High Electron Mobility Transistor Structures Research Articles

A simulation study of the impact of traps in the GaN substrate on the electrical characteristics of an AlGaN/GaN HEMT with a thin channel layer

Gallium nitride (GaN) substrates are promising candidates for GaN high-electron-mobility transistors (HEMTs) because of their epitaxial layer growth with low defect density. We perform device simulations to study the influence of substrate acceptor traps in GaN HEMTs on semiinsulating GaN substrates with a thin (0.02 μm) channel layer. When the trap concentration in the GaN substrate increases at a constant channel trap concentration of 1.0 × 1015 cm−3, the drain leakage current decreases but the transient response worsens. These phenomena result from an increase of the conduction-band energy due to ionized acceptor traps with negative charge. For GaN buffer structures, we obtain moderate suppression of the drain leakage current (1.7 × 10−9 A/mm) and a good transient response (with a normalized drain current of 0.86 at 1 ms for the transient response from the off- to on-state condition) at a substrate trap concentration of 5.0 × 1015 cm−3. The aluminum gallium nitride (AlGaN) back-barrier structure is highly effective for suppressing the drain leakage current at a low trap concentration of 1.0 × 1015 cm−3 in GaN substrates. Although its maximum drain current is decreased, this structure exhibits a low drain leakage current (4.7 × 10−11 A/mm) and a high normalized drain current (0.95) in the transient response. Increasing the Al content in the barriers of the GaN HEMT structure increases its maximum drain current to 1.2 A/mm, whereas the drain leakage current and transient response are well maintained. Moreover, the traps in the GaN substrate affect the low-frequency S21, which is important for the linearity of power amplifiers, and the characteristics of S21 are similar to those of the transient responses.

(Invited) AlGaN Transistors for Digital Logic Applications in High-Temperature Environments

Al-rich AlGaN HEMTs (high electron mobility transistors) are promising for extreme environment applications1. Over wide temperature ranges III-N HEMTs with ternary channels (AlGaN) exhibit more stable device behavior, compared to GaN, due to their alloy scattering-dominant transport, rather than the polar optical phonon scattering that dominates binaries (GaN). Additionally, Al(x>50%)Ga(1-x)N’s Ultra-Wide Band Gap enables superior suppression of intrinsic carrier density effects and thermionic emission-induced leakage2, making them prime candidates for high temperature electronics. While wide band gap (WBG) materials, such as SiC and GaN, have been previously investigated for high temperature device operability, the ultra-wide bandgap of Al-rich AlGaN potentially offers further advantages. We report on AlGaN 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 compositions, (b) developing a suitable gate metal scheme for use in high temperatures, and (c) over-temperature testing of discrete devices and inverters.For the carrier transport characterization over temperature, Hall mobility data were collected for four different (Al)GaN HEMT structures with different Barrier/Channel AlxGa1-xN aluminum compositions. As the temperature increased from room temperature to 500°C, the GaN-channel HEMT experienced a 6.9× reduction in mobility, whereas the AlGaN channel HEMTs experienced about a 3.1x reduction. 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. For the gate metal scheme three stacks selected for low resistivity, high melting point, low thermal expansion coefficient, and high expected barrier height were evaluated. HEMTs were fabricated on Al0.85Ga0.15N barrier / Al0.7Ga0.3N channel epitaxy and used a Ti/Al/Ni/Au Ohmic metal (annealed at 1100°C). The gate metal stacks evaluated were: (1) 20 nm Pt / 200 nm Au, (2) 200 nm Pd, and (3) 200 nm W. The impact of thermal cycling was examined through electrical characterization of samples measured before and after rapid thermal anneal (500°C, 10 minutes, 1 mTorr nitrogen atmosphere). 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 improvement 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. P. H. Carey, F. Ren, A. G. Baca, B. A. Klein, A. A. Allerman, A. M. Armstrong, E. A. Douglas, R. J. Kaplar, P. G. Kotula and S. J. Pearton, Journal of the Electron Devices Society 7, 444-452 (2019).P. G. Neudeck, R. S. Okojie and L.-Y. Chen, Proceedings of the IEEE 90 (6), 1065 - 1076 (2002). Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government. SAND2021-4920 A

