High Electron Mobility Transistor Structures Research Articles

Impact of Al profile in high-Al content AlGaN/GaN HEMTs on the 2DEG properties

Ultra-thin high-Al content barrier layers can enable improved gate control and high-frequency operation of AlGaN/GaN high electron mobility transistors (HEMTs) but the precise composition control is very challenging. In this work, we investigate the compositional profiles of AlxGa1−xN/GaN HEMT structures with targeted Al content in the barrier layer, x = 0.50, 0.70, and 1, and thickness in the sub-10 nm range in correlation with the two-dimensional electron gas (2DEG) properties. The HEMT structures are grown by metal-organic chemical vapor deposition on SiC. The maximum Al content in the barrier layer, experimentally determined by scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy, is found to be lower than that intended and the deviations from the designed structures increase progressively with increasing x. Compositionally sharp interface between GaN and Al0.46Ga0.54N and box-like Al profile is achieved for intended x∼0.50 while pronounced Al grading is found in the samples with intended x of 0.70 and 1, with a maximum Al content of 0.78 reached for the HEMT structure with intended AlN barrier layer. The impact of the experimentally determined Al profiles on the 2DEG properties, obtained by contactless and electrical Hall effect measurements and coupled with self-consistent solution of the Poisson–Schrödinger equation, is evaluated and discussed. It is shown that the observed deviations from the intended Al profiles have a negative effect on the 2DEG confinement and result in reduced mobility parameters, which have significant implications for the implementation of high-Al content AlGaN/GaN structures in high-frequency devices.

Design and optimization of high electron mobility transistor with high-k dielectric material integration

We have developed and simulated a high electron mobility transistor (HEMT) operating in the 5 nm regime. This HEMT uses hafnium oxide (HfO2), a high-k dielectric material, to create an undoped region (UR) beneath the gate. While the gate and undoped regions share equal thickness, the channel length differs. This innovative undoped under the gate dielectric HEMT design mitigates the maximum electric field (V) within the channel area, leading to a significant increase in drain current. The utilization of a high-k dielectric in the HEMT structure results in a saturated Ion current that is 60% higher compared to conventional structures. Specifically, we use an AlGaN/GaN/SiC-based HEMT with an intrinsic section below the gate, using HfO2 as the high-k dielectric substantial, for applications requiring high power and high-frequency power amplifiers. Compare this advanced HEMT design to conventional HEMTs and you will see improved conductivity, a greater drain current (Id), a 54% increase in transconductance (Gm), and a lower on-resistance (Ron). Additionally, advancements in the electric field in the Y direction are seen. This HEMT structure exhibits superior performance compared to alternative materials analyzed. The integration of AlGaN/GaN materials in HEMTs opens up extensive opportunities in the realms of radio frequency very large-scale integration (VLSI) and power electronics.

Charge Trapping and Emission during Bias Temperature Stressing of Schottky Gate GaN-on-Silicon HEMT Structures Targeting RF/mm Wave Power Amplifiers.

For operation as power amplifiers in RF applications, high electron mobility transistor (HEMT) structures are subjected to a range of bias conditions, applied at both the gate and drain terminals, as the device is biased from the OFF- to ON-state conditions. The stability of the device threshold voltage (Vt) condition is imperative from a circuit-design perspective and is the focus of this study, where stresses in both the ON and OFF states are explored. We see rapid positive threshold voltage increases under negative bias stress and subsequent recovery (i.e., Vt reduces), whereas conversely, we see a negative Vt shift under positive stress and Vt increase during the subsequent relaxation phase. These effects are correlated with the thickness of the GaN layer and ultimately result from the deep carbon-acceptor levels in the C-GaN back barrier incorporated to screen the buffer between the silicon substrate and the epitaxially grown GaN layer. Methods to mitigate this effect are explored, and the consequences are discussed.

Threshold voltage mapping at the nanoscale of GaN-based high electron mobility transistor structures using hyperspectral scanning capacitance microscopy

Threshold voltage mapping at the nanoscale of GaN-based high electron mobility transistor structures using hyperspectral scanning capacitance microscopy

Optical and Electrical Properties of AlGaN-Based High Electron Mobility Transistors and Photodetectors with AlGaN/AlN/GaN Channel-Stacking Structure.

