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3 pages, 179 KiB

Open AccessEditorial

Editorial for the Special Issue on Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II

by Giovanni Verzellesi

Micromachines 2022, 13(3), 403; https://doi.org/10.3390/mi13030403 - 1 Mar 2022

Viewed by 1771

Abstract

Wide bandgap (WBG) semiconductors are becoming a key enabling technology for several strategic fields of human activities [...] Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

Research

Jump to: Editorial, Review

9 pages, 2327 KiB

Open AccessArticle

Improvement of Crystal Quality of AlN Films with Different Polarities by Annealing at High Temperature

by Yang Yue, Maosong Sun, Jie Chen, Xuejun Yan, Zhuokun He, Jicai Zhang and Wenhong Sun

Micromachines 2022, 13(1), 129; https://doi.org/10.3390/mi13010129 - 14 Jan 2022

Cited by 11 | Viewed by 2450

Abstract

High-quality AlN film is a key factor affecting the performance of deep-ultraviolet optoelectronic devices. In this work, high-temperature annealing technology in a nitrogen atmosphere was used to improve the quality of AlN films with different polarities grown by magnetron sputtering. After annealing at [...] Read more.

High-quality AlN film is a key factor affecting the performance of deep-ultraviolet optoelectronic devices. In this work, high-temperature annealing technology in a nitrogen atmosphere was used to improve the quality of AlN films with different polarities grown by magnetron sputtering. After annealing at 1400–1650 °C, the crystal quality of the AlN films was improved. However, there was a gap between the quality of non-polar and polar films. In addition, compared with the semi-polar film, the quality of the non-polar film was more easily improved by annealing. The anisotropy of both the semi-polar and non-polar films decreased with increasing annealing temperature. The results of Raman spectroscopy, scanning electron microscopy and X-ray photoelectron spectroscopy revealed that the annihilation of impurities and grain boundaries during the annealing process were responsible for the improvement of crystal quality and the differences between the films with different polarities. Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

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16 pages, 43467 KiB

Open AccessArticle

Design and Implementation of a Flexible Photovoltaic Emulator Using a GaN-Based Synchronous Buck Converter

by Chao-Tsung Ma, Zhen-Yu Tsai, Hung-Hsien Ku and Chin-Lung Hsieh

Micromachines 2021, 12(12), 1587; https://doi.org/10.3390/mi12121587 - 20 Dec 2021

Cited by 4 | Viewed by 2661

Abstract

In order to efficiently facilitate various research works related to power converter design and testing for solar photovoltaic (PV) generation systems, it is a great merit to use advanced power-converter-based and digitally controlled PV emulators in place of actual PV modules to reduce [...] Read more.

In order to efficiently facilitate various research works related to power converter design and testing for solar photovoltaic (PV) generation systems, it is a great merit to use advanced power-converter-based and digitally controlled PV emulators in place of actual PV modules to reduce the space, cost, and time to obtain the required scenarios of solar irradiances for various functional tests. This paper presents a flexible PV emulator based on gallium nitride (GaN), a wide-bandgap (WBG) semiconductor, and a based synchronous buck converter and controlled with a digital signal processor (DSP). With the help of GaN-based switching devices, the proposed emulator can accurately mimic the dynamic voltage-current characteristics of any PV module under normal irradiance and partial shading conditions. With the proposed PV emulator, it is possible to closely emulate any PV module characteristic both theoretically, based on manufacturer’s datasheets, and experimentally, based on measured data from practical PV modules. A curve fitting algorithm is used to handle the real-time generation of control signals for the digital controller. Both simulation with computer software and implementation on 1 kW GaN-based experimental hardware using Texas Instruments DSP as the controller have been carried out. Results show that the proposed emulator achieves efficiency as high as 99.05% and exhibits multifaceted application features in tracking various PV voltage and current parameters, demonstrating the feasibility and excellent performance of the proposed PV emulator. Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

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10 pages, 3611 KiB

Open AccessArticle

Dual Laser Beam Asynchronous Dicing of 4H-SiC Wafer

by Zhe Zhang, Zhidong Wen, Haiyan Shi, Qi Song, Ziye Xu, Man Li, Yu Hou and Zichen Zhang

Micromachines 2021, 12(11), 1331; https://doi.org/10.3390/mi12111331 - 29 Oct 2021

Cited by 23 | Viewed by 5174

Abstract

SiC wafers, due to their hardness and brittleness, suffer from a low feed rate and a high failure rate during the dicing process. In this study, a novel dual laser beam asynchronous dicing method (DBAD) is proposed to improve the cutting quality of [...] Read more.

