Channel High Electron Mobility Transistors Research Articles

Acoustic and optical plasmons excitation in double-channel AlGaN/GaN HEMT under asymmetric boundaries

We have simulatively studied the electrical excitation of acoustic and optical plasmons in double-channel AlGaN/GaN high electron mobility transistors (HEMT) under asymmetric boundaries. By solving the self-consistent hydrodynamic model with Maxwell’s equations, it is found that the drift plasmons are excited in the bilayer 2DEGs under asymmetric boundaries. The oscillation intensity in a bilayer system is much stronger than that in a monolayer 2DEG system because of the simultaneous excitation of the fundamental and the second harmonic components. The fundamental component always corresponds to the acoustic plasmon, while the optical plasmon can be excited when it is resonant with the second harmonic of the acoustic plasmon. Because of the out-of-phase properties of the acoustic plasmons, their radiated power is limited by the current cancellation. On the other hand, as the optical plasmons are excited, the radiated powers are much higher than those of the single channel HEMT due to the in-phase currents generated in the double layers. The effects of different parameters such as gate lengths, 2DEG spacings, and electron concentrations on plasmon excitations and THz emissions of double-channel HEMTs are analyzed in detail. The numerical results provide a more powerful terahertz radiation mechanism in double-channel HEMT compared with the Dyakonov–Shur instability in traditional single-channel HEMT.

Al‐Rich AlGaN Transistors with Regrown p‐AlGaN Gate Layers and Ohmic Contacts

AbstractEpitaxial regrowth processes are presented for achieving Al‐rich aluminum gallium nitride (AlGaN) high electron mobility transistor (HEMTs) with p‐type gates with large, positive threshold voltage for enhancement mode operation and low resistance Ohmic contacts. Utilizing a deep gate recess etch into the channel and an epitaxial regrown p‐AlGaN gate structure, an Al0.85Ga0.15N barrier/Al0.50Ga0.50N channel HEMT with a large positive threshold voltage (VTH = +3.5 V) and negligible gate leakage is demonstrated. Epitaxial regrowth of AlGaN avoids the use of gate insulators which can suffer from charge trapping effects observed in typical dielectric layers deposited on AlGaN. Low resistance Ohmic contacts (minimum specific contact resistance = 4 × 10−6 Ω cm2, average = 1.8 × 10−4 Ω cm2) are demonstrated in an Al0.85Ga0.15N barrier/Al0.68Ga0.32N channel HEMT by employing epitaxial regrowth of a heavily doped, n‐type, reverse compositionally graded epitaxial structure. The combination of low‐leakage, large positive threshold p‐gates and low resistance Ohmic contacts by the described regrowth processes provide a pathway to realizing high‐current, enhancement‐mode, Al‐rich AlGaN‐based ultra‐wide bandgap transistors.

Composite channel 100 nm InP HEMT with ultrathin barrier for millimetre wave applications

This study introduces a High Electron Mobility Transistor (HEMT) designed for millimeter-wave applications, utilizing a composite channel structure based on InP and InGaAs-InAs-InGaAs. The proposed device incorporates an ultra-thin 2 nm barsrier layer, a distinctive composite channel topology, and a judicious selection of III-V materials. These features collectively contribute to an improved confinement of electrons within the channel, thereby improving the concentration of two-dimensional electron gas (2DEG), and consequently, enhancing the mobility and speed of the device. The proposed device exhibits a unity current gain frequency (f T) of 249 GHz and a maximum oscillation frequency (f MAX) of 523.9 GHz, accompanied by a current gain of 67.7 dB at 0.1 GHz. The off-state leakage current is maintained within the nanoampere range, and the minimum noise figure (NF MIN) is merely 0.76 dB at 10 GHz. A comparative analysis of DC and RF performance, along with an examination of associated parasitic elements, is conducted among various composite channel HEMTs proposed in recent literature. A quantitative justification is provided for the superiority of InGaAs-InAs-InGaAs channel HEMTs, establishing their heightened f T and f MAX. The proposed InGaAs-InAs-InGaAs channel HEMTs exhibit 1.4 times improved f T and f MAX, coupled with only half the NF MIN in comparison to their InGaAs-InP-InGaAs channel counterparts. To further comprehend the device’s behavior under varying RF conditions, a frequency-dependent intrinsic Field-Effect Transistor (FET) model is presented. This model facilitates the analysis of the device’s performance and allows the identification of the impact of individual parameters on the overall system.

