AlGaN Channel Research Articles

Understanding and Quantifying the Benefit of Graded Aluminum Gallium Nitride Channel High-Electron Mobility Transistors

Graded AlGaN channel High-Electron Mobility Transistor (HEMT) technology is emerging as a strong candidate for millimeter-wave applications, as superior efficiency and linearity performances can be achieved. In this paper, graded channel AlGaN/GaN HEMTs are investigated with the aim of further understanding the benefit of the graded AlGaN channel compared to more conventional GaN channel HEMTs. Our study employed a comprehensive simulation workflow including an extensive calibration of direct current (DC), S-parameter, large signal, and linearity characteristics at 30 GHz. Through device modeling and implementation of circuit-level simulation using Advanced Design System (ADS, 2023) software, both linearity and large signal performances could be mimicked remarkably. In agreement with previous studies, the results show that graded channel technology allows for a modified electron confinement leading to a 3D electron gas (3DEG). Consequently, the electric field peak inside of the channel is reduced without degrading the radio frequency (RF) performance, as the electron velocity is improved, thus offering a more linear transconductance and better linearity performances. As a result, for graded AlGaN channel HEMTs, a 6 dB output power back-off from peak power-added efficiency (PAE) is needed to achieve a carrier with a third-order intermodulation (C/IM3) ratio of 30 dBc against 9 dB for conventional AlGaN/GaN HEMTs with a lower associated PAE.

Hetero dielectric based dual material gate AlGaN channel MISHEMT for enhanced electrical characteristics

Herein, we report an AlGaN channel metal insulator semiconductor high electron mobility transistor (MISHEMT) in dual material gate (DMG) architecture with two different dielectrics as gate insulator for enhancing the DC performance of the device. The use of a high-k dielectric material as gate insulator below gate metal 1 and a low-k dielectric below gate metal 2 results in significant threshold voltage variation along the channel. This contributes to improved average carrier velocity which in turn results in better current drive. The proposed device exhibits a peak current of 162 mA/mm at zero gate bias, whereas, the DMG and single material gate (SMG) AlGaN channel HEMTs offer currents of 137 mA/mm and 125 mA/mm respectively. The peak transconductances are about 24.66 mS/mm, 23.68 mS/mm and 22.71 mS/mm respectively for the proposed device, DMG and SMG HEMTs. The proposed device reduces the OFF current by an order of 106 compared to that of DMG HEMT.

Analysis of the high-temperature transport property in GaN-based single and double heterostructures

GaN-based heterostructures have been proved to be excellent candidates for high-voltage, large-power, high-frequency and high-temperature application fields, thanks to their superior material properties. To further improve the carriers’ confinement, using InGaN channel layer or introducing AlGaN back barrier layer is promoted. These structures are also proved to be favorable for the robust electron mobility at high temperature. However, the reason solely rests on the longitudinal optical (LO) phonon scattering and the behind mechanism is still absent. This work compares the temperature-dependent electronic transport properties in three groups of GaN-based heterostructures, including GaN-based single heterostructure (SH) with/without AlN interlayer, GaN-based SH and double heterostructure (DH), and GaN-based DH with AlGaN or GaN channel layer. The advantageous high-temperature mobility is attributed to the enhanced LO phonon energy in heterostructures. By self-consistent calculations of the Schrödinger–Poisson equations, a close relation between the narrow electron distribution in the quantum well with the high LO phonon energy, and consequently superior mobility at high temperature is implied. The results provide a simple strategy for the optimized design of the GaN-based heterostructures outperforming at high temperature.

