silicon-based alternatives in cryogenic and room-temperature setups used for front-ends. Monolithic microwave integrated circuits (MMICs) incorporating HEMTs facilitate compact integration for and base stations, with GaAs pseudomorphic HEMTs (pHEMTs) dominating due to their balance of cost, reliability, and performance. A 0.10-μm GaAs pHEMT E-band (60–90 GHz) transmit/receive MMIC achieves 20 dB gain and 15 dBm output power, supporting phased-array for automotive and defense applications as well as millimeter-wave . These MMICs enable multi-function modules that handle signal amplification, mixing, and phase shifting, reducing system size while maintaining efficiency in base stations operating at 28 GHz and above. GaAs pHEMTs hold a leading position in this domain, comprising over 39% of the MMIC market driven by deployment. Advanced HEMTs push operational limits through high cutoff frequencies, with InP metamorphic HEMTs (mHEMTs) reaching up to 1 THz, facilitating terahertz-scale circuits. A 25-nm InP HEMT process has shown 3.5 dB gain at 1 THz, with extrapolated maximum oscillation (f_max) of 1.5 THz, underscoring their role in future high-speed transceivers. HEMTs are widely used in mm-wave transceivers for and telecom, bolstered by post-2020 research into prototypes exploiting sub-THz bands for ultra-high data rates exceeding 100 Gbps. This positions HEMTs as a cornerstone for emerging sensing and communication networks.

Power and Optoelectronics

Gallium nitride () high-electron-mobility transistors (HEMTs) have become pivotal in high-power applications due to their superior and efficiency compared to traditional devices. In power amplifiers, GaN HEMTs achieve output power densities exceeding 10 W/mm, enabling compact designs for demanding systems such as base stations and (EV) on-board chargers (OBCs). For instance, multi-level topologies incorporating 600 V-rated GaN HEMTs have demonstrated enhanced power density in EV OBCs, reaching levels up to 4 kW/L while maintaining high thermal performance. These devices typically exhibit specific on-resistances below 10 mΩ·cm², which minimizes conduction losses and supports efficient operation at high voltages. In switching applications, enhancement-mode GaN HEMTs (eHEMTs) offer breakdown voltages greater than 600 V, making them ideal for power adapters and converters where they surpass MOSFETs in switching speed and loss reduction. These eHEMTs enable higher operating frequencies with lower gate charge and output , resulting in improved system for point-of-load (POL) regulators and AC-DC adapters. Recent advancements in the have pushed power conversion efficiencies beyond 90%, as seen in buck converters using GaN HEMTs that achieve over 90% at 10 MHz switching frequencies under various voltage ratios. Similarly, GaN-based LED drivers have reported peak efficiencies of 96.1% with low . For , HEMTs are integrated into photonic devices to leverage their high-speed electron transport for modulation and detection. AlGaN/GaN HEMT structures serve as ultraviolet (UV) photodetectors, exhibiting enhanced near-UV responsivity through nanohole on the barrier surface, which improves response and UV/visible discrimination. p-GaN HEMT-based UV photodetectors demonstrate high photoresponsivity at low temperatures, benefiting from the (2DEG) for sensitive detection. In integration with light-emitting diodes (LEDs), voltage-controlled GaN HEMT-LED devices enable fast switching up to 15 MHz and dimmable emission, suitable for systems. Monolithic epitaxial approaches further allow on-chip μLED-HEMT pairs with modulation bandwidths exceeding 1 GHz, enhancing high-speed data transmission. Emerging applications include quantum cascade lasers (QCLs) utilizing InGaAs/InAlAs HEMT-like structures, which have advanced mid-infrared emission since the 2000s through strain-balanced designs grown by (MBE) or metal-organic chemical vapor deposition (MOCVD). These structures enable low-threshold currents and multi-gigahertz modulation speeds, with ongoing optimizations in the 2020s focusing on performance for compact, high-reliability sources.

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