SiC wafers, due to their hardness and brittleness, suffer from a low feed rate and a high failure rate during the dicing process. In this study, a novel dual laser beam asynchronous dicing method (DBAD) is proposed to improve the cutting quality of SiC wafers, where a pulsed laser is firstly used to introduce several layers of micro-cracks inside the wafer, along the designed dicing line, then a continuous wave (CW) laser is used to generate thermal stress around cracks, and, finally, the wafer is separated. A finite-element (FE) model was applied to analyze the behavior of CW laser heating and the evolution of the thermal stress field. Through experiments, SiC samples, with a thickness of 200 μm, were cut and analyzed, and the effect of the changing of continuous laser power on the DBAD system was also studied. According to the simulation and experiment results, the effectiveness of the DBAD method is certified. There is no more edge breakage because of the absence of the mechanical breaking process compared with traditional stealth dicing. The novel method can be adapted to the cutting of hard-brittle materials. Specifically for materials thinner than 200 μm, the breaking process in the traditional SiC dicing process can be omitted. It is indicated that the dual laser beam asynchronous dicing method has a great engineering potential for future SiC wafer dicing applications. Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

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9 pages, 28283 KiB

Open AccessArticle

Crystal Growth of Cubic and Hexagonal GaN Bulk Alloys and Their Thermal-Vacuum-Evaporated Nano-Thin Films

by Marwa Fathy, Sara Gad, Badawi Anis and Abd El-Hady B. Kashyout

Micromachines 2021, 12(10), 1240; https://doi.org/10.3390/mi12101240 - 13 Oct 2021

Cited by 5 | Viewed by 2706

Abstract

In this study, we investigate a novel simple methodology to synthesize gallium nitride nanoparticles (GaN) that could be used as an active layer in light-emitting diode (LED) devices by combining the crystal growth technique with thermal vacuum evaporation. The characterizations of structural and [...] Read more.

In this study, we investigate a novel simple methodology to synthesize gallium nitride nanoparticles (GaN) that could be used as an active layer in light-emitting diode (LED) devices by combining the crystal growth technique with thermal vacuum evaporation. The characterizations of structural and optical properties are carried out with different techniques to investigate the main featured properties of GaN bulk alloys and their thin films. Field emission scanning electron microscopy (FESEM) delivered images in bulk structures that show micro rods with an average diameter of 0.98 µm, while their thin films show regular microspheres with diameter ranging from 0.13 µm to 0.22 µm. X-ray diffraction (XRD) of the bulk crystals reveals a combination of 20% hexagonal and 80% cubic structure, and in thin films, it shows the orientation of the hexagonal phase. For HRTEM, these microspheres are composed of nanoparticles of GaN with diameter of 8–10 nm. For the optical behavior, a band gap of about from 2.33 to 3.1 eV is observed in both cases as alloy and thin film, respectively. This article highlights the fabrication of the major cubic structure of GaN bulk alloy with its thin films of high electron lifetime. Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

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8 pages, 3736 KiB

Open AccessArticle

Effect of High-Temperature Nitridation and Buffer Layer on Semi-Polar (10–13) AlN Grown on Sapphire by HVPE

by Qian Zhang, Xu Li, Jianyun Zhao, Zhifei Sun, Yong Lu, Ting Liu and Jicai Zhang

Micromachines 2021, 12(10), 1153; https://doi.org/10.3390/mi12101153 - 25 Sep 2021

Cited by 6 | Viewed by 1799

Abstract

We have investigated the effect of high-temperature nitridation and buffer layer on the semi-polar aluminum nitride (AlN) films grown on sapphire by hydride vapor phase epitaxy (HVPE). It is found the high-temperature nitridation and buffer layer at 1300 °C are favorable for the [...] Read more.

We have investigated the effect of high-temperature nitridation and buffer layer on the semi-polar aluminum nitride (AlN) films grown on sapphire by hydride vapor phase epitaxy (HVPE). It is found the high-temperature nitridation and buffer layer at 1300 °C are favorable for the formation of single (10–13) AlN film. Furthermore, the compressive stress of the (10–13) single-oriented AlN film is smaller than polycrystalline samples which have the low-temperature nitridation layer and buffer layer. On the one hand, the improvement of (10–13) AlN crystalline quality is possibly due to the high-temperature nitridation that promotes the coalescence of crystal grains. On the other hand, as the temperature of nitridation and buffer layer increases, the contents of N-Al-O and Al-O bonds in the AlN film are significantly reduced, resulting in an increase in the proportion of Al-N bonds. Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

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8 pages, 1859 KiB

Open AccessArticle

On the Modeling of the Donor/Acceptor Compensation Ratio in Carbon-Doped GaN to Univocally Reproduce Breakdown Voltage and Current Collapse in Lateral GaN Power HEMTs

by Nicolò Zagni, Alessandro Chini, Francesco Maria Puglisi, Paolo Pavan and Giovanni Verzellesi

Micromachines 2021, 12(6), 709; https://doi.org/10.3390/mi12060709 - 16 Jun 2021

Cited by 13 | Viewed by 2788

Abstract

The intentional doping of lateral GaN power high electron mobility transistors (HEMTs) with carbon (C) impurities is a common technique to reduce buffer conductivity and increase breakdown voltage. Due to the introduction of trap levels in the GaN bandgap, it is well known [...] Read more.