RF operation of AlN/Al0.25Ga0.75N/AlN HEMTs with f T /f max of 67/166 GHz

We report on highly-scaled Al0.25Ga0.75N channel high electron mobility transistors. Regrown ohmic contacts covering the sidewall of the compressively strained Al0.25Ga0.75N channel exhibited a low contact resistance of R c = 0.23 Ω · mm. Scaled devices with a T-shaped gate showed record high speed for any AlGaN-based transistors, f T/f max = 67/166 GHz, while simultaneously achieving high average breakdown field exceeding 2 MV cm−1. The load-pull measurements performed at 10 GHz revealed a 20% peak power added efficiency with an output power density of 2 W mm−1, which is mainly limited by the RF dispersion.

Growth and mobility characterization of N-polar AlGaN channel high electron mobility transistors

All-AlGaN based high electron mobility transistors (HEMTs) are promising for increasing the power density in both RF and power devices, improving overall efficiency. Nitrogen (N) polar GaN/AlGaN HEMTs offer lower contact resistance compared to its metal-polar counterpart. In this work, we report the metal organic chemical vapor deposition (MOCVD)-based growth of N-polar AlGaN channel HEMT structures with a varying Al mole fraction in the AlxGa1−xN channel (x = 20%, 30%, 59%, and 73%). We confirmed the high-quality morphology and the Al composition of the grown structures using atomic force microscopy and x-ray diffraction spectra, respectively. We measured a mobility of ∼160 cm2/(V.s) in our N-polar AlGaN HEMT stack (20% Al in the channel) structure and found an alloy-scattering dominated transport with increasing Al mole fraction, further supported by our simulations that consider both alloy-scattering and optical phonon-scattering mechanisms. From 20% to 59% Al composition, we found a decreasing trend in mobility while for 59%–73% Al composition in the channel, both the simulated and the experimental mobility showed a nearly saturating trend. The structures were then fabricated into HEMTs with Al0.20Ga0.80N (channel)/Al0.59Ga0.41N (barrier), showing 320 mA/mm drain current for a 4 μm long-channel device.

Demonstration of N-Polar All-AlGaN High Electron Mobility Transistors With 375 mA/mm Drive Current

Devices made from ultrawide bandgap (UWBG) materials are being widely investigated for high-power and radio frequency (RF) electronics. High electron mobility transistor (HEMT) is one of the most effective designs to implement heterostructures in III-Nitrides and III-Oxides that leverage a 2D-channel with high conductivity and large breakdown electric field. Nitrogen (N)-polarity in GaN channel HEMTs have shown remarkable performance advantage in both power and RF applications compared to its metal-polar counterpart. Here, UWBG N-polar AlGaN channel HEMT can bring further performance benefits due to an increase in the channel’s breakdown electric field. In this work, we report the first experimental demonstration of N-polar all-AlGaN HEMT devices with two different Al compositions (20% and 30%) in the channel. The HEMT with 3 μm long-channel (20% Al) showed a drive current of 375 mA/mm (at 0 V gate voltage). These devices also show low on-state leakage current of ~0.5 nA/mm, and large on/off ratio of ~2 x 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> . Furthermore, >400 V breakdown voltage was achieved without any field plate structures.

AlN/AlGaN/AlN quantum well channel HEMTs

We present a compositional dependence study of electrical characteristics of AlxGa1−xN quantum well channel-based AlN/AlGaN/AlN high electron mobility transistors (HEMTs) with x=0.25,0.44, and 0.58. This ultra-wide bandgap heterostructure is a candidate for next-generation radio frequency and power electronics. The use of selectively regrown n-type GaN Ohmic contacts results in contact resistance that increases as the Al content of the channel increases. The DC HEMT device characteristics reveal that the maximum drain current densities progressively reduce from 280 to 30 to 1.7 mA/mm for x=0.25,0.44, and 0.58, respectively. This is accompanied by a simultaneous decrease (in magnitude) in threshold voltage from −5.2 to −4.9 to −2.4 V for the three HEMTs. This systematic experimental study of the effects of Al composition x on the transistor characteristics provides valuable insights for engineering AlGaN channel HEMTs on AlN for extreme electronics at high voltages and high temperatures.

Polarization-induced 2D electron gases in N-polar AlGaN/AlN heterostructures on single-crystal AlN substrates

Polarization-induced carriers play an important role in achieving high electrical conductivity in ultrawide bandgap semiconductor AlGaN, which is essential for various applications ranging from radio frequency and power electronics to deep UV photonics. Despite significant scientific and technological interest, studies on polarization-induced carriers in N-polar AlGaN are rare. We report the observation and properties of polarization-induced two-dimensional electron gases (2DEGs) in N-polar AlGaN/AlN heterostructures on single-crystal AlN substrates by systematically varying the Al content in the 8 nm top layers from x = 0 to x = 0.6, spanning energy bandgaps from 3.56 to 4.77 eV. The 2DEG density drops monotonically with increasing Al content, from 3.8 × 1013/cm2 in the GaN channel, down to no measurable conductivity for x = 0.6. Alloy scattering limits the 2DEG mobility to below 50 cm2/V s for x = 0.49. These results provide valuable insights for designing N-polar AlGaN channel high electron mobility transistors on AlN for extreme electronics at high voltages and high temperatures, and for UV photonic devices.