High-Temperature Characterization of AlGaN Channel High Electron Mobility Transistor Based on Silicon Substrate

In this paper, it is demonstrated that the AlGaN high electron mobility transistor (HEMT) based on silicon wafer exhibits excellent high-temperature performance. First, the output characteristics show that the ratio of on-resistance (RON) only reaches 1.55 when the working temperature increases from 25 °C to 150 °C. This increase in RON is caused by a reduction in optical phonon scattering-limited mobility (μOP) in the AlGaN material. Moreover, the device also displays great high-performance stability in that the variation of the threshold voltage (ΔVTH) is only 0.1 V, and the off-state leakage current (ID,off-state) is simply increased from 2.87 × 10−5 to 1.85 × 10−4 mA/mm, under the operating temperature variation from 25 °C to 200 °C. It is found that the two trap states are induced at high temperatures, and the trap state densities (DT) of 4.09 × 1012~5.95 × 1012 and 7.58 × 1012~1.53 × 1013 cm−2 eV−1 are located at ET in a range of 0.46~0.48 eV and 0.57~0.61 eV, respectively, which lead to the slight performance degeneration of AlGaN HEMT. Therefore, this work provides experimental and theoretical evidence of AlGaN HEMT for high-temperature applications, pushing the development of ultra-wide gap semiconductors greatly.

Ultra-wide Bandgap AlGaN Channel HEMTs for Portable Power Electronics Applications

AlGaN channel (Eg>3.4 eV) is the most effective method for enhancing the breakdown field of the group III-nitride based HEMTs. This work demonstrates the potential of AlGaN double channel HEMTs on Silicon carbide substrate. The device DC characteristics are investigated using numerical simulator by using drift-diffusion transport model. The AlGaN double channel HEMTs enhances the total 2DEG density due to double potential well and shows better current driving capability (IDS) of 0.714 A/mm, transconductance (gm) of 116 mS/mm, and low specific ON-resistance (Ron) of 3.262 Ω.mm. The AlGaN double channel HEMT on Silicon carbide substrate exhibited 680 V blocking voltage (VBR) and gate field plate HEMT shows 532 V. The effective reduction in electric field at the gate edge is the major source for elevated breakdown voltage in field plate HEMTs. The superior DC characteristics indicates the proposed wide bandgap channel HEMT is suitable device for future portable power converters.

Dual-gate AlGaN channel HEMTs: advancements in E-mode performance with N-shaped graded composite barriers

Abstract This work evaluates the performance of dual gate AlGaN channel HEMTs on SiC substrate. The study analyzes two HEMT structures that differ only in barrier design, one design consists of fixed composite barriers (FCB-HEMT) i.e. there is fixed Aluminum composition x = 0.48 in the first Al x Ga1−x N barrier and x = 0.42 in the second Al x Ga1−xN barrier while the other design comprises N-shaped graded composite barriers (NGCB-HEMT) i.e. the Aluminum content varies gradually from 0.4 to 0.48 in the first AlGaN barrier and from 0.34 to 0.42 in the second AlGaN barrier. The paper concentrates on energy band diagrams, electron concentration profile, electric field distribution, drain and transfer characteristics, and the effects of high temperature on drain characteristics and mobility of the NGCB-HEMT. It has been reported that at zero gate bias, the FCB-HEMT has a drain current density of 0.137 A mm−1 while it decreases to 0.0058 A mm−1 in the case of NGCB-HEMT, thus presenting a novel approach towards enhancement-mode AlGaN HEMTs. Hence, grading can be optimized in the composite barriers to achieve enhancement mode operation of AlGaN channel HEMTs. Furthermore, the study reveals that the critical electric field of FCB-HEMT is 6.9975 M V cm−1, while that of NGCB-HEMT is 5.3124 M V cm−1, demonstrating their usefulness in electronic devices that operate at high voltages and harsh temperatures. Moreover, at higher temperatures, the phenomenon of optical phonon scattering leads to decreased mobilities, which in turn causes low drain currents relative to the drain voltage.

