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Control over the quantization of electrons in quantum wells is at the heart of the functioning of modern advanced electronics; high electron mobility transistors, semiconductor and Capasso terahertz lasers, and many others. However, this avenue has not been explored in the case of 2D materials. Here we apply this concept to van der Waals heterostructures using the thickness of exfoliated crystals to control the quantum well dimensions in few-layer semiconductor InSe. This approach realizes precise control over the energy of the subbands and their uniformity guarantees extremely high quality electronic transport in these systems. Using tunnelling and light emitting devices, we reveal the full subband structure by studying resonance features in the tunnelling current, photoabsorption and light emission spectra. In the future, these systems could enable development of elementary blocks for atomically thin infrared and THz light sources based on intersubband optical transitions in few-layer van der Waals materials. A plethora of solid-state nanodevices rely on engineering the quantization of electrons in quantum wells. Here, the authors leverage the thickness of exfoliated 2D crystals to control the quantum well dimensions in few-layer semiconductor InSe and investigate the resonance features in the tunnelling current, photoabsorption and light emission spectra.

The electron mobility influenced by optical phonons in AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors with different gate dielectrics around room temperature is investigated theoretically. The electronic states are obtained by the finite difference method in consideration of built-in electric fields and the conduction band bending. The optical phonons are analyzed using the dielectric continuum model. Based on the theory of force balance equation, the electron mobility of two-dimensional electron gas is obtained for the structures with four different gate dielectrics of Al 2 O 3 , HfO 2 , SiO 2 and Si 3 N 4 . Our results show that the electron mobility is the highest in HfO 2 systems when Al composition in AlGaN is small, whereas the mobility is the highest in Al 2 O 3 systems as Al composition increases to a certain value. The effects of the ternary mixed crystals, each layer’s size and the fixed charges on the sheet density and electron mobility are also discussed for different gate dielectric materials.

A new generation of high-efficiency power devices is being developed using wide bandgap (WBG) semiconductors, like GaN and SiC, which are emerging as attractive alternatives to silicon. The recent interest in GaN has been piqued by its excellent material characteristics, including its high critical electric field, high saturation velocity, high electron mobility, and outstanding thermal stability. Therefore, the superior performance is represented by GaN-based high electron mobility transistor (HEMT) devices. They can perform at higher currents, voltages, temperatures, and frequencies, making them suitable devices for the next generation of high-efficiency power converter applications, including electric vehicles, phone chargers, renewable energy, and data centers. Thus, this review article will provide a basic overview of the various technological and scientific elements of the current GaN HEMTs technology. First, the present advancements in the GaN market and its primary application areas are briefly summarized. After that, the GaN is compared with other devices, and the GaN HEMT device’s operational material properties with different heterostructures are discussed. Then, the normally-off GaN HEMT technology with their different types are considered, especially on the recessed gate metal insulator semiconductor high electron mobility transistor (MISHEMT) and p-GaN. Hereafter, this review also discusses the reliability concerns of the GaN HEMT which are caused by trap effects like a drain, gate lag, and current collapse with numerous types of degradation. Eventually, the breakdown voltage of the GaN HEMT with some challenges has been studied.

GaN based high electron mobility transistors (HEMTs) have demonstrated extraordinary features in the applications of high power and high frequency devices. In this paper, we review recent progress in AlGaN/GaN HEMTs, including the following sections. First, challenges in device fabrication and optimizations will be discussed. Then, the latest progress in device fabrication technologies will be presented. Finally, some promising device structures from simulation studies will be discussed.

In this work, AlGaN double channel heterostructure is proposed and grown by metal organic chemical vapor deposition (MOCVD), and high-performance AlGaN double channel high electron mobility transistors (HEMTs) are fabricated and investigated. The implementation of double channel feature effectively improves the transport properties of AlGaN channel heterostructures. On one hand, the total two dimensional electron gas (2DEG) density is promoted due to the double potential wells along the vertical direction and the enhanced carrier confinement. On the other hand, the average 2DEG density in each channel is reduced, and the mobility is elevated resulted from the suppression of carrier-carrier scattering effect. As a result, the maximum drain current density (Imax) of AlGaN double channel HEMTs reaches 473 mA/mm with gate voltage of 0 V. Moreover, the superior breakdown performance of the AlGaN double channel HEMTs is also demonstrated. These results not only show the great application potential of AlGaN double channel HEMTs in microwave power electronics but also develop a new thinking for the studies of group III nitride-based electronic devices.

GaN HEMT has attracted a lot of attention in recent years owing to its wide applications from the high-frequency power amplifier to the high voltage devices used in power electronic systems. Development of GaN HEMT on Si-based substrate is currently the main focus of the industry to reduce the cost as well as to integrate GaN with Si-based components. However, the direct growth of GaN on Si has the challenge of high defect density that compromises the performance, reliability, and yield. Defects are typically nucleated at the GaN/Si heterointerface due to both lattice and thermal mismatches between GaN and Si. In this article, we will review the current status of GaN on Si in terms of epitaxy and device performances in high frequency and high-power applications. Recently, different substrate structures including silicon-on-insulator (SOI) and engineered poly-AlN (QST®) are introduced to enhance the epitaxy quality by reducing the mismatches. We will discuss the development and potential benefit of these novel substrates. Moreover, SOI may provide a path to enable the integration of GaN with Si CMOS. Finally, the recent development of 3D hetero-integration technology to combine GaN technology and CMOS is also illustrated.