The Radiation Hardness of Quantum Dot Embedded in High Electron Mobility Transistor Force-Sensitive Structure

This paper studied the radiation hardness of InAs Quantum Dot(QD) embedded in high electron mobility transistor (HEMT) for force-sensitive structure, and further verified its high irradiation properties and high sensitivity. The transfer characteristics, output characteristics, structural changes and force-sensitivity of InAs QD-HEMT and HEMT structure are analyzed after irradiated with 1900-Mev <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">181</sup> Ta ions at different fluences. The experiment results of electrical characterization indicated that the radiation hardness of InAs QD-HEMT was 3.5 times higher than that of HEMT. The mechanical sensitivity of pristine InAs QD-HEMT structure was 0.153 mA/KPa, which was higher than that of pristine HEMT structure by 0.088mA/KPa, and the force-sensitivity of InAs QD-HEMT structure was less affected by irradiation. The results indicate that InAs QD-HEMT force-sensitive structure has a strong tolerance to heavy ion irradiation. The irradiation hardens, and force-sensitive characteristics of the device can be improved by using InAs QD-HEMT structure, which can provide radiation protection for MEMS sensors applied in a harsh radiation environment.

Excimer laser liftoff of AlGaN/GaN HEMTs on thick AlN heat spreaders

We report on 193 nm excimer laser-based liftoff (LLO) of Al0.26Ga0.74N/GaN high electron mobility transistors (HEMTs) with thick (t &amp;gt; 10 μm) AlN heat spreading buffer layers grown over sapphire substrates. The use of the thick AlN heat spreading layer resulted in thermal resistance (Rth) of 16 K mm/W for as-fabricated devices on sapphire, which is lower than the value of ∼25–50 K mm/W for standard HEMT structures on sapphire without the heat-spreaders. Soldering the LLO devices onto a copper heat sink led to a further reduction of Rth to 8 K mm/W, a value comparable to published measurements on bulk SiC substrates. The reduction in Rth by LLO and bonding to copper led to significantly reduced self-heating and drain current droop. A drain current density as high as 0.9 A/mm was observed despite a marginal reduction of the carrier mobility (∼1800 to ∼1500 cm2/V s). This is the highest drain current density and mobility reported to-date for LLO AlGaN/GaN HEMTs.

Electron and hole mobilities of GaN with bulk, quantum well, and HEMT structures

Electron and hole mobilities of GaN are calculated for three-dimensional (3D, bulk) and two-dimensional [2D, quantum well (QW), and HEMT] structures, including scattering processes of acoustic deformation potential, polar optical phonon, piezoelectric, ionized impurity, and so on. The calculated mobilities for 2D structures are strongly dependent on quantum well structures and impurity densities, although the temperature dependence of the mobilities behaves in a similar way to bulk values. In the present analysis, energy band structures of GaN are calculated by the empirical pseudopotential method including spin–orbit interaction, and then the electron effective mass of the conduction band and the hole effective masses of the valence bands are evaluated, which are used for the calculations of electron and hole mobilities. The calculated valence band structure of the heavy, light, and crystal field splitted valence bands reveal complicated dispersion due to the spin–orbit interaction. The obtained electron effective mass mc=0.145m is isotropic, and the heavy hole effective mass in the c∥ plane is mhh∥=1.20m, while in the c⊥ plane, the band edge effective mass is mhh0⊥=0.55m and the over all fitted heavy hole effective mass is mhh⊥=1.20m. The light hole effective masses are mlh∥=1.35m and mlh⊥=0.165m. Both of the electron and hole mobilities are limited by ionized impurity scattering at low temperatures and by polar optical phonon scattering at high temperatures. Calculated electron mobilities are 7100 cm2/Vs for bulk, 4600 cm2/Vs for high electron mobility transistor (HEMT), and 3600 cm2/Vs for QW at room temperature and calculated hole mobilities are 450 cm2/Vs for bulk, 450 cm2/Vs for HEMT, and 500 cm2/Vs for QW at room temperature. All the expressions for scattering rates and respective mobilities are derived for 3D and 2D (QW and HEMT) structures and enable readers to calculate electron and hole mobilities in different structures with parameters given in the table or modified ones.