Molecular beam epitaxial growth and physical properties of AlN/GaN superlattices with an average 50% Al composition

Abstract We report molecular beam epitaxial growth and electrical and ultraviolet light emitting properties of (AlN)m/(GaN)n superlattices (SLs), where m and n represent the numbers of monolayers. Clear satellite peaks observed in XRD 2θ-ω scans and TEM images evidence the formation of clear periodicity and atomically sharp interfaces. For (AlN)m/(GaN)n SLs with an average Al composition of 50%, we have obtained an electron density up to 4.48×1019 cm-3 and a resistivity of 0.002 Ω·cm, and a hole density of 1.83×1018 cm-3 with a resistivity of 3.722 Ω·cm, both at room temperature. Furthermore, the (AlN)m/(GaN)n SLs exhibit a blue shift for their photoluminescence peaks, from 403 nm to 318 nm as GaN reduced from n=11 to n=4 MLs, reaching the challenging UVB wavelength range. The results demonstrate that the (AlN)m/(GaN)n SLs have the potential to enhance the conductivity and avoid the usual random alloy scattering of the high-Al-composition ternary AlGaN, making them promising functional components in both UVB emitter and AlGaN channel high electron mobility transistor applications.

Electronic Properties of Group-III Nitride Semiconductors and Device Structures Probed by THz Optical Hall Effect.

Group-III nitrides have transformed solid-state lighting and are strategically positioned to revolutionize high-power and high-frequency electronics. To drive this development forward, a deep understanding of fundamental material properties, such as charge carrier behavior, is essential and can also unveil new and unforeseen applications. This underscores the necessity for novel characterization tools to study group-III nitride materials and devices. The optical Hall effect (OHE) emerges as a contactless method for exploring the transport and electronic properties of semiconductor materials, simultaneously offering insights into their dielectric function. This non-destructive technique employs spectroscopic ellipsometry at long wavelengths in the presence of a magnetic field and provides quantitative information on the charge carrier density, sign, mobility, and effective mass of individual layers in multilayer structures and bulk materials. In this paper, we explore the use of terahertz (THz) OHE to study the charge carrier properties in group-III nitride heterostructures and bulk material. Examples include graded AlGaN channel high-electron-mobility transistor (HEMT) structures for high-linearity devices, highlighting the different grading profiles and their impact on the two-dimensional electron gas (2DEG) properties. Next, we demonstrate the sensitivity of the THz OHE to distinguish the 2DEG anisotropic mobility parameters in N-polar GaN/AlGaN HEMTs and show that this anisotropy is induced by the step-like surface morphology. Finally, we present the temperature-dependent results on the charge carrier properties of 2DEG and bulk electrons in GaN with a focus on the effective mass parameter and review the effective mass parameters reported in the literature. These studies showcase the capabilities of the THz OHE for advancing the understanding and development of group-III materials and devices.

GaN-based E-mode p-FETs with polarization-doped p-type graded AlGaN channels

Herein, a novel enhancement-mode (E-mode) GaN-based p-channel FETs (p-FETs) with a linearly graded AlGaN (LGA) p-channel is proposed and numerically studied by Silvaco TCAD. Thanks to strong polarization-induced doping, three-dimensional hole gas (3DHG) can be uniformly generated in LGA to form a continuous p-channel with a hole concentration over 1018 cm–3. Combined with an optimized recessed gate structure, the LGA p-FET can simultaneously achieve a large threshold voltage (|V TH|) > 2 V and a high current density (|J DS|) of ∼10 mA mm−1 at V DS = −10 V. Additionally, two critical parameters of the LGA p-FETs, i.e. the depth of recessed gate and initial Al composition of LGA, are specifically studied to reveal the unique carrier behavior of 3DHG in the devices. Importantly, the LGA structure is further optimized and implemented as the p-type cap layer to construct an E-mode GaN n-FET. Thereby, based on the same LGA configuration, a GaN-based inverter with the matched complementary n- and p-FETs is monolithically constructed, showing sharp voltage transition. The reported novel LGA structure and its availability in both GaN-based E-mode n- and p-FETs provides valuable insights and guidance to construct highly efficient GaN p-type devices and All-GaN-based integrated circuits for compact power electronic systems.