The intentional doping of lateral GaN power high electron mobility transistors (HEMTs) with carbon (C) impurities is a common technique to reduce buffer conductivity and increase breakdown voltage. Due to the introduction of trap levels in the GaN bandgap, it is well known that these impurities give rise to dispersion, leading to the so-called “current collapse” as a collateral effect. Moreover, first-principles calculations and experimental evidence point out that C introduces trap levels of both acceptor and donor types. Here, we report on the modeling of the donor/acceptor compensation ratio (CR), that is, the ratio between the density of donors and acceptors associated with C doping, to consistently and univocally reproduce experimental breakdown voltage (V_BD) and current-collapse magnitude (ΔI_CC). By means of calibrated numerical device simulations, we confirm that ΔI_CC is controlled by the effective trap concentration (i.e., the difference between the acceptor and donor densities), but we show that it is the total trap concentration (i.e., the sum of acceptor and donor densities) that determines V_BD, such that a significant CR of at least 50% (depending on the technology) must be assumed to explain both phenomena quantitatively. The results presented in this work contribute to clarifying several previous reports, and are helpful to device engineers interested in modeling C-doped lateral GaN power HEMTs. Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

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8 pages, 1236 KiB

Open AccessArticle

Fabrication of All-GaN Integrated MIS-HEMTs with High Threshold Voltage Stability Using Supercritical Technology

by Meihua Liu, Yang Yang, Changkuan Chang, Lei Li and Yufeng Jin

Micromachines 2021, 12(5), 572; https://doi.org/10.3390/mi12050572 - 18 May 2021

Cited by 2 | Viewed by 3049

Abstract

In this paper, a novel method to achieve all-GaN integrated MIS-HEMTs in a Si-CMOS platform by self-terminated and self-alignment process is reported. Furthermore, a process of repairing interface defects by supercritical technology is proposed to suppress the threshold voltage shift of all GaN [...] Read more.

In this paper, a novel method to achieve all-GaN integrated MIS-HEMTs in a Si-CMOS platform by self-terminated and self-alignment process is reported. Furthermore, a process of repairing interface defects by supercritical technology is proposed to suppress the threshold voltage shift of all GaN integrated MIS-HEMTs. The threshold voltage characteristics of all-GaN integrated MIS-HEMTs are simulated and analyzed. We found that supercritical NH

_{3}

fluid has the characteristics of both liquid NH

_{3}

and gaseous NH

_{3}

simultaneously, i.e., high penetration and high solubility, which penetrate the packaging of MIS-HEMTs. In addition, NH

_{2}^{-}

produced via the auto coupling ionization of NH

_{3}

has strong nucleophilic ability, and is able to fill nitrogen vacancies near the GaN surface created by high temperature process. The fabricated device delivers a threshold voltage of 2.67 V. After supercritical fluid treatment, the threshold voltage shift is reduced from 0.67 V to 0.13 V. Our demonstration of the supercritical technology to repair defects of wide-bandgap family of semiconductors may bring about great changes in the field of device fabrication. Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

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12 pages, 6176 KiB

Open AccessArticle

An Experimental and Systematic Insight into the Temperature Sensitivity for a 0.15-µm Gate-Length HEMT Based on the GaN Technology

by Mohammad Abdul Alim, Christophe Gaquiere and Giovanni Crupi

Micromachines 2021, 12(5), 549; https://doi.org/10.3390/mi12050549 - 12 May 2021

Cited by 11 | Viewed by 2628

Abstract

Presently, growing attention is being given to the analysis of the impact of the ambient temperature on the GaN HEMT performance. The present article is aimed at investigating both DC and microwave characteristics of a GaN-based HEMT versus the ambient temperature using measured [...] Read more.

Presently, growing attention is being given to the analysis of the impact of the ambient temperature on the GaN HEMT performance. The present article is aimed at investigating both DC and microwave characteristics of a GaN-based HEMT versus the ambient temperature using measured data, an equivalent-circuit model, and a sensitivity-based analysis. The tested device is a 0.15-μm ultra-short gate-length AlGaN/GaN HEMT with a gate width of 200 μm. The interdigitated layout of this device is based on four fingers, each with a length of 50 μm. The scattering parameters are measured from 45 MHz to 50 GHz with the ambient temperature varied from −40 °C to 150 °C. A systematic study of the temperature-dependent performance is carried out by means of a sensitivity-based analysis. The achieved findings show that by the heating the transistor, the DC and microwave performance are degraded, due to the degradation in the electron transport properties. Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

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6 pages, 4266 KiB

Open AccessArticle

High Thermal Dissipation of Normally off p-GaN Gate AlGaN/GaN HEMTs on 6-Inch N-Doped Low-Resistivity SiC Substrate

by Yu-Chun Huang, Hsien-Chin Chiu, Hsuan-Ling Kao, Hsiang-Chun Wang, Chia-Hao Liu, Chong-Rong Huang and Si-Wen Chen

Micromachines 2021, 12(5), 509; https://doi.org/10.3390/mi12050509 - 1 May 2021

Cited by 8 | Viewed by 2360

Abstract

Efficient heat removal through the substrate is required in high-power operation of AlGaN/GaN high-electron-mobility transistors (HEMTs). Thus, a SiC substrate was used due to its popularity. This article reports the electrical characteristics of normally off p-GaN gate AlGaN/GaN high-electron-mobility transistors (HEMTs) on a [...] Read more.