Investigation to Enhance the DC and RF Performances of Nitride-Based Nanoelectronic HEMTs

In this work, a structural optimisation is performed on the basis of DC and RF performances of high electron mobility transistors (HEMTs) in nano-scale regime by simulation results obtained from SILVACO-ATLAS physical simulator. The formation of quantum well is demonstrated at unbiased condition in each type of these HEMT structures. In DC analysis of each structure, the variations in drain current are studied with respect to drain voltage, gate voltage and doping concentration. Also, the variations in transconductance are studied with gate voltage and doping concentration corresponding to each structure. In RF analysis, the variations in current gain cut-off frequency and power gain cut-off frequency are studied with gate voltage and doping concentration corresponding to each structure. The DC and RF performances are observed to be the highest for modified HEMT structure with inserted InN atomic layer (0.36 nm thick) having AlGaN doping concentration of 4×1018 cm-3 among all designed structures in this work. To investigate the effect of doping concentration on DC and RF performances in nano-scale regime is one novelty in this work. Our work may be helpful in the applications related to biomedical sensors. Also, our work may be suitable for high frequency applications.

Single Event Burnout Hardening of Enhancement Mode HEMTs With Double Field Plates

Single event burnout (SEB) of enhancement mode GaN high-electron mobility transistors (HEMTs) under heavy ion irradiation is systematically studied based on simulations in this work. Susceptibility to SEB of different particle incidence environments is investigated to determine the device's sensitive position. Under these conditions, the mechanism of SEB in enhancement mode HEMTs is mainly associated with the charge congestion and subsequent continuous impact ionization. A hardened HEMT structure with the gate and drain double field plates is proposed correspondingly. Apart from better output characteristics and almost 25% improvement of breakdown voltage, the hardened structure also demonstrates better tolerance of SEB. This hardened structure can modify the electric field in the high field region so that charge accumulation is reduced significantly, thus effectively decreasing the possibility of SEB's occurrence. With a vertical heavy ion strike of 63.8 MeV ·cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /mg linear energy transfer (LET), SEB threshold voltage of the conventional structure HEMT is 160 V, while that of the hardened structure HEMT is much higher, reaching 249 V. Besides, the hardened device exhibits better SEB tolerance than the conventional device even at different higher LET conditions. Furthermore, the contribution to SEB tolerance of each field plate is also discussed.

2DEG characteristics of InAlAs/InP based HEMTs by solving Schrödinger and Poisson equations followed by device characteristics

AbstractIn this paper the 2DEG characteristics study of InAlAs/InP based high electron mobility transistor (HEMT) structures with two different channel materials (InAlAs and InGaAs) are presented through self‐consistent solutions of Schrodinger and Poisson equations which results in energy bands, sheet charge density, electric field, sheet resistance and CV charactristics. The 2DEG (two dimensional electron gas) is created at the heterointerface of InAlAs/In(Al,Ga)As due to conduction energy band discontinuity. The 2DEG is realised by the presence of subbands at the quantum well. The subbands represent the wave function of electrons. The innovation of this work is that InAlAs and InGaAs with x = 0.75 composition can be best utilised as channel material due to low bandgaps 0.821 and 0.639 eV respectively towards improved 2DEG density, subbands, channel sheet resistance, electric field in the 2DEG and CV characteristics. The DC characteristics comprised of Id ~ Vg, Id ~ Vd, transconductance, cut‐off frequency, maximum frequency of oscillation are extracted through TCAD simulations followed by experimental validation. The InGaAs/InP based HEMT can be a potential candidate for ultra‐high‐speed microwave and millimetre wave applications.

Optical Performance of Two Dimensional Electron Gas and GaN:C Buffer Layers in AlGaN/AlN/GaN Heterostructures on SiC Substrate

Terahertz time-domain spectroscopy and Fourier-transform infrared spectroscopy were developed as the method for the investigation of high-frequency characteristics of two-dimensional electron gas and GaN:C buffer layers in AlGaN/AlN/GaN heterostructures grown on a semi-insulating SiC substrate. The reflectance and transmittance spectra of the selected heterostructure layers were studied after the top layers were removed by a reactive ion etching. Results were numerically analyzed using the transfer matrix method taking into account the high-frequency electron conductivity via a Drude model and complex dielectric permittivity of each epitaxial layer via a one-phonon-resonance approximation. Good agreement between the experiment and theory was achieved revealing the temperature dependent electron effective mass in AlGaN/AlN/GaN high electron mobility transistor structures and the small damping factors of optical phonons due to high crystal quality of the epitaxial layers fabricated on the SiC substrate.