High channel current of the high electron mobility transistors (HEMTs) and high relative responsivity of the photodetectors (PDs) were demonstrated in the AlGaN/AlN/GaN channel-stacking epitaxial structures. The interference properties of the X-ray curves indicated high-quality interfaces of the conductive channels. The AlGaN/AlN/GaN interfaces were observed clearly in the transmission electron microscope micrograph. The saturation I ds currents of the HEMT structures were increased by adding a number of channels. The conductive properties of the channel-stacking structures corresponded to the peaks of the transconductance (g m) spectra in the HEMT structures. The depletion-mode one- and two-channel HEMT structures can be operated at the cutoff region by increasing the reverse V gs bias voltages. Higher I ds current in the active state and lower current in the cutoff state were observed in the two-channel HEMT structure compared with one- and three-channel HEMT structures. For the channel-stacking metal-semiconductor-metal photodetector structures, the peak responsivity was observed at almost 300 nm incident monochromic light, which was increased by adding a number of channel layers. The channel current of the HEMT devices and the photocurrent in the PD devices were increased by adding a number of two-dimensional electron gas (2DEG) channels. By using a flat gate metal layer, the two-channel AlGaN/AlN/GaN HEMT structures exhibited a high I ds current, a low cutoff current, and a high peak g m value and have the potential for GaN-based power devices, fast portable chargers, and ultraviolet PD applications.

Enhanced Performance of Ultraviolet AlGaN/GaN Photo‐HEMTs by Optimized Channel Isolation Schemes

AbstractSolar blind ultraviolet (UV) photodetectors utilizing AlGaN/GaN high electron mobility transistor (HEMT) structure offer a very high responsivity, photo‐to‐dark current ratio (PDCR), and detectivity. However, the performance of conventional mesa‐isolated photo‐HEMT is limited due to current leakage paths through the side wall and AlGaN barrier. In this work, apart from conventional mesa‐type isolation, ion implantation and metal‐insulator‐semiconductor (MIS) schemes are adopted to isolate the channels of AlGaN/GaN photo‐HEMTs. A significant reduction in dark current is achieved for MIS photo‐HEMT of around four and three orders of magnitude as compared to those of conventional mesa‐ and ion‐implanted photo‐HEMTs, respectively. Moreover, the responsivity and PDCR of MIS photo‐HEMT are around three and four orders of magnitude higher than those of conventional mesa‐isolated photo‐HEMT. Subsequently, MIS photo‐HEMT on Si substrate has achieved a very high detectivity of 4.63 × 1016 Jones at the UV incidence wavelength of 360 nm. The results presented in this work establish the advantages of MIS photo‐HEMT over other types of photo‐HEMTs. In addition, compatibility with GaN device fabrication technology and significantly enhanced optoelectronic performance suggest AlGaN/GaN MIS photo‐HEMTs as a promising candidate for a variety of advanced UV sensing applications.

A Comprehensive Study on AlGaN/GaN-Based HEMT for High-Speed

GaN-based solutions can handle emerging technological demands from today’s fast-changing electronics industry. AlGaN/GaN heterostructures offer the possibility of a variety of applications. The primary issues in commercializing GaN-based technology are reliability and performance enhancement. This research focuses on recent improvements in AlGaN/GaN-based high-electron-mobility transistors (HEMTs) for diverse high-speed applications. This review will assist researchers in gathering all relevant knowledge on AlGaN/GaN HEMT structure in one location, allowing them to focus on developing high-speed applications.

Enhanced Performance of Metal‐Semiconductor‐Metal UV Photodetectors on Algan/Gan Hemt Structure via Periodic Nanohole Patterning

AbstractAlGaN/GaN High Electron Mobility Transistor (HEMT) structures offer superior electrical and material properties that make them ideal for the fabrication of high‐performance Ultraviolet photodetectors (UV PDs), especially using the metal‐semiconductor‐metal (MSM) configuration. However, the metal layout of the MSM design and crystal defects in multi‐stack HEMTs can reduce photocurrent and degrade device performance. Nano‐structuring of the AlGaN/GaN surface with different nanofeatures is a promising approach to improve light absorption efficiency and increase device response. In this work, AlGaN/GaN HEMT MSM UV photodetectors with enhanced performance parameters by engineering the surface with periodic nanohole arrays are demonstrated. Optical simulations are used to optimize the design of the nanoholes' periodicity and depth. Unpatterned and nanohole‐patterned devices with varying nanohole depths are fabricated, and their performance shows substantial enhancement with the incorporation of nanoholes. The device with 40 nm deep nanoholes and 230 nm array periodicity shows the highest performance in terms of photocurrent (0.15 mA), responsivity (1.4 × 105 A W−1), UV/visible rejection ratio (≈103), and specific detectivity (4.9 × 1014 Jones). These findings present a HEMT‐compatible strategy to enhance UV photodetector performance for power optoelectronic applications, highlighting that nanohole patterning is a promising prospect for advancements in UV photodetection technology.