Al‐Rich AlGaN Channel High Electron Mobility Transistors on Silicon: A Relevant Approach for High Temperature Stability of Electron Mobility

AbstractUltrawide bandgap (UWBG) semiconductors offer new possibilities to develop power electronics. High voltage operation for the off‐state as well as high temperature stability of the devices in on‐state are required. More than AlGaN/GaN heterostructures, AlGaN/AlGaN heterostructures are promising candidates to meet these criteria. Furthermore, the possibility to choose the Al molar fraction of AlGaN paves the way to more tunable heterostructures. In this study, the electronic transport properties of AlGaN channel heterostructures grown on silicon substrates with various aluminum contents, focusing on the temperature dependence of the electron mobility, is investigated. Experimental results from Hall effect measurements are confronted with carrier scattering models and deep level transient spectroscopy analysis to quantify limiting effects. These results demonstrated the significant potential of Al‐rich AlGaN channel heterostructures grown on silicon substrates for high power and high temperature applications.

Achieving semi-metallic conduction in Al-rich AlGaN: Evidence of Mott transition

The development of high performance wide-bandgap AlGaN channel transistors with high current densities and reduced Ohmic losses necessitates extremely highly doped, high Al content AlGaN epilayers for regrown source/drain contact regions. In this work, we demonstrate the achievement of semi-metallic conductivity in silicon (Si) doped N-polar Al0.6Ga0.4N grown on C-face 4H-SiC substrates by molecular beam epitaxy. Under optimized conditions, the AlGaN epilayer shows smooth surface morphology and a narrow photoluminescence spectral linewidth, without the presence of any secondary peaks. A favorable growth window is identified wherein the free electron concentration reaches as high as ∼1.8 × 1020 cm−3 as obtained from Hall measurements, with a high mobility of 34 cm2/V·s, leading to a room temperature resistivity of only 1 mΩ·cm. Temperature-dependent Hall measurements show that the electron concentration, mobility, and sheet resistance do not depend on temperature, clearly indicating dopant Mott transition to a semi-metallic state, wherein the activation energy (Ea) falls to 0 meV at this high value of Si doping for the AlGaN films. This achievement of semi-metallic conductivity in Si doped N-polar high Al content AlGaN is instrumental for advancing ultrawide bandgap electronic and optoelectronic devices.

Impact of Graded AlGaN Buffer Layer Thickness on the DC, RF, Linearity, Intermodulation and Off-State Breakdown Characteristics of AlGaN Channel HEMT

This study examines the extraction of small-signal equivalent circuit parameters, linearity and intermodulation metrics, and capacitance–voltage characteristics for both AlGaN and GaN channel HEMTs. Compared to GaN channel HEMTs, AlGaN channel HEMTs have better linearity and intermodulation distortion. In the instance of AlGaN channel HEMTs, the device parasitics are decreased. Additionally, the effect of graded AlGaN buffer layer thickness on breakdown characteristics and linearity is examined. For breakdown characteristics, the 200 nm-graded AlGaN buffer layer thickness yields superior results with VBOff 157 V for a thickness of 200 nm (147 V in the case of 500 nm). At 200 nm thickness, the linearity and intermodulation distortion FOMs are also reasonably good, and the remaining device properties are essentially unchanged. Hence, the graded AlGaN buffer thickness can be reduced for high-voltage applications. For higher-voltage applications, the graded AlGaN buffer thickness can, therefore, be decreased.

