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This article proposes a highly efficient current-fed half-bridge resonant converter over a wide range of the output voltages. By adding a bidirectional switch across center nodes of the voltage doubler and applying corresponding switching modulation, the converter accomplishes almost zero-voltage switching at the turn-off instant under the wide output voltage variations. Moreover, the circuit experiences very little instantaneous reactive current even when the output voltage varies widely. Primary-side duty cycle fixed at 0.5 balances the voltages across the capacitors at the primary side and at the secondary side each irrespective of the output voltage variation; it also limits the voltage stress at the active power components. Therefore, the proposed converter delivers high efficiency and compact component size. We fabricated a 400-W experimental prototype to validate the superior properties of the proposed converter.

While two-dimensional (2D) hexagonal boron nitride (h-BN) is emerging as an atomically thin and dangling bond-free insulating layer for next-generation electronics and optoelectronics, its practical implementation into miniaturized integrated circuits has been significantly limited due to difficulties in large-scale growth directly on epitaxial semiconductor wafers. Herein, the realization of a wafer-scale h-BN van der Waals heterostructure with a 2 in. AlGaN/GaN high-electron mobility transistor (HEMT) wafer using metal-organic chemical vapor deposition is presented. The combination of state-of-the-art microscopic and spectroscopic analyses and theoretical calculations reveals that the heterointerface between ∼2.5 nm-thick h-BN and AlGaN layers is atomically sharp and exhibits a very weak van der Waals interaction without formation of a ternary or quaternary alloy that can induce undesired degradation of device performance. The fabricated AlGaN/GaN HEMT with h-BN shows very promising performance including a cutoff frequency (fT) and maximum oscillation frequency (fMAX) as high as 28 and 88 GHz, respectively, enabled by an effective passivation of surface defects on the HEMT wafer to deliver accurate information with minimized power loss. These findings pave the way for practical implementation of 2D materials integrated with conventional microelectronic devices and the realization of future all-2D electronics.

This article presents a bidirectional soft-switching push–pull resonant converter that is highly efficient over a wide range of battery voltages. It is derived by integrating a current-fed push–pull circuit and an active voltage doubler circuit with a bidirectional switch. The converter operates as a pulsewidth modulation (PWM) current-fed push–pull resonant-boost converter in the forward direction, but as a PWM half-bridge resonant converter in the backward direction. The most attractive feature of the proposed converter is that, by employing the bidirectional switch and corresponding switching modulation, it can achieve almost ZVS at turn- off instants under wide variations of battery voltages. Therefore, the proposed converter achieves high efficiency even during high-frequency operation. Moreover, no instantaneous reactive current flows through the circuit under wide variations of battery voltages and loads. Furthermore, the proposed bidirectional resonant converter has a high conversion ratio in both power flow directions without using a transformer that has high turns-ratio. The operating principles and characteristics of the proposed bidirectional resonant converter are presented in detail. Experimental results using a 1-kW prototype verified the feasibility and efficiency of the proposed converter.

We propose a stray inductance extraction method on power modules of the few‐kilovolts/several‐hundred‐amperes class using only low voltages and low currents. The method incorporates a double‐pulse generator, a level shifter, a switching device, and a load inductor. The conventional approach generally requires a high voltage of more than half the power module's rated voltage and a high current of around half the rated current. In contrast, the proposed method requires a low voltage and low current environment regardless of the power module's rated voltage because the module is measured in a turn‐off state. Both theoretical and experimental results are provided. A physical circuit board was fabricated, and the method was applied to three commercial power modules with EconoDUAL3 cases. The obtained stray inductance values differed from the manufacturer‐provided values by less than 1.65 nH, thus demonstrating the method's accuracy. The greatest advantage of the proposed approach is that high voltages or high currents are not required.

Devices based on AlGaN/GaN heterostructures, for example, Schottky barrier diodes (SBDs) and high electron mobility transistors (HEMTs), have been intensively investigated for applications to high-frequency and high-power areas. Presently, the substrates widely distributed are AlGaN/GaN on SiC for its high performance in radio frequency (RF) applications, for examples high cutoff frequency (fT) or high maximum oscillation frequency (fmax), and AlGaN/GaN on Si for its high power performance, for examples high breakdown voltage or high voltage operation. Chemical vapor deposition (CVD) diamond substrates have a thermal conductivity of 12 W/cm·K, and this is a remarkable point because HEMTs or SBDs on AlGaN/GaN on CVD diamonds are one of the promising alternatives for power and RF applications. In comparison, the thermal conductivity of AlGaN/GaN on a sapphire substrate is 0.33 W/cm·K while that of AlGaN/GaN on a Si substrate is 1.3 W/cm·K and that of AlGaN/GaN on a SiC substrate is 4.9 W/cm·K. In this work, we fabricated SBDs with a 137 mm Schottky channel length on AlGaN/GaN on Si and also on a CVD diamond substrate. We also compared the thermal behaviors of these fabricated large scale SBDs on Si and a CVD diamond substrate.

GaN-based metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs) are intensively investigated for high power, high frequency [1] and aerospace application [2] due to its wide bandgap, high breakdown field, and high carrier density and mobility at AlGaN/GaN interface [3]. For the demonstration of high power radio-frequency and aerospace applicable devices, the passivation layer introduced on the AlGaN barrier plays