Al 0.3 Ga 0.7 N/GaN HEMT Yapısı için QMSA Metodu Uygulanması

In this study, Al 0.3 Ga 0.7 N/GaN high electron mobility transistor (HEMT) structure is grown on a sapphire (Al2O3) substrate by using metal-organic vapor phase epitaxy (MOVPE), and its electron transport and magnetic transport properties are investigated. Resistivity is measured in the 20-350 K temperature range. Hall mobility and Hall carrier concentration are measured in the 0-1.5 T magnetic field range and the same temperature range. Magnetic transport properties are analyzed using quantitative mobility spectrum analysis (QMSA). 2DEG and 3DEG transport mechanisms are separated by using QMSA results.

Enhancement of electron transport mobility in GaAs/InGaAs asymmetrically doped narrow quantum well pHEMT structure

We study the effect of asymmetric doping concentrations on the electron mobility μ in GaAs/InGaAs-based single quantum well (SQW) as well as double quantum well (DQW) pseudomorphic high electron mobility transistor (pHEMT) structures. Unequal doping in the substrate and surface barriers (n d1 and n d2 ) causes asymmetric distributions of subband wave functions, ψ 0 and ψ 1, which influence the subband scattering rate matrix elements (SSRME), thereby affecting the subband mobility μ n . For narrow well widths (w w ), in SQW structures, mostly a single subband is occupied. We show that an increase in n d2 , keeping n d1 fixed, enhances μ nonlinearly. The interface roughness (ir-) scattering mostly dominates μ in thin wells (w w < 70 Å), while generally, μ is determined by ionized impurity (ii-) scattering and to some extent by alloy disorder (ad-) scattering. The influence of ir-scattering enhances, while ad-scattering diminishes, by reducing n d2 . For DQW, a double subband is occupied. In a symmetric DQW structure at resonance, n d1 = n d2 , ψ 0 and ψ 1 equally extend into both the wells. For a minor variation, say n d1 > n d2 , ψ 0 mostly lies in one well while ψ 1 is in the other well. In the case of n d1 < n d2 , the distribution reverts. The substantial changes in ψ 0 and ψ 1 influence the intra- and inter-SSRME differently through intersubband effects, leading to nonlinear μ n as a function of n d2 . Taking n d1 + n d2 = 3 × 1018 cm−3, we show that for w w1 = w w2 = 80 Å, a shallow dip in μ occurs at n d1 = n d2 = 1.5 × 1018 cm−3. Whereas, for w w1 = 60 Å and w w2 = 100 Å, the dip in μ occurs near the corresponding resonance, n d1 = 2.3 × 1018 cm−3 and n d2 = 0.7 × 1018 cm−3. Our results of nonlinear μ can be utilized for performance analysis of pHEMT.

Room temperature two-dimensional electron gas scattering time, effective mass, and mobility parameters in AlxGa1−xN/GaN heterostructures (0.07 ≤ x ≤ 0.42)

AlxGa1−xN/GaN high-electron-mobility transistor (HEMT) structures are key components in electronic devices operating at gigahertz or higher frequencies. In order to optimize such HEMT structures, understanding their electronic response at high frequencies and room temperature is required. Here, we present a study of the room temperature free charge carrier properties of the two-dimensional electron gas (2DEG) in HEMT structures with varying Al content in the AlxGa1−xN barrier layers between x=0.07 and x=0.42. We discuss and compare 2DEG sheet density, mobility, effective mass, sheet resistance, and scattering times, which are determined by theoretical calculations, contactless Hall effect, capacitance-voltage, Eddy current, and cavity-enhanced terahertz optical Hall effect (THz-OHE) measurements using a low-field permanent magnet (0.6 T). From our THz-OHE results, we observe that the measured mobility reduction from x=0.13 to x=0.42 is driven by the decrease in 2DEG scattering time, and not the change in effective mass. For x&amp;lt;0.42, the 2DEG effective mass is found to be larger than for electrons in bulk GaN, which in turn, contributes to a decrease in the principally achievable mobility. From our theoretical calculations, we find that values close to 0.3m0 can be explained by the combined effects of conduction band nonparabolicity, polarons, and hybridization of the electron wavefunction through penetration into the barrier layer.

Simulation of Single-Event Transient Effect for GaN High-Electron-Mobility Transistor.