Terahertz emission spectroscopy of GaN-based heterostructures

This study investigates the band bending profile of gallium-nitride-based heterostructures with respect to the change in the direction of the induced current in the out-of-plane plane direction, which gives the phase of the terahertz emission peak and can be detected by laser THz emission microscopy. We use a wavelength-tunable laser THz emission spectroscope to observe the transport of charges within the band bending regions, excitation of nearby photocarriers, and carrier scattering. We observe flip-flop peak-to-peak THz emission waveforms for a GaN heterostructure field-effect transistor (FET) compared to a strong THz emission radiated from an Al0.3Ga0.7N/GaN high-electron-mobility-transistor (HEMT). The flip-flop THz emission indicates the emission from various interfaces inside the FET structure, and intense THz emission from the HEMT structure indicates band-edge excitation. Our results provide a valuable perspective for characterizing complex heterostructures that provide insight into possible defects, carrier mobilities, and band bending of multilayer interface electronic devices.

Transport properties of AlGaN/GaN HEMT structures with back barrier: impact of dislocation density and improved design

The influence of dislocation density on the transport properties of high electron mobility transistor (HEMT) structures is reported in this work. Experimental results, obtained on HEMT structures prepared in the same growth run on templates with different dislocation densities, are compared. However, the direct comparison of structures is complicated, since parallel parasitic currents through deeper parts of the heterostructure were observed in samples with lower dislocation density. To suppress this phenomenon, the addition of an AlGaN back barrier (BB) beneath the two dimensional electron gas (2DEG) channel is proposed and optimized and the most suitable design based on both modeling and experimental results is suggested. Subsequently, the comparison of electron mobility in 2DEG for structures with AlGaN BB and different dislocation densities was possible. We show experimentally that lowering the dislocation density considerably increases the electron mobility in 2DEG. It is shown that the dislocation density also has a significant influence on the Fermi level position in GaN. The presented results suggest that reduction of dislocation density can improve the performance of high-frequency GaN HEMT applications.

Demonstration of AlGaN/GaN-based ultraviolet phototransistor with a record high responsivity over 3.6 × 107 A/W

In this work, we demonstrate a high-performance ultraviolet phototransistor (UVPT) based on the AlGaN/GaN high-electron mobility transistor (HEMT) configuration. When the device is biased at off state, the peak photoresponsivity (R) of 3.6 × 107 A/W under 265 nm illumination and 1.0 × 106 A/W under 365 nm illumination can be obtained. Those two R values are one of the highest among the reported UVPTs at the same detection wavelength under off-state conditions. In addition, we investigate the gate-bias (VGS) dependent photoresponse of the fabricated device with the assistance of band structure analysis. It was found that a more negative VGS can significantly reduce the rise/decay time for 265 nm detection, especially under weak illumination. This can be attributed to a largely enhanced electric field in the absorptive AlGaN barrier that pushes the photo-generated carriers rapidly into the GaN channel. In contrast, the VGS has little impact on the switching time for 365 nm photodetection, since the GaN channel has a larger absorption depth and the entire UVPT simply acts as a photoconductive-type device. In short, the proposed AlGaN/GaN HEMT structure with the superior photodetection performance paves the way for the development of next generation UVPTs.