Efficient heat removal through the substrate is required in high-power operation of AlGaN/GaN high-electron-mobility transistors (HEMTs). Thus, a SiC substrate was used due to its popularity. This article reports the electrical characteristics of normally off p-GaN gate AlGaN/GaN high-electron-mobility transistors (HEMTs) on a low-resistivity SiC substrate compared with the traditional Si substrate. The p-GaN HEMTs on the SiC substrate possess several advantages, including electrical characteristics and good qualities of epitaxial crystals, especially on temperature performance. Additionally, the price of the low-resistivity SiC substrate is three times lower than the ordinary SiC substrate. Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

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12 pages, 9939 KiB

Open AccessArticle

Laser Processing of Transparent Wafers with a AlGaN/GaN Heterostructures and High-Electron Mobility Devices on a Backside

by Simonas Indrišiūnas, Evaldas Svirplys, Justinas Jorudas and Irmantas Kašalynas

Micromachines 2021, 12(4), 407; https://doi.org/10.3390/mi12040407 - 6 Apr 2021

Cited by 5 | Viewed by 2161

Abstract

Sapphire and silicon carbide substrates are used for growth of the III-N group heterostructures to obtain the electronic devices for high power and high frequency applications. Laser micromachining of deep channels in the frontside of the transparent wafers followed by mechanical cleavage along [...] Read more.

Sapphire and silicon carbide substrates are used for growth of the III-N group heterostructures to obtain the electronic devices for high power and high frequency applications. Laser micromachining of deep channels in the frontside of the transparent wafers followed by mechanical cleavage along the ablated trench is a useful method for partitioning of such substrates after the development of the electronics on a backside. However, in some cases damage to the component performance occurs. Therefore, the influence of various parameters of the laser processing, such as fluence in the spot size, substrate thickness, orientation, and the polarization of focused laser beam, to the formation of damage zones at both sides of the transparent substrate with thin coatings when ablating the trenches from one side was investigated. The vicinity effect of the ablated trenches on the performance of the electronics was also evaluated, confirming the laser micromachining suitability for the dicing of transparent wafers with high accuracy and flexibility. Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

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9 pages, 5020 KiB

Open AccessArticle

Performance Optimization of Nitrogen Dioxide Gas Sensor Based on Pd-AlGaN/GaN HEMTs by Gate Bias Modulation

by Van Cuong Nguyen, Kwangeun Kim and Hyungtak Kim

Micromachines 2021, 12(4), 400; https://doi.org/10.3390/mi12040400 - 5 Apr 2021

Cited by 15 | Viewed by 2792

Abstract

We investigated the sensing characteristics of NO₂ gas sensors based on Pd-AlGaN/GaN high electron mobility transistors (HEMTs) at high temperatures. In this paper, we demonstrated the optimization of the sensing performance by the gate bias, which exhibited the advantage of the FET-type [...] Read more.

We investigated the sensing characteristics of NO₂ gas sensors based on Pd-AlGaN/GaN high electron mobility transistors (HEMTs) at high temperatures. In this paper, we demonstrated the optimization of the sensing performance by the gate bias, which exhibited the advantage of the FET-type sensors compared to the diode-type ones. When the sensor was biased near the threshold voltage, the electron density in the channel showed a relatively larger change with a response to the gas exposure and demonstrated a significant improvement in the sensitivity. At 300 °C under 100 ppm concentration, the sensor’s sensitivities were 26.7% and 91.6%, while the response times were 32 and 9 s at V_G = 0 V and V_G = −1 V, respectively. The sensor demonstrated the stable repeatability regardless of the gate voltage at a high temperature. Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

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8 pages, 6941 KiB

Open AccessArticle

Improved Performance of GaN-Based Light-Emitting Diodes Grown on Si (111) Substrates with NH₃ Growth Interruption

by Sang-Jo Kim, Semi Oh, Kwang-Jae Lee, Sohyeon Kim and Kyoung-Kook Kim

Micromachines 2021, 12(4), 399; https://doi.org/10.3390/mi12040399 - 5 Apr 2021

Cited by 7 | Viewed by 3110

Abstract

We demonstrate the highly efficient, GaN-based, multiple-quantum-well light-emitting diodes (LEDs) grown on Si (111) substrates embedded with the AlN buffer layer using NH₃ growth interruption. Analysis of the materials by the X-ray diffraction omega scan and transmission electron microscopy revealed a remarkable [...] Read more.