In this paper, we propose an enhancement-mode (E-mode)-operated Al0.15Ga0.85N/GaN/Al0.07Ga0.93N metal–insulator–semiconductor high-electron-mobility transistor (MIS-HEMT) device structure by varying the gate angles, θ from 40° to 110°. Initially, a recessed field-plated T-gate Al0.15Ga0.85N/GaN/Al0.07Ga0.93N MIS-HEMT device structure above the GaN substrate with a gate angle of 90° simulated in this work, operating in E-mode with Vth of 5.82 V, achieves enhanced drain current Ids of 0.39 A/mm and optimized VBD of 1038 V, with ft and fmax of 44.25 GHz and 70.7 GHz. Decreasing the gate angle from 90° to 40° reduces the gate surface area, enhancing the electric field strength and electron confinement in the two-dimensional electron gas (2DEG) channel, improving carrier mobility, and providing stronger gate control with lower gate capacitance. This results in an increase in Ids of 0.437 A/mm, Vth of 5.73 V, reduction in RON of 9.54 Ω mm, and VBD of 867 V, with enhancement in ft and fmax of 60.04 GHz and 86.02 GHz at θ = 40°. Increasing the gate angle from 90° to 110° increases the gate surface area, broadens the electric field, reduces electron confinement and gate control, and decreases the ability to modulate channel charge density while allowing the material to withstand a higher voltage before breakdown. This results in a decrease in Ids of 0.328 A/mm, Vth of 5.97 V, higher RON of 15.07 Ω mm, and optimized VBD of 1210 V, with ft and fmax of 25.52 GHz and 57.37 GHz when θ = 110°. The increase in VBD at θ = 110° with the optimized Ids, ft, and fmax values makes it suitable for high-voltage switching applications, including power electronic converters and low-noise amplifiers. The increase in ft and fmax at θ = 40° with the optimized Ids and VBD values makes it suitable for radio frequency (RF) transmitters and receivers, as well as amplifiers that operate at microwave and millimeter-wave frequencies. Finally, the switching performance of the proposed device structures was analyzed using an ultralow-loss boost converter circuit, and the results indicate that the proposed device structure with θ = 110° is a suitable candidate for use in high-power/low-loss switching applications.

Because of the global trends of energy demand increase and decarbonization, developing green energy sources and increasing energy conversion efficiency are recently two of the most urgent topics in energy fields. The requirements for power level and performance of converter systems are continuously growing for the fast development of modern technologies such as the Internet of things (IoT) and Industry 4.0. In this regard, power switching devices based on wide-bandgap (WBG) materials such as silicon carbide (SiC) and gallium nitride (GaN) are fast maturing and expected to greatly benefit power converters with complex switching schemes. In low- and medium-voltage applications, GaN-based high-electron-mobility transistors (HEMTs) are superior to conventional silicon (Si)-based devices in terms of switching frequency, power rating, thermal capability, and efficiency, which are crucial factors to enhance the performance of advanced power converters. Previously published review papers on GaN HEMT technology mainly focused on fabrication, device characteristics, and general applications. To realize the future development trend and potential of applying GaN technology in various converter designs, this paper reviews a total of 162 research papers focusing on GaN HEMT applications in mid- to high-power (over 500 W) converters. Different types of converters including direct current (DC)–DC, alternating current (AC)–DC, and DC–AC conversions with various configurations, switching frequencies, power densities, and system efficiencies are reviewed.

A physics-based analytical model for GaN high-electron-mobility transistors (HEMTs) with non-recessed- and recessed-gate structure is presented. Based on this model, the two-dimensional electron gas density (2DEG) and thereby the on-state resistance and breakdown voltage can be controlled by varying the barrier layer thickness and Al mole fraction in non-recessed depletion-mode GaN HEMTs. The analytical model indicates that the 2DEG charge density in the channel increases from 2.4 × 10 12  cm −2 to 1.8 × 10 13  cm −2 when increasing the Al mole fraction from x  = 0.1 to 0.4 for an experimental non-recessed-gate GaN HEMT. In the recessed-gate GaN HEMT, in addition to these parameters, the recess height can also control the 2DEG to achieve high-performance power electronic devices. The model also calculates the critical recess height for which a normally-ON GaN switch becomes normally-OFF. This model shows good agreement with reported experimental results and promises to become a useful tool for advanced design of GaN HEMTS.

AlGaN/GaN HEMTs were fabricated using two special gate metals, Hf and W, as gate Schottky contact materials. Based on the measured I-V data and two-dimensional scattering theory, the electron mobility corresponding to polarization Coulomb field (PCF) scattering and other scattering mechanisms was calculated. The additional polarization charge (APC) due to the inverse piezoelectric effect (IPE) was also calculated. The differences in the effects of two special gate metals, Hf and W, on the PCF scattering intensity are analyzed from the aspects of Young’s modulus and work function of the metal. It was found that compared with W metal, Hf metal has a smaller Young’s modulus and weaker ability to resist elastic deformation, which makes the AlGaN barrier layer in contact with Hf more easily deformed due to IPE, resulting in more APC under the Hf metal Schottky contact. On the other hand, Hf has a smaller work function compared to W metal, so the 2DEG density is higher. The influence of special gate metals Hf and W on the PCF scattering intensity is the result of the combined effect of these two factors. This study is of great significance for understanding the electron transport mechanisms of AlGaN/GaN HEMTs with special gate metals such as Hf and W, and for further improving the electrical performance and stability of the devices.