Computational Analysis of Joule Heating Effect in Triple Material Gate AlGaN/GaN High Electron Mobility Transistor

Aluminum gallium nitride (AlGaN)/gallium nitride (GaN) high electron mobility transistor (HEMT) has shown great potential and superiority over other devices in high power and high speed-based systems [1-4]. Recently, a novel AlGaN/GaN HEMT structure named triple material gate (TMG) HEMT has been proposed in [5] in which the gate contact is composed of three metals with different work functions. TMG HEMT ensures high carrier velocity and better suppression of short channel effects. In this work, a two-dimensional electro-thermal model has been developed to investigate the joule heating effect in TMG AlGaN/GaN HEMT using the software COMSOL [6]. The current continuity and heat transport equations are solved self-consistently to obtain the temperature distribution profile in various layers of the device. Thermal efficiency of TMG HEMT is compared with dual material gate and single material gate HEMT. It has been found that, besides providing better current carrying capability, TMG HEMT shows optimum thermal performance. Moreover, the variation of peak temperature in the channel with drain and gate voltages is analyzed to study the bias voltage dependence of the device temperature rise. Effects of several device parameters like total channel length, spontaneous and piezoelectric polarization induced charges and ratio of lengths of control and screen gates, on the thermal performance have also been explored. Simulations are carried out for the device lying on different substrate materials (Sapphire, GaN, Si and SiC) with varying thermal conductivity to demonstrate their performance in heat removal capacity. This simulation model can serve as a very useful tool to comprehend the thermal behavior of the device for high power and high frequency applications.

Suppressing Buffer-Induced Current Collapse in GaN HEMTs with a Source-Connected p-GaN (SCPG): A Simulation Study

Carbon doping in the buffer of AlGaN/GaN high-electron-mobility transistors (HEMTs) leads to the notorious current collapse phenomenon. In this paper, an HEMT structure with a source-connected p-GaN (SCPG) embedded in the carbon-doped semi-insulating buffer is proposed to suppress the buffer-induced current collapse effect. Two-dimensional transient simulation was carried out to show the successful suppression of buffer-induced current collapse in the SCPG-HEMTs compared with conventional HEMTs. The mechanism of suppressing dynamic on-resistance degradation by ejecting holes from the SCPG into the high resistive buffer layer after off-state stress is illustrated based on energy band diagrams. This paper contributes an innovative device structure to potentially solve the buffer-induced degradation of the dynamic on-resistance in GaN power devices.

Numerical Study of Two HEMTs, AlGaN and InGaN, by Sharing the Drain Area for Power Application

In this paper we simulate a high electron mobility transistor structure of two HEMTs, by sharing the drain, with the using Silvaco-Tcad software. Our two HEMTs AlGaN and InGaN, on H4-SiC substrate and 50 nm gates length shows a very good scalability in different applications. It demonstrates a drain current of about 991.4 mA, a maximum transconductance of 859.1 ms/mm at Vds = 3V, a cut-off frequency of 115.6 GHz, a maximum oscillation frequency of 168.8 GHz, a drain-induced barrier lowering (DIBL) = 40 mV/V, a sub-threshold swing of 130 mV/dec, an On/Off current ratio higher than 6.82 × 107 and a leakage gate current of 2 × 10−8 A.

Mercury (II) Ion Detection Using AgNWs-MoS2 Nanocomposite on GaN HEMT for IoT-Enabled Smart Water Quality Analysis

We have demonstrated a highly sensitive novel platform for real-time detection of mercury (Hg <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ) ions after successfully making silver nanowires (AgNWs)-MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanocomposite and functionalizing it over ungated AlGaN/GaN high-electron-mobility transistor (HEMT). The AlGaN/GaN HEMT structures were grown over the sapphire substrate using molecular beam epitaxy. AgNWs-MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanocomposites were optimized for the device functionalization and 1:4 ratio was found highly sensitive for Hg <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ions. The sensor exhibits high sensitivity toward Hg <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ions of 1.604 mA/ppb and calculated its Limit of Detection (LoD) up to the range of 20 parts per trillion. The observed sensitivity is highest among previously reported AlGaN/GaN fabricated HEMT-based sensors for mercury (Hg <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ) ions detection and is well below the standard permissible limits as set by World Health Organization (WHO) and Environmental Protection Agency (EPA). The enhancement in sensitivity is due to the enhanced surface to volume ratio of AgNW-MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanocomposite and the highly conductive nature of AgNWs incorporated in MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . Moreover, we also performed sensing on real water samples of tap water and lake water. Furthermore, we showed the smart sensing capability of our developed sensor by illustrating the Internet of Things (IoT)-enabled system for next-generation heavy metal ion sensing.