We demonstrate the highly efficient, GaN-based, multiple-quantum-well light-emitting diodes (LEDs) grown on Si (111) substrates embedded with the AlN buffer layer using NH₃ growth interruption. Analysis of the materials by the X-ray diffraction omega scan and transmission electron microscopy revealed a remarkable improvement in the crystalline quality of the GaN layer with the AlN buffer layer using NH₃ growth interruption. This improvement originated from the decreased dislocation densities and coalescence-related defects of the GaN layer that arose from the increased Al migration time. The photoluminescence peak positions and Raman spectra indicate that the internal tensile strain of the GaN layer is effectively relaxed without generating cracks. The LEDs embedded with an AlN buffer layer using NH₃ growth interruption at 300 mA exhibited 40.9% higher light output power than that of the reference LED embedded with the AlN buffer layer without NH₃ growth interruption. These high performances are attributed to an increased radiative recombination rate owing to the low defect density and strain relaxation in the GaN epilayer. Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

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16 pages, 9372 KiB

Open AccessArticle

First-Principles Studies for Electronic Structure and Optical Properties of Strontium Doped β-Ga₂O₃

by Loh Kean Ping, Mohd Ambri Mohamed, Abhay Kumar Mondal, Mohamad Fariz Mohamad Taib, Mohd Hazrie Samat, Dilla Duryha Berhanuddin, P. Susthitha Menon and Raihana Bahru

Micromachines 2021, 12(4), 348; https://doi.org/10.3390/mi12040348 - 24 Mar 2021

Cited by 29 | Viewed by 4407

Abstract

The crystal structure, electron charge density, band structure, density of states, and optical properties of pure and strontium (Sr)-doped β-Ga₂O₃ were studied using the first-principles calculation based on the density functional theory (DFT) within the generalized-gradient approximation (GGA) with the [...] Read more.

The crystal structure, electron charge density, band structure, density of states, and optical properties of pure and strontium (Sr)-doped β-Ga₂O₃ were studied using the first-principles calculation based on the density functional theory (DFT) within the generalized-gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE). The reason for choosing strontium as a dopant is due to its p-type doping behavior, which is expected to boost the material’s electrical and optical properties and maximize the devices’ efficiency. The structural parameter for pure β-Ga₂O₃ crystal structure is in the monoclinic space group (C2/m), which shows good agreement with the previous studies from experimental work. Bandgap energy from both pure and Sr-doped β-Ga₂O₃ is lower than the experimental bandgap value due to the limitation of DFT, which will ignore the calculation of exchange-correlation potential. To counterbalance the current incompatibilities, the better way to complete the theoretical calculations is to refine the theoretical predictions using the scissor operator’s working principle, according to literature published in the past and present. Therefore, the scissor operator was used to overcome the limitation of DFT. The density of states (DOS) shows the hybridization state of Ga 3d, O 2p, and Sr 5s orbital. The bonding population analysis exhibits the bonding characteristics for both pure and Sr-doped β-Ga₂O₃. The calculated optical properties for the absorption coefficient in Sr doping causes red-shift of the absorption spectrum, thus, strengthening visible light absorption. The reflectivity, refractive index, dielectric function, and loss function were obtained to understand further this novel work on Sr-doped β-Ga₂O₃ from the first-principles calculation. Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

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7 pages, 1912 KiB

Open AccessArticle

Unidirectional Operation of p-GaN Gate AlGaN/GaN Heterojunction FET Using Rectifying Drain Electrode

by Tae-Hyeon Kim, Won-Ho Jang, Jun-Hyeok Yim and Ho-Young Cha

Micromachines 2021, 12(3), 291; https://doi.org/10.3390/mi12030291 - 10 Mar 2021

Cited by 2 | Viewed by 2620

Abstract

In this study, we proposed a rectifying drain electrode that was embedded in a p-GaN gate AlGaN/GaN heterojunction field-effect transistor to achieve the unidirectional switching characteristics, without the need for a separate reverse blocking device or an additional process step. The rectifying drain [...] Read more.

In this study, we proposed a rectifying drain electrode that was embedded in a p-GaN gate AlGaN/GaN heterojunction field-effect transistor to achieve the unidirectional switching characteristics, without the need for a separate reverse blocking device or an additional process step. The rectifying drain electrode was implemented while using an embedded p-GaN gating electrode that was placed in front of the ohmic drain electrode. The embedded p-GaN gating electrode and the ohmic drain electrode are electrically shorted to each other. The concept was validated by technology computer aided design (TCAD) simulation along with an equivalent circuit, and the proposed device was demonstrated experimentally. The fabricated device exhibited the unidirectional characteristics successfully, with a threshold voltage of ~2 V, a maximum current density of ~100 mA/mm, and a forward drain turn-on voltage of ~2 V. Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

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18 pages, 7661 KiB

Open AccessArticle

A Novel Ultrasonic TOF Ranging System Using AlN Based PMUTs

by Yihsiang Chiu, Chen Wang, Dan Gong, Nan Li, Shenglin Ma and Yufeng Jin

Micromachines 2021, 12(3), 284; https://doi.org/10.3390/mi12030284 - 8 Mar 2021

Cited by 17 | Viewed by 3928

Abstract

This paper presents a high-accuracy complementary metal oxide semiconductor (CMOS) driven ultrasonic ranging system based on air coupled aluminum nitride (AlN) based piezoelectric micromachined ultrasonic transducers (PMUTs) using time of flight (TOF). The mode shape and the time-frequency characteristics of PMUTs are simulated [...] Read more.