Fabrication and characterization of a high-electron-mobility transistor using a sputtering buffer layer on a Si substrate

A high-electron-mobility transistor (HEMT) was developed using a high-purity gallium nitride (GaN) sputtering target (oxygen content of <0.4% and metal impurities of 99.999% (5 N)). Two layers of sputtered GaN/aluminum nitride (AlN) were used instead of conventional superlattices (SLs) as stress-relaxing structures in GaN on Si devices. AlGaN/GaN layers were deposited on the GaN/AlN/Si templates by metal–organic chemical vapor deposition to construct a channel of HEMTs. To the best of our knowledge, this is the first demonstration of HEMTs without SLs achieved using sputtered GaN/AlN buffer layers, instead of normal buffer layers obtained via chemical vapor deposition on Si substrates. Sputter epitaxy has a significant potential for fabricating low-cost and high-producibility GaN-on-Si devices.

Optimization of Channel Structures in InP HEMT Technology for Cryogenic Low-Noise and Low-Power Operation

We report the impact from channel composition on the cryogenic low-noise performance at low dc power for a 100-nm gate-length InGaAs-InAlAs-InP high-electron mobility transistor (HEMT). Two indium (In) channel compositions, 65% and 80%, were studied by dc and RF characterization at 300 and 5 K. For the cryogenic low-noise optimization, it was important to increase the transconductance to gate–source capacitance ratio in the weak inversion region implying that a higher maximum cutoff frequency in the HEMT does not guarantee lower noise. The HEMT noise performance was obtained from noise measurements in a hybrid three-stage 4–8-GHz ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{C}$</tex-math> </inline-formula> -band) low-noise amplifier (LNA) down to 300- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu$</tex-math> </inline-formula> W dc power dissipation. While the HEMT LNA noise performance for both the channel compositions at 300 K was found to be comparable, the HEMT LNA at 5 K with 65% In channel showed a minimum noise temperature of 1.4 K, whereas the noise temperature in the HEMT LNA with 80% In channel HEMTs increased to 2.4 K. The difference in the noise became more pronounced at reduced dc power dissipation. The ultralow dc power of 300 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu$</tex-math> </inline-formula> W demonstrated for a cryogenic <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\textit{C}$</tex-math> </inline-formula> -band LNA with an average noise temperature of 2.9 K and 24-dB gain is of interest for future qubit read-out electronics at 4 K.

Effects of GaN channel downscaling in AlGaN–GaN high electron mobility transistor structures grown on AlN bulk substrate

In this work, two series of AlGaN/GaN/AlN high electron mobility transistor (HEMT) heterostructures have been grown by molecular beam epitaxy on AlN bulk substrates. The effects of reduction in the GaN channel thickness as well as the AlGaN barrier thickness and composition on structural and electrical properties of the heterostructures have been studied. The material analysis involved high-resolution x-ray diffraction, atomic force microscopy, and cross-sectional transmission electron microscopy. In a first series of HEMT structures grown with an aluminum content of 30% in the AlGaN barrier, the channel downscaling results in a reduction in the GaN strain relaxation rate but at the expense of degradation in the mean crystal quality and in the electron mobility with a noticeable increase in the sheet resistance. An opposite trend is noticed for the three-terminal breakdown voltage of transistors, so that a trade-off is obtained for a 50 nm width GaN channel HEMT, which exhibits a sheet resistance of 1700 Ω/sq. with transistors demonstrating three-terminal breakdown voltage up to 1400 V for 40 μm gate to drain spacing with static on resistance Ron = 32 mΩ cm2. On the other hand, a second series of HEMT structures with high aluminum content AlGaN barriers and sub-10 nm GaN channels have been grown perfectly strained with high sheet carrier densities allowing to preserve sheet resistances in the range of 880–1050 Ω/sq.