A GaN high-electron-mobility transistor (HEMT) was simulated using the semiconductor simulation software Silvaco TCAD in this paper. By constructing a two-dimensional structure of GaN HEMT, combined with key models such as carrier mobility, the effects of a different state, different incidence position, different drain voltage, different LET values, and a different incidence angle on the single-event transient effect of GaN HEMT are simulated. LET stands for the linear energy transfer capacity of a particle, which refers to the amount of energy transferred by the particle to the irradiated substance on the unit path. The simulation results show that for GaN HEMTs, the single-event transient effect is more obvious when the device is in off-state than in on-state. The most sensitive location of GaN HEMTs to the single-event effect is in the region near the drain. The peak transient current increases with the increase in the drain bias and incident ion LET values. The drain charge collection time increases with the angle of incidence of heavy ion.

Structural, optical, and electrical characterization and performance comparison of AlGaN/GaN HEMT structures with different buffer layers

AlGaN/GaN high electron mobility transistor (HEMT) structures are promising for high power and high speed electronic applications. The selection of a good buffer structure is crucial for the achievement of high quality HEMT devices. Here, we present a comparative analysis of structural, optical, and electrical properties of AlGaN/GAN HEMT structures with three different buffer configurations: step-graded AlGaN and superlattice-based buffers on Si, and no buffer on SiC substrates. The analysis of structural properties showed that the structure on SiC has better crystal quality. The Analysis also provided an insight of the role superlattice buffer to cut-off the dislocations and enhance the GaN channel crystallinity. Optical and electrical characterizations showed the enhanced 2DEG quality in the superlattice-based structure, due to the better stress management in the top layers. The electronic and optoelectronic quality of the structures were studied by fabricating HEMT and metal-semiconductor-metal photodetector devices. The devices analysis showed that the superlattice based structure outperformed the other two structures with drain current and transconductance of 406.3 mA/mm and 81.5 mS/mm, respectively, as well as photo-to-dark current ratio that is 2–3 orders of magnitude higher than the other two structures, under 325 nm light illumination. The results provide insights into the impact of different buffer configurations on the performance of AlGaN/GaN HEMT structures, suggesting that the superlattice buffer structure is the optimal choice for fabricating high-performance AlGaN/GaN HEMT-based electronics.

Structural, electrical, morphological, and interfacial characteristics of lattice-matched InAlN/GaN HEMT structure on SiC substrate

This work highlights the influence of surface properties, on the characteristics of InAlN/GaN based high electron mobility transistor (HEMT) structures grown on the SiC substrate by metalorganic vapor phase epitaxy. The growth parameters, i.e., reactor pressure and V/III ratio were tuned to improve the morphological and two-dimensional electron gas (2DEG) characteristics of the HEMT structure. It was found that V/III ratio plays a significant role in improving surface morphology and 2DEG properties without altering average indium composition. It was also found that 2DEG properties are highly sensitive to surface morphology and its features. The step flow smooth surface morphology with very low surface and interface roughness was observed in optimized lattice-matched InAlN/GaN HEMT structures. The sheet resistance of ∼170 Ω/sq with good 2DEG concentration (∼2.4 × 1013 cm−2) and 2DEG mobility (∼1500 cm2/V s) was achieved in the optimized lattice-matched InAlN/GaN HEMT structure. A comparison between different barrier-based HEMT structures, i.e., lattice-matched InAlN/GaN and strained AlGaN/GaN, was also discussed. Their structural, electrical, morphological, and interfacial characteristics were compared.

Microstructural characterization of AlxGa1−xN/GaN high electron mobility transistor layers on 200 mm Si(111) substrates

Realization of high crystal quality GaN layers on silicon substrates requires a great control over epitaxy as well as detailed characterization of the buried defects and propagating dislocations within the epilayers. In this Letter, we present the microstructural characterization of AlxGa1−xN/GaN high electron mobility transistor (HEMT) structures epitaxially grown on 200 mm Si (111) substrates with superlattice (SL) buffers. The HEMT stack is also composed of an intermediate thick AlxGa1−xN layer sandwiched between the short-period and long-period AlxGa1−xN/AlN SLs. Structural and morphological characteristics of the GaN-based epilayers are studied to assess the quality of the HEMT stack grown on such 200 mm diameter substrates. The detailed microstructural uniformity of the epilayers is addressed by the transmission electron microscopy (TEM) technique. In depth scanning TEM with electron energy loss spectroscopy (EELS) investigations have been carried out to probe the microstructural quality of the HEMT stack comprising of such short- and long-period AlxGa1−xN/AlN SLs. The EELS-plasmon maps are utilized to address the sharp interface characteristics of the SL layers as well as the top active p-GaN/AlxGa1−xN/GaN layers. This work highlights the capability of high-resolution TEM as a complementing characterization method to produce reliable AlxGa1−xN/GaN HEMTs on such a large diameter silicon substrate.

