High-electron-mobility transistor

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Cross section of a GaAs/AlGaAs/InGaAs pHEMT HEMT-scheme-en.svg
Cross section of a GaAs/AlGaAs/InGaAs pHEMT
Band diagram of GaAs/AlGaAs heterojunction-based HEMT, at equilibrium. HEMT-band structure scheme-en.svg
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 are 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.

Contents

Applications

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

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 Spring 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

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

The advantages of HEMTs over other transistor topologies like the Bipolar junction transistor and the MOSFET are higher operating temperatures of GaN-based HEMTs compared to Si-based MOSFETS, [10] higher breakdown strengths in the case of GaN-based HEMTs versus Si-based MOSFETS, lower specific on-state resistances when comparing GaN-based HEMTs to Si-based MOSFETs, [3] and low noise performance/higher switching speeds in the case of InP HEMTs. [13]

2DEG channel creation

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

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:

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

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 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

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 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

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

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

Related Research Articles

A semiconductor is a material that has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity generally falls as its temperature rises; metals behave in the opposite way. In many cases their conducting properties may be altered in useful ways by introducing impurities ("doping") into the crystal structure. When two differently doped regions exist in the same crystal, a semiconductor junction is created. The behavior of charge carriers, which include electrons, ions, and electron holes, at these junctions is the basis of diodes, transistors, and most modern electronics. Some examples of semiconductors are silicon, germanium, gallium arsenide, and elements near the so-called "metalloid staircase" on the periodic table. After silicon, gallium arsenide is the second-most common semiconductor and is used in laser diodes, solar cells, microwave-frequency integrated circuits, and others. Silicon is a critical element for fabricating most electronic circuits.

<span class="mw-page-title-main">Transistor</span> Solid-state electrically operated switch also used as an amplifier

A transistor is a semiconductor device used to amplify or switch electrical signals and power. It is one of the basic building blocks of modern electronics. It is composed of semiconductor material, usually with at least three terminals for connection to an electronic circuit. A voltage or current applied to one pair of the transistor's terminals controls the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal. Some transistors are packaged individually, but many more in miniature form are found embedded in integrated circuits. Because transistors are the key active components in practically all modern electronics, many people consider them one of the 20th century's greatest inventions.

<span class="mw-page-title-main">Semiconductor device</span> Electronic component that exploits the electronic properties of semiconductor materials

A semiconductor device is an electronic component that relies on the electronic properties of a semiconductor material for its function. Its conductivity lies between conductors and insulators. Semiconductor devices have replaced vacuum tubes in most applications. They conduct electric current in the solid state, rather than as free electrons across a vacuum or as free electrons and ions through an ionized gas.

<span class="mw-page-title-main">MOSFET</span> Type of field-effect transistor

The metal–oxide–semiconductor field-effect transistor is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which determines the conductivity of the device. This ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals. The term metal–insulator–semiconductor field-effect transistor (MISFET) is almost synonymous with MOSFET. Another near-synonym is insulated-gate field-effect transistor (IGFET).

<span class="mw-page-title-main">Gallium arsenide</span> Chemical compound

Gallium arsenide (GaAs) is a III-V direct band gap semiconductor with a zinc blende crystal structure.

<span class="mw-page-title-main">Monolithic microwave integrated circuit</span>

Monolithic microwave integrated circuit, or MMIC, is a type of integrated circuit (IC) device that operates at microwave frequencies. These devices typically perform functions such as microwave mixing, power amplification, low-noise amplification, and high-frequency switching. Inputs and outputs on MMIC devices are frequently matched to a characteristic impedance of 50 ohms. This makes them easier to use, as cascading of MMICs does not then require an external matching network. Additionally, most microwave test equipment is designed to operate in a 50-ohm environment.

Wide-bandgap semiconductors are semiconductor materials which have a larger band gap than conventional semiconductors. Conventional semiconductors like silicon have a bandgap in the range of 0.6 – 1.5 electronvolt (eV), whereas wide-bandgap materials have bandgaps in the range above 2 eV. Generally, wide-bandgap semiconductors have electronic properties which fall in between those of conventional semiconductors and insulators.

<span class="mw-page-title-main">Gallium nitride</span> Chemical compound

Gallium nitride is a binary III/V direct bandgap semiconductor commonly used in blue light-emitting diodes since the 1990s. The compound is a very hard material that has a Wurtzite crystal structure. Its wide band gap of 3.4 eV affords it special properties for applications in optoelectronic, high-power and high-frequency devices. For example, GaN is the substrate which makes violet (405 nm) laser diodes possible, without requiring nonlinear optical frequency-doubling.