Tuning composition in graded AlGaN channel HEMTs toward improved linearity for low-noise radio-frequency amplifiers

Compositionally graded channel AlGaN/GaN high electron mobility transistors (HEMTs) offer a promising route to improve device linearity, which is necessary for low-noise radio-frequency amplifiers. In this work, we demonstrate different grading profiles of a 10-nm-thick AlxGa1−xN channel from x = 0 to x = 0.1 using hot-wall metal-organic chemical vapor deposition (MOCVD). The growth process is developed by optimizing the channel grading and the channel-to-barrier transition. For this purpose, the Al-profiles and the interface sharpness, as determined from scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy, are correlated with specific MOCVD process parameters. The results are linked to the channel properties (electron density, electron mobility, and sheet resistance) obtained by contactless Hall and terahertz optical Hall effect measurements coupled with simulations from solving self-consistently Poisson and Schrödinger equations. The impact of incorporating a thin AlN interlayer between the graded channel and the barrier layer on the HEMT properties is investigated and discussed. The optimized graded channel HEMT structure is found to have similarly high electron density (∼ 9  × 10 12 cm−2) as the non-graded conventional structure, though the mobility drops from ∼ 2360 cm2/V s in the conventional to ∼ 960 cm2/V s in the graded structure. The transconductance gm of the linearly graded channel HEMTs is shown to be flatter with smaller g m ′ and g m ″ as compared to the conventional non-graded channel HEMT implying improved device linearity.

A room-temperature, low-impedance and high-IF-bandwidth terahertz heterodyne detector based on bowtie-antenna-coupled AlGaN/GaN HEMT

In this paper, a 330 GHz terahertz heterodyne detector based on bowtie-antenna-coupled AlGaN/GaN high-electron-mobility transistor (HEMT) is designed and demonstrated. The bowtie antenna and a silicon lens couple the terahertz wave into a transmission line, in which the HEMT's channel generates both self-mixing and heterodyne signals. Compared to field-effect detectors without front low-noise amplifiers and output impedance matching, this detector boosts the intermediate-frequency (IF) bandwidth to 2.9 GHz due to a low output impedance of 505 Ω while maintaining a comparable sensitivity. With further sensitivity enhancement, such detectors would be developed into room-temperature, high-sensitivity, and high-IF-bandwidth heterodyne arrays.

Studies on the coupling correlation and strain state of AlGaN/GaN double channel heterostructures grown by metal organic chemical vapor deposition

In this work, high quality AlGaN/GaN double channel heterostructures were grown by metal organic chemical vapor deposition and the coupling correlation between the two channels, as well as the strain state of this multilayer structure are investigated in detail. It was revealed that the stronger coupling correlation affect the 2DEG distribution and leads to the lower channel is similar to the upper channel with the thinner thickness of the upper channel. In addition, the impact of the multilayer strain state on the characteristics of double channel heterostructures was discussed. Moreover, the effects of coupling correlation and strain state on the performance of double channel high electron mobility transistor (HEMT) was shown by 2-D simulation. This work is not only beneficial for the further study of the double (multiple) channel heterostructures and devices, but also instructive to the design and realization of the future atomic hyperfine structures of nitride semiconductors.

High Al-content AlGaN channel high electron mobility transistors on silicon substrate

The rapidly increasing power demand, downsizing of power electronics and material specific limitation of silicon has led to development of AlGaN/GaN heterostructures. Commercial GaN power devices are best available for radio frequency (RF) and high voltage switching applications. Emerging AlxGa1-xN channel based heterostructures are promising to enhance the limits of next generation high voltage GaN power switching devices. In this work, we report on the study of electrical performance of AlGaN channel HEMTs-on-Silicon using various Al content. The fabricated devices exhibited outstanding buffer breakdown electric field above 2.5 MV/cm considering the submicron thin heterostructures grown on silicon substrate. Furthermore, we also experimentally demonstrate high temperature operation of fabricated AlGaN channel HEMTs.

Effect of Strain in Channel on Electron Transport Properties of Ga1−xInxSb High Electron Mobility Transistor Structures with Strained‐Al0.40In0.60Sb/Al0.25In0.75Sb Stepped Buffer

GaInSb is one of the attractive Sb‐based channel materials for high electron mobility transistors (HEMTs) that can operate in the terahertz band. The effect of strain in the channel on the electron transport properties of Ga1−x In x Sb channel HEMT structures (x = 0.60, 0.78, 0.85, 0.90, and 0.94) with the strained‐Al0.40In0.60Sb/Al0.25In0.75Sb stepped buffer is investigated. The strain in the Ga1−x In x Sb channel layer is determined by the lattice constant of Al0.25In0.75Sb lower buffer layer. The electron mobility (μ) shows the maximum value of 15 100 cm2 V−1 s−1 at x = 0.78 (unstrained), which has the minimum threading dislocation density (TDD). The value for sheet electron density (N s) increases with decreasing x, and saturates at about 2.1 × 1012 cm−2 when unstrained or tensile strained. The minimum sheet resistance (R s) is 202 Ω/□ for the unstrained Ga0.22In0.78Sb channel.