Crack-Free High-Composition (&gt;35%) Thick-Barrier (&gt;30 nm) AlGaN/AlN/GaN High-Electron-Mobility Transistor on Sapphire with Low Sheet Resistance (&lt;250 Ω/□)

In this article, a high-composition (&gt;35%) thick-barrier (&gt;30 nm) AlGaN/AlN/GaN high-electron-mobility transistor (HEMT) structure grown on a sapphire substrate with ultra-low sheet resistivity (&lt;250 Ω/□) is reported. The optimization of growth conditions, such as reduced deposition rate, and the thickness optimization of different epitaxial layers allowed us to deposit a crack-free high-composition and thick AlGaN barrier layer HEMT structure. A significantly high two-dimensional electron gas (2DEG) density of 1.46 × 1013 cm−2 with a room-temperature mobility of 1710 cm2/V·s was obtained via Hall measurement using the Van der Pauw method. These state-of-the-art results show great potential for high-power Ga-polar HEMT design on sapphire substrates.

Scanning capacitance microscopy of GaN-based high electron mobility transistor structures: A practical guide

The scanning capacitance microscope (SCM) is a powerful tool to characterise local electrical properties in GaN-based high electron mobility transistor (HEMT) structures with nanoscale resolution. We investigated the experimental setup and the imaging conditions to optimise the SCM contrast. As to the experimental setup, we show that the desired tip should be sharp (e.g., with the tip radius of ≤25nm) and its coating should be made of conductive doped diamond. Most importantly, its spring constant should be large to achieve stable tip-sample contact. The selected tip should be positioned close to both the edge and Ohmic contact of the sample. Regarding the imaging conditions, we also show that a dc bias should be applied in addition to an ac bias because the latter alone is not sufficient to deplete the two-dimensional electron gas (2DEG) in the AlGaN/GaN heterostructure. The approximate range of the effective dc bias values was found by measuring the local dC/dV-V curves, yielding, after further optimisation, two optimised dc bias values which provide strong, but opposite, SCM contrast. In comparison, the selected ac bias value has no significant impact on the SCM contrast. The described methodology could potentially also be applied to other types of HEMT structures, and highly-doped samples.

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.

Study on the Point‐Contact Gate AlGaN/GaN High Electron Mobility Transistor with 0.1 μm Gate Length

The current collapse has been one of the most challenging problems in AlGaN/GaN high electron mobility transistors (HEMTs). In recent years, the virtual gate effect has been the most widely accepted cause of the current collapse effect. To effectively improve the current collapse effect and reduce the influence of the virtual gate effect on the device, a point‐contact gate AlGaN/GaN HEMT structure is proposed in this article. Tests show that the device has high transconductance, high cut‐off frequency, and good transfer, output, and breakdown characteristics. The maximum saturation drain current is 18 mA mm−1, the maximum transconductance is 200 mS mm−1, the breakdown voltage is 994 V, the maximum current gain is about 190 dB, and the cut‐off frequency is up to 206 GHz when the gate length and width are 0.1 μm × 0.001 μm. The experimental analysis shows that the point‐contact gate AlGaN/GaN HEMT structure shortens the gate length, thereby improving the virtual gate effect and effectively suppressing the current collapse effect, increasing the breakdown voltage of the device, and further improving the device performance, which lays the foundation for the future research of AlGaN/GaN HEMT.

Fabrication of Low On‐Resistance and Normally Off AlGaN/GaN Metal Oxide Semiconductor Heterojunction Field‐Effect Transistors with AlGaN Back Barrier by the Selective Area Regrowth Technique

Herein, a recessed‐gate AlGaN/GaN metal oxide semiconductor heterojunction field‐effect transistor (MOS‐HFET) with an AlGaN back‐barrier layer fabricated by a selective area regrowth (SAG) technique is investigated. A recessed‐gate structure enables normally off operation required for power‐switching applications. A thin AlGaN/GaN channel and the AlGaN back‐barrier structures are fabricated on a Si substrate by metal–organic chemical vapor deposition. The 50 nm thick, thin GaN channel layer with a smooth surface is grown on the AlGaN back‐barrier layer. A recessed‐gate structure is successfully formed by SAG of an AlGaN/GaN layer on the thin AlGaN/GaN layer. The regrown AlGaN/GaN high electron mobility transistor structure shows lower sheet resistance owing to high concentration and high mobility of a two‐dimensional electron gas. Transfer characteristics of the thin AlGaN/GaN channel MOS‐HFETs show normally off operation as a consequence of using the AlGaN back‐barrier structure. Channel mobility becomes five times higher than that of GaN channel in the case of using the thin AlGaN/GaN channel. These results indicate that the regrown thin AlGaN/GaN channel MOS‐HFET has the potential to realize low on‐resistance and normally off operation.