<span class="mw-page-title-main">Schottky barrier</span> Potential energy barrier in metal–semiconductor junctions

A Schottky barrier, named after Walter H. Schottky, is a potential energy barrier for electrons formed at a metal–semiconductor junction. Schottky barriers have rectifying characteristics, suitable for use as a diode. One of the primary characteristics of a Schottky barrier is the Schottky barrier height, denoted by ΦB. The value of ΦB depends on the combination of metal and semiconductor.

A MESFET is a field-effect transistor semiconductor device similar to a JFET with a Schottky (metal–semiconductor) junction instead of a p–n junction for a gate.

<span class="mw-page-title-main">Doping (semiconductor)</span> Intentional introduction of impurities into an intrinsic semiconductor

In semiconductor production, doping is the intentional introduction of impurities into an intrinsic (undoped) semiconductor for the purpose of modulating its electrical, optical and structural properties. The doped material is referred to as an extrinsic semiconductor.

<span class="mw-page-title-main">Aluminium nitride</span> Chemical compound

Aluminium nitride (AlN) is a solid nitride of aluminium. It has a high thermal conductivity of up to 321 W/(m·K) and is an electrical insulator. Its wurtzite phase (w-AlN) has a band gap of ~6 eV at room temperature and has a potential application in optoelectronics operating at deep ultraviolet frequencies.

The heterojunction bipolar transistor (HBT) is a type of bipolar junction transistor (BJT) which uses differing semiconductor materials for the emitter and base regions, creating a heterojunction. The HBT improves on the BJT in that it can handle signals of very high frequencies, up to several hundred GHz. It is commonly used in modern ultrafast circuits, mostly radio frequency (RF) systems, and in applications requiring a high power efficiency, such as RF power amplifiers in cellular phones. The idea of employing a heterojunction is as old as the conventional BJT, dating back to a patent from 1951. Detailed theory of heterojunction bipolar transistor was developed by Herbert Kroemer in 1957.

Indium gallium arsenide (InGaAs) is a ternary alloy of indium arsenide (InAs) and gallium arsenide (GaAs). Indium and gallium are group III elements of the periodic table while arsenic is a group V element. Alloys made of these chemical groups are referred to as "III-V" compounds. InGaAs has properties intermediate between those of GaAs and InAs. InGaAs is a room-temperature semiconductor with applications in electronics and photonics.

A two-dimensional electron gas (2DEG) is a scientific model in solid-state physics. It is an electron gas that is free to move in two dimensions, but tightly confined in the third. This tight confinement leads to quantized energy levels for motion in the third direction, which can then be ignored for most problems. Thus the electrons appear to be a 2D sheet embedded in a 3D world. The analogous construct of holes is called a two-dimensional hole gas (2DHG), and such systems have many useful and interesting properties.

IQE PLC is a British semiconductor company founded 1988 in Cardiff, Wales, which manufactures advanced epitaxial wafers for a wide range of technology applications for wireless, optoelectronic, electronic and solar devices. IQE specialises in advanced silicon and compound semiconductor materials based on gallium arsenide (GaAs), indium phosphide (InP), gallium nitride (GaN) and silicon. The company is the largest independent outsource producer of epiwafers manufactured by metalorganic vapour phase epitaxy (MOCVD), molecular beam epitaxy (MBE) and chemical vapor deposition (CVD).

<span class="mw-page-title-main">Field-effect transistor</span> Type of transistor

The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor. It comes in two types: junction FET (JFET) and metal-oxide-semiconductor FET (MOSFET). FETs have three terminals: source, gate, and drain. FETs control the flow of current by the application of a voltage to the gate, which in turn alters the conductivity between the drain and source.

Indium aluminium nitride (InAlN) is a direct bandgap semiconductor material used in the manufacture of electronic and photonic devices. It is part of the III-V group of semiconductors, being an alloy of indium nitride and aluminium nitride, and is closely related to the more widely used gallium nitride. It is of special interest in applications requiring good stability and reliability, owing to its large direct bandgap and ability to maintain operation at temperatures of up to 1000 °C., making it of particular interest to areas such as the space industry. InAlN high-electron-mobility transistors (HEMTs) are attractive candidates for such applications owing to the ability of InAlN to lattice-match to gallium nitride, eliminating a reported failure route in the closely related aluminium gallium nitride HEMTs.

Roger John Malik is a physicist, engineer and inventor.