This paper presents a high-accuracy complementary metal oxide semiconductor (CMOS) driven ultrasonic ranging system based on air coupled aluminum nitride (AlN) based piezoelectric micromachined ultrasonic transducers (PMUTs) using time of flight (TOF). The mode shape and the time-frequency characteristics of PMUTs are simulated and analyzed. Two pieces of PMUTs with a frequency of 97 kHz and 96 kHz are applied. One is used to transmit and the other is used to receive ultrasonic waves. The Time to Digital Converter circuit (TDC), correlating the clock frequency with sound velocity, is utilized for range finding via TOF calculated from the system clock cycle. An application specific integrated circuit (ASIC) chip is designed and fabricated on a 0.18 μm CMOS process to acquire data from the PMUT. Compared to state of the art, the developed ranging system features a wide range and high accuracy, which allows to measure the range of 50 cm with an average error of 0.63 mm. AlN based PMUT is a promising candidate for an integrated portable ranging system. Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

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9 pages, 2293 KiB

Open AccessArticle

Effect of Gas Annealing on the Electrical Properties of Ni/AlN/SiC

by Dong-Hyeon Kim, Michael A. Schweitz and Sang-Mo Koo

Micromachines 2021, 12(3), 283; https://doi.org/10.3390/mi12030283 - 8 Mar 2021

Cited by 7 | Viewed by 2600

Abstract

It is shown in this work that annealing of Schottky barrier diodes (SBDs) in the form of Ni/AlN/SiC heterojunction devices in an atmosphere of nitrogen and oxygen leads to a significant improvement in the electrical properties of the structures. Compared to the non-annealed [...] Read more.

It is shown in this work that annealing of Schottky barrier diodes (SBDs) in the form of Ni/AlN/SiC heterojunction devices in an atmosphere of nitrogen and oxygen leads to a significant improvement in the electrical properties of the structures. Compared to the non-annealed device, the on/off ratio of the annealed SBD devices increased by approximately 100 times. The ideality factor, derived from the current-voltage (IV) characterization, decreased by a factor of ~5.1 after annealing, whereas the barrier height increased from ~0.52 to 0.71 eV. The bonding structure of the AlN layer was characterized by X-ray photoelectron spectroscopy. Examination of the N 1 s and O 1 s peaks provided direct indication of the most prevalent chemical bonding states of the elements. Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

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9 pages, 2850 KiB

Open AccessArticle

Bias Stress Stability of Solution-Processed Nano Indium Oxide Thin Film Transistor

by Rihui Yao, Xiao Fu, Wanwan Li, Shangxiong Zhou, Honglong Ning, Biao Tang, Jinglin Wei, Xiuhua Cao, Wei Xu and Junbiao Peng

Micromachines 2021, 12(2), 111; https://doi.org/10.3390/mi12020111 - 22 Jan 2021

Cited by 7 | Viewed by 2594

Abstract

In this paper, the effects of annealing temperature and other process parameters on spin-coated indium oxide thin film transistors (In₂O₃-TFTs) were studied. The research shows that plasma pretreatment of glass substrate can improve the hydrophilicity of glass substrate and [...] Read more.

In this paper, the effects of annealing temperature and other process parameters on spin-coated indium oxide thin film transistors (In₂O₃-TFTs) were studied. The research shows that plasma pretreatment of glass substrate can improve the hydrophilicity of glass substrate and stability of the spin-coating process. With Fourier transform infrared (FT-IR) and X-ray diffraction (XRD) analysis, it is found that In₂O₃ thin films prepared by the spin coating method are amorphous, and have little organic residue when the annealing temperature ranges from 200 to 300 °C. After optimizing process conditions with the spin-coated rotating speed of 4000 rpm and the annealing temperature of 275 °C, the performance of In₂O₃-TFTs is best (average mobility of 1.288 cm²·V⁻¹·s⁻¹, I_on/I_off of 5.93 × 10⁶, and SS of 0.84 V·dec⁻¹). Finally, the stability of In₂O₃-TFTs prepared at different annealing temperatures was analyzed by energy band theory, and we identified that the elimination of residual hydroxyl groups was the key influencing factor. Our results provide a useful reference for high-performance metal oxide semiconductor TFTs prepared by the solution method. Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

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11 pages, 3210 KiB

Open AccessArticle

Temperature-Controlled Crystal Size of Wide Band Gap Nickel Oxide and Its Application in Electrochromism

by Muyang Shi, Tian Qiu, Biao Tang, Guanguang Zhang, Rihui Yao, Wei Xu, Junlong Chen, Xiao Fu, Honglong Ning and Junbiao Peng

Micromachines 2021, 12(1), 80; https://doi.org/10.3390/mi12010080 - 14 Jan 2021

Cited by 45 | Viewed by 4284

Abstract

Nickel oxide (NiO) is a wide band gap semiconductor material that is used as an electrochromic layer or an ion storage layer in electrochromic devices. In this work, the effect of annealing temperature on sol-gel NiO films was investigated. Fourier transform infrared spectroscopy [...] Read more.