High Breakdown Voltage and Low-Current Dispersion in AlGaN/GaN HEMTs With High-Quality AlN Buffer Layer

We have successfully grown AlGaN/GaN high electron mobility transistor (HEMT) structure on the high-quality undoped thick AlN buffer layer with large band offset to replace the conventional high-resistivity GaN buffer layer. The AlGaN/GaN HEMT fabricated on this AlN buffer layer exhibits low OFF-state leakage current with high ${I}_{\mathrm{\scriptscriptstyle ON}}/{I}_{\mathrm{\scriptscriptstyle OFF}}$ of $\sim 10^{{6}}$ due to enhanced confinement of the electrons in the 2-D electron gas (2-DEG) channel. The undoped AlN buffer layer is responsible for suppressing the trapping effects to greatly reduce the current dispersion in pulsed ${I}_{\text {D}}$ – ${V}_{\text {D}}$ characteristics, which is hardly avoided in conventional deep acceptor-doped GaN buffer layer. The device also demonstrates high breakdown voltage of 2154 V with very high figure of merit (FOM) of ~1.8 GV $^{2-1}$ cm $^{-{2}}$ , one of the highest ever reported, suggesting that the AlGaN/GaN-based Hemts WITH AlN buffer layer are promising for high-performance RF and power applications.

Abnormal increase of 2DEG density in AlGaN/GaN HEMT grown on free-standing GaN substrate

In this study, AlGaN/GaN high-electron-mobility transistors (HEMTs) were grown on a GaN template and GaN substrate under the same growth conditions. It was observed that, in the HEMT structure grown on the GaN substrate, mobility decreased because of an increase in the two-dimensional electron-gas (2DEG) density; the origin of these redundant electrons was studied. The 2DEG density decreased with decreasing temperature, this phenomenon closely related to unintentionally induced shallow donors with ionization energy calculated to be around 67.8 meV. After Ⅴ/Ⅲ ratio regulation, the 2DEG density returned to a normal level; this combined with photoluminescence, confirmed for the first time that the abnormal increase of 2DEG density in HEMT structure grown on the GaN substrate is associated with nitrogen vacancies. Therefore, increasing the Ⅴ/Ⅲ ratio is beneficial for obtaining higher mobility by returning the 2DEG density to a normal level.

HfO2 as gate insulator on N-polar GaN–AlGaN heterostructures

To further increase the operation frequency of GaN high electron mobility transistors (HEMTs) with high power gain in the mm-wave band (30–300 GHz), channel thickness must be scaled. High-k dielectrics such as HfO2 can be inserted as gate insulator to reduce the leakage, while maintaining good gate control. In this work, we investigated HfO2 on N-polar HEMT structure by performing frequency dependent capacitance–voltage measurements on metal-oxide-semiconductor capacitors (MOSCAPs). The impact of annealing on the quality of HfO2 was studied. Moreover, we compared the CV characteristics of MOSCAPs with HfO2 deposited on the GaN channel with that deposited on the AlGaN cap layer. We showed that HfO2 deposition on AlGaN cap leads to Fermi level pinning, which prevents the channel being pinched off by applying gate voltage. On the other hand, well-behaved CV profiles were achieved by depositing HfO2 directly on GaN or SiN dielectric.

Low-sheet-resistance high-electron-mobility transistor structures with strain-controlled high-Al-composition AlGaN barrier grown by MOVPE

We demonstrated low-sheet-resistance AlGaN high electron mobility transistors (HEMTs) using a strain-controlled high-Al-composition AlGaN barrier grown by MOVPE. We systematically investigated the effects of the AlGaN-barrier’s growth conditions on the electrical characteristics. We found that growing an AlGaN barrier with nitrogen as a carrier gas increases dislocation density in the AlGaN barrier, resulting in reduction of strain in the AlGaN barrier and the top of the GaN channel. Reduction strain in the GaN channel can improve electron mobility. Moreover, dislocation and impurity scattering are prevented using an AlN spacer. Finally, we achieved low-sheet-resistance HEMT structures with a high Al composition of over 0.60. The lowest sheet resistance is 211 Ω/sq. with electron mobility of 1820 cm2 V−1 s−1 using an Al0.68Ga0.32N barrier.