AlGaN High Electron Mobility Transistor for High Temperature Logic

We report on AlGaN HEMT-based logic development, using combined enhancement- and depletion-mode transistors to fabricate inverters with operation from room temperature up to 500°C. Our development approach included: (a) characterizing temperature dependent carrier transport for different AlGaN HEMT heterostructures, (b) developing a suitable gate metal scheme for use in high temperatures, and (c) over-temperature testing of discrete devices and inverters. Hall mobility data revealed the GaN-channel HEMT experienced a 6.9× reduction in mobility, whereas the AlGaN channel HEMTs experienced about a 3.1x reduction. Furthermore, a greater aluminum contrast between the barrier and channel enabled higher carrier densities in the two-dimensional electron gas for all temperatures. The combination of reduced variation in mobility with temperature and high sheet carrier concentration showed that an Al-rich AlGaN-channel HEMT with a high barrier-to-channel aluminum contrast is the best option for an extreme temperature HEMT design. Three gate metal stacks were selected for low resistivity, high melting point, low thermal expansion coefficient, and high expected barrier height. The impact of thermal cycling was examined through electrical characterization of samples measured before and after rapid thermal anneal. The 200 nm tungsten gate metallization was the top performer with minimal reduction in drain current, a slightly positive threshold voltage shift, and about an order of magnitude advantage over the other gates in on-to-off current ratio. After incorporating the tungsten gate metal stack in device fabrication, characterization of transistors and inverters from room temperature up to 500°C was performed. The enhancement-mode (e-mode) devices’ resistance started increasing at about 200°C, resulting in drain current degradation. This phenomenon was not observed in depletion-mode (d-mode) devices but highlights a challenge for inverters in an e-mode driver and d-mode load configuration.

Research Progress in Breakdown Enhancement for GaN-Based High-Electron-Mobility Transistors

The breakdown characteristics are very important for GaN high-electron-mobility transistors (HEMTs), which affect the application voltage, power density, efficiency, etc. In order to further enhance the breakdown voltage of the device, it is necessary to carry out research on the breakdown mechanisms of the device. This article summarizes several breakdown mechanisms of GaN devices, including electric field concentration, buffer leakage current, gate leakage current, and vertical breakdown. In order to suppress the breakdown mechanisms, techniques such as the use of a field plate, reduced surface field (RESURF), back barrier, gate dielectric, substrate removal, and addition of AlGaN channels can be developed. With the continuous development of various technologies, the breakdown characteristics of GaN devices can be fully explored, laying the foundation for improving the performance of power electronic systems.

Linearity improvement in graded channel AlGaN/GaN HEMTs for high-speed applications

AlGaN HEMTs are popular devices for high-frequency applications. At higher frequencies, nonlinearity effects are major concerns and need serious attention. In this work, we have attempted to improve the linearity performance of graded channel Al x Ga1−x N/GaN HEMTs in comparison with abrupt channel GaN HEMTs through Technology Computer Aided (TCAD) simulation. In graded channel HEMTs, linearly grading of Al composition in the AlGaN layer, reduces the local carrier densities and provides improvement in the carrier saturation velocity. This provides an avenue to improve the overall linearity performance. A complete set of figures of merits (FOMs) such as and for both devices are presented. The simulation results have been calibrated with reported experimental results available in the literature. The graded devices are observed to have better transconductance (g m ) and drain current for gate voltages greater than −2 V. The drop in g m is reduced by nearly 50% at the higher gate-to-source voltages. Moreover, the RF figure of merits such as the current gain cutoff frequency (f T) and a maximum frequency of oscillation (f max) have been calculated for both devices. Due to the graded channel f T increases by 20% and f max becomes twice which makes it a better choice for high-voltage and high-power applications. The impact of interface trap and leakage current performance on the graded channel device are also reported. The cutoff frequency reduces by 25% with an increase in trap charge concentration from 9e17 cm−2 to 1e19 cm−2. Also, it has been found that the graded channel device shows better linearity and lower intermodulation distortion in comparison to the abrupt device which has been calculated from the linearity parameters. The high gate leakage current in the case of the graded channel device can be reduced by adding a thin GaN cap layer on the AlGaN channel which can be a future prospect of improving the performance of the graded GaN device.

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.

Si‐Doping Effects in AlGaN Channel Layer on Performance of N‐Polar AlGaN/AlN FETs

The nitrogen‐polar (N‐polar) AlGaN/AlN structure is expected to have higher carrier density than conventional metal‐polar AlGaN/GaN electronic devices, and the AlN substrate offers various advantages, such as high breakdown voltage and high‐temperature operation. Herein, a N‐polar AlGaN/AlN‐heterostructured field‐effect transistor (FET) with static FET characteristics is successfully fabricated. However, the drain current density, IDS, remains significantly small. This study aims to improve IDS by doping Si in the topmost AlGaN channel layer under various conditions.

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.