Nickel oxide (NiO) is a wide band gap semiconductor material that is used as an electrochromic layer or an ion storage layer in electrochromic devices. In this work, the effect of annealing temperature on sol-gel NiO films was investigated. Fourier transform infrared spectroscopy (FTIR) showed that the formation of NiO via decomposition of the precursor nickel acetate occurred at about 300 °C. Meanwhile, an increase in roughness was observed by Atomic force microscope (AFM), and precipitation of a large number of crystallites was observed at 500 °C. X-ray Diffraction (XRD) showed that the NiO film obtained at such a temperature showed a degree of crystallinity. The film crystallinity and crystallite size also increased with increasing annealing temperature. An ultraviolet spectrophotometer was used to investigate the optical band gap of the colored NiO films, and it was found that the band gap increased from 3.65 eV to 3.74 eV with the increase in annealing temperature. An electrochromic test further showed that optical modulation density and coloring efficiency decreased with the increase in crystallite size. The electrochromic reaction of the nickel oxide film is more likely to occur at the crystal interface and is closely related to the change of the optical band gap. An NiO film with smaller crystallite size is more conducive to ion implantation and the films treated at 300 °C exhibit optimum electrochromic behavior. Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

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13 pages, 3466 KiB

Open AccessArticle

AlGaN/GaN on SiC Devices without a GaN Buffer Layer: Electrical and Noise Characteristics

by Justinas Jorudas, Artūr Šimukovič, Maksym Dub, Maciej Sakowicz, Paweł Prystawko, Simonas Indrišiūnas, Vitalij Kovalevskij, Sergey Rumyantsev, Wojciech Knap and Irmantas Kašalynas

Micromachines 2020, 11(12), 1131; https://doi.org/10.3390/mi11121131 - 20 Dec 2020

Cited by 21 | Viewed by 5294

Abstract

We report on the high-voltage, noise, and radio frequency (RF) performances of aluminium gallium nitride/gallium nitride (AlGaN/GaN) on silicon carbide (SiC) devices without any GaN buffer. Such a GaN–SiC hybrid material was developed in order to improve thermal management and to reduce trapping [...] Read more.

We report on the high-voltage, noise, and radio frequency (RF) performances of aluminium gallium nitride/gallium nitride (AlGaN/GaN) on silicon carbide (SiC) devices without any GaN buffer. Such a GaN–SiC hybrid material was developed in order to improve thermal management and to reduce trapping effects. Fabricated Schottky barrier diodes (SBDs) demonstrated an ideality factor n at approximately 1.7 and breakdown voltages (fields) up to 780 V (approximately 0.8 MV/cm). Hall measurements revealed a thermally stable electron density at N_2DEG = 1 × 10¹³ cm⁻² of two-dimensional electron gas in the range of 77–300 K, with mobilities μ = 1.7 × 10³ cm²/V∙s and μ = 1.0 × 10⁴ cm²/V∙s at 300 K and 77 K, respectively. The maximum drain current and the transconductance were demonstrated to be as high as 0.5 A/mm and 150 mS/mm, respectively, for the transistors with gate length L_G = 5 μm. Low-frequency noise measurements demonstrated an effective trap density below 10¹⁹ cm⁻³ eV⁻¹. RF analysis revealed f_T and f_max values up to 1.3 GHz and 6.7 GHz, respectively, demonstrating figures of merit f_T × L_G up to 6.7 GHz × µm. These data further confirm the high potential of a GaN–SiC hybrid material for the development of thin high electron mobility transistors (HEMTs) and SBDs with improved thermal stability for high-frequency and high-power applications. Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

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23 pages, 17392 KiB

Open AccessArticle

Design and Implementation of a SiC-Based VRFB Power Conditioning System

by Chao-Tsung Ma and Yi-Hung Tian

Micromachines 2020, 11(12), 1099; https://doi.org/10.3390/mi11121099 - 12 Dec 2020

Cited by 1 | Viewed by 2290

Abstract

An energy storage system using secondary batteries combined with advanced power control schemes is considered the key technology for the sustainable development of renewable energy-based power generation and smart micro-grids. The performance of energy storage systems in practical application mainly depends on their [...] Read more.