Electrical characteristics of metal–semiconductor–metal (MSM) varactors fabricated on In0.53Ga0.47As/InAs/In0.53Ga0.47As composite channel HEMT structure

In this paper, we have fabricated and characterized metal–semiconductor–metal (MSM) varactors with gate lengths ranging from 120, 180, and 290 nm on In0.53Ga0.47As/InAs/In0.53Ga0.47As composite channel high electron mobility transistor structure. We applied the buried-Pt gate technology to study the electrical characteristics of the MSM varactors depending on the gate-to-channel distance. The fabricated MSM varactors exhibited a cutoff frequency of 185 GHz for annealed varactors and 174 GHz for non-annealed varactors with L g of 120 nm, respectively. Also, non-annealed MSM varactors show an excellent capacitance ratio (=C max/C min) of 5.1 with L g of 290 nm, this is because of the longer gate-to-channel distance compared to annealed MSM varactors. The figure of merit (FOM = f 0·C max/C min) of the MSM varactors, which represents the performance of the MSM varactors, shows 451 and 438 for non-annealed and annealed devices, respectively.

Electron mobility in GaN layers and HEMT structure optimized by MOVPE technological parameters

Although a lot of attention was devoted to different aspects of GaN high electron mobility transistor (HEMT) structure such as buffer layer architecture, AlGaN barrier, surface passivation and dielectric choice, not much attention was paid to optimize the technology of GaN channel. We show in this work that by optimizing the channel technology the electron mobility can be significantly improved. We have studied the influence of technological parameters on transport properties on the series of GaN layers resembling a HEMT channel. We pay most attention to the layer growth using TEG precursor. The examined parameters were type of reactor atmosphere, growth temperature, growth rate influenced by precursor concentration and reactor pressure. Using optimized parameters for growth of a HEMT structure, we have succeeded in increasing the electron mobility in 2DEG by 30 %.

Toward AlGaN channel HEMTs on AlN: Polarization-induced 2DEGs in AlN/AlGaN/AlN heterostructures

Due to its high breakdown electric field, the ultra-wide bandgap semiconductor AlGaN has garnered much attention recently as a promising channel material for next-generation high electron mobility transistors (HEMTs). A comprehensive experimental study of the effects of Al composition x on the transport and structural properties is lacking. We report the charge control and transport properties of polarization-induced 2D electron gases (2DEGs) in strained AlGaN quantum well channels in molecular-beam-epitaxy-grown AlN/AlxGa1−xN/AlN double heterostructures by systematically varying the Al content from x = 0 (GaN) to x = 0.74, spanning energy bandgaps of the conducting HEMT channels from 3.49 to 4.9 eV measured by photoluminescence. This results in a tunable 2DEG density from 0 to 3.7 × 1013 cm2. The room temperature mobilities of x ≥ 0.25 AlGaN channel HEMTs were limited by alloy disorder scattering to below 50 cm2/(V.s) for these 2DEG densities, leaving ample room for further heterostructure design improvements to boost mobilities. A characteristic alloy fluctuation energy of ≥1.8 eV for electron scattering in AlGaN alloy is estimated based on the temperature dependent electron transport experiments.

Waveguide-coupled heterodyne terahertz detector based on AlGaN/GaN high-electron-mobility transistor

We report a room-temperature, low output impedance, broad intermediate-frequency (IF) bandwidth field-effect terahertz detector based on an AlGaN/GaN high-electron-mobility transistor (HEMT) integrated in a metal waveguide. The waveguide detector equips a pair of quasi-Yagi antenna probes that are used to couple the terahertz energy to the HEMT channel. The gate is configured as an asymmetric edge-coupled coplanar waveguide transmission line. This terahertz electric field is asymmetrically distributed in the channel along the edges of the transmission lines. The responsivity and noise for direct and heterodyne detections are characterized and analyzed at different local oscillator (LO) powers. The noise-equivalent power in direct detection is below 189 pW/Hz1/2. Operated in a heterodyne mode with a LO power of −3 dBm, the detector offers a conversion loss less than 55 dB in a frequency band of 320–340 GHz. The channel in a form of transmission line performs the broad IF bandwidth, which is increased to gigahertz range (3 GHz), and reduces the output impedance to 377 Ω which is about 20 times lower than previously reported. The transmission-line impedance could be optimized together with the distribution of the terahertz electric field in the gated channel to reduce the conversion loss.