<span class="mw-page-title-main">Aristos Christou</span> American engineer

Aristos Christou is an American engineer and scientist, academic professor and researcher. He is a Professor of Materials Science, Professor of Mechanical Engineering and Professor of Reliability Engineering at the University of Maryland.

References

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  4. 1 2 3 4 Mimura, Takashi (March 2002). "The early history of the high electron mobility transistor (HEMT)". IEEE Transactions on Microwave Theory and Techniques. 50 (3): 780–782. Bibcode:2002ITMTT..50..780M. doi:10.1109/22.989961.
  5. ↑ US 4163237,Ray Dingle, Arthur Gossard and Horst Störmer,"High mobility multilayered heterojunction devices employing modulated doping"
  6. ↑ Mimura, Takashi (8 December 2005). "Development of High Electron Mobility Transistor" (PDF). Japanese Journal of Applied Physics. 44 (12R): 8263–8268. Bibcode:2005JaJAP..44.8263M. doi:10.1143/JJAP.44.8263. ISSN   1347-4065. S2CID   3112776. Archived from the original (PDF) on 8 March 2019.
  7. ↑ US 4471366,Daniel Delagebeaudeuf and Tranc L. Nuyen,"Field effect transistor with high cut-off frequency and process for forming same" (Google Patents)
  8. ↑ Asif Khan, M.; Bhattarai, A.; Kuznia, J. N.; Olson, D. T. (1993). "High electron mobility transistor based on a GaN-AlxGa1−xN heterojunction". Applied Physics Letters. 63 (9): 1214–1215. Bibcode:1993ApPhL..63.1214A. doi:10.1063/1.109775.
  9. ↑ Ye, P. D.; Yang, B.; Ng, K. K.; Bude, J.; Wilk, G. D.; Halder, S.; Hwang, J. C. M. (1 September 2004). "GaN MOS-HEMT USING ATOMIC LAYER DEPOSITION Al2O3 AS GATE DIELECTRIC AND SURFACE PASSIVATION". International Journal of High Speed Electronics and Systems. 14 (3): 791–796. doi:10.1142/S0129156404002843. ISSN   0129-1564.
  10. 1 2 Meneghini, Matteo; De Santi, Carlo; Abid, Idriss; Buffolo, Matteo; Cioni, Marcello; Khadar, Riyaz Abdul; Nela, Luca; Zagni, Nicolò; Chini, Alessandro; Medjdoub, Farid; Meneghesso, Gaudenzio; Verzellesi, Giovanni; Zanoni, Enrico; Matioli, Elison (2021). "GaN-based power devices: Physics, reliability, and perspectives". Journal of Applied Physics. 130 (18): 181101. Bibcode:2021JAP...130r1101M. doi: 10.1063/5.0061354 . hdl: 11380/1255364 .
  11. ↑ Pattnaik, Geeta; Mohapatra, Meryleen (2021). Sabut, Sukanta Kumar; Ray, Arun Kumar; Pati, Bibudhendu; Acharya, U. Rajendra (eds.). Design of AlGaAs/InGaAs/GaAs-Based PHEMT for High Frequency Application. Springer Singapore. pp. 329–337. ISBN   978-981-33-4866-0.
  12. ↑ Kasamatsu, A; Kasai, K; Hikosaka, K; Matsui, T; Mimura, T (2004). "60nm gate-length Si/SiGe HEMT". Applied Surface Science. 224 (1): 382–385. Bibcode:2004ApSS..224..382K. doi:10.1016/j.apsusc.2003.08.064.
  13. ↑ Ajayan, J.; Nirmal, D.; Mathew, Ribu; Kurian, Dheena; Mohankumar, P.; Arivazhagan, L.; Ajitha, D. (2021). "A critical review of design and fabrication challenges in InP HEMTs for future terahertz frequency applications". Materials Science in Semiconductor Processing. 128: 105753. doi:10.1016/j.mssp.2021.105753.
  14. ↑ "Indium Phosphide: Transcending frequency and integration limits. Semiconductor TODAY Compounds&AdvancedSilicon • Vol. 1 • Issue 3 • September 2006" (PDF).
  15. ↑ Hu, Zhixiang; Zhou, Licheng; Li, Long; Ying, Binzhou; Zhao, Yunong; Wang, Peng; Li, Huayao; Zhang, Yang; Liu, Huan (18 April 2023). "Quantum Dots-Sensitized High Electron Mobility Transistor (HEMT) for Sensitive NO2 Detection". Chemosensors. 11 (4): 252. doi: 10.3390/chemosensors11040252 . ISSN   2227-9040.
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