An energy storage system using secondary batteries combined with advanced power control schemes is considered the key technology for the sustainable development of renewable energy-based power generation and smart micro-grids. The performance of energy storage systems in practical application mainly depends on their power conditioning systems. This paper proposes a silicon carbide-based multifunctional power conditioning system for the vanadium redox flow battery. The proposed system is a two-stage circuit topology, including a three-phase grid-tie inverter that can perform four-quadrant control of active and reactive power and a bi-directional multi-channel direct current converter that is responsible for the fast charging and discharging control of the battery. To achieve the design objectives, i.e., high reliability, high efficiency, and high operational flexibility, silicon carbide-based switching devices, and advanced digital control schemes are used in the construction of a power conditioning system for the vanadium redox flow battery. This paper first describes the proposed system topologies and controller configurations and the design methods of controllers for each converter in detail, and then results from both simulation analyses and experimental tests on a 5 kVA hardware prototype are presented to verify the feasibility and effectiveness of the proposed system and the designed controllers. Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

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Review

Jump to: Editorial, Research

13 pages, 2184 KiB

Open AccessReview

GaN Heterostructures as Innovative X-ray Imaging Sensors—Change of Paradigm

by Stefan Thalhammer, Andreas Hörner, Matthias Küß, Stephan Eberle, Florian Pantle, Achim Wixforth and Wolfgang Nagel

Micromachines 2022, 13(2), 147; https://doi.org/10.3390/mi13020147 - 19 Jan 2022

Cited by 6 | Viewed by 2745

Abstract

Direct conversion of X-ray irradiation using a semiconductor material is an emerging technology in medical and material sciences. Existing technologies face problems, such as sensitivity or resilience. Here, we describe a novel class of X-ray sensors based on GaN thin film and GaN/AlGaN [...] Read more.

Direct conversion of X-ray irradiation using a semiconductor material is an emerging technology in medical and material sciences. Existing technologies face problems, such as sensitivity or resilience. Here, we describe a novel class of X-ray sensors based on GaN thin film and GaN/AlGaN high-electron-mobility transistors (HEMTs), a promising enabling technology in the modern world of GaN devices for high power, high temperature, high frequency, optoelectronic, and military/space applications. The GaN/AlGaN HEMT-based X-ray sensors offer superior performance, as evidenced by higher sensitivity due to intensification of electrons in the two-dimensional electron gas (2DEG), by ionizing radiation. This increase in detector sensitivity, by a factor of 10⁴ compared to GaN thin film, now offers the opportunity to reduce health risks associated with the steady increase in CT scans in today’s medicine, and the associated increase in exposure to harmful ionizing radiation, by introducing GaN/AlGaN sensors into X-ray imaging devices, for the benefit of the patient. Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

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32 pages, 8361 KiB

Open AccessReview

Development of GaN HEMTs Fabricated on Silicon, Silicon-on-Insulator, and Engineered Substrates and the Heterogeneous Integration

by Lung-Hsing Hsu, Yung-Yu Lai, Po-Tsung Tu, Catherine Langpoklakpam, Ya-Ting Chang, Yu-Wen Huang, Wen-Chung Lee, An-Jye Tzou, Yuh-Jen Cheng, Chun-Hsiung Lin, Hao-Chung Kuo and Edward Yi Chang

Micromachines 2021, 12(10), 1159; https://doi.org/10.3390/mi12101159 - 27 Sep 2021

Cited by 42 | Viewed by 15369

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

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28 pages, 1765 KiB

Open AccessReview

Review on Driving Circuits for Wide-Bandgap Semiconductor Switching Devices for Mid- to High-Power Applications

by Chao-Tsung Ma and Zhen-Huang Gu

Micromachines 2021, 12(1), 65; https://doi.org/10.3390/mi12010065 - 8 Jan 2021

Cited by 24 | Viewed by 5550

Abstract

Wide-bandgap (WBG) material-based switching devices such as gallium nitride (GaN) high electron mobility transistors (HEMTs) and silicon carbide (SiC) metal-oxide-semiconductor field-effect transistors (MOSFETs) are considered very promising candidates for replacing conventional silicon (Si) MOSFETs for various advanced power conversion applications, mainly because of [...] Read more.

Wide-bandgap (WBG) material-based switching devices such as gallium nitride (GaN) high electron mobility transistors (HEMTs) and silicon carbide (SiC) metal-oxide-semiconductor field-effect transistors (MOSFETs) are considered very promising candidates for replacing conventional silicon (Si) MOSFETs for various advanced power conversion applications, mainly because of their capabilities of higher switching frequencies with less switching and conduction losses. However, to make the most of their advantages, it is crucial to understand the intrinsic differences between WBG- and Si-based switching devices and investigate effective means to safely, efficiently, and reliably utilize the WBG devices. This paper aims to provide engineers in the power engineering field a comprehensive understanding of WBG switching devices’ driving requirements, especially for mid- to high-power applications. First, the characteristics and operating principles of WBG switching devices and their commercial products within specific voltage ranges are explored. Next, considerations regarding the design of driving circuits for WBG switching devices are addressed, and commercial drivers designed for WBG switching devices are explored. Lastly, a review on typical papers concerning driving technologies for WBG switching devices in mid- to high-power applications is presented. Full article

(This article belongs to the Special Issue Wide Bandgap Based Devices: Design, Fabrication and Applications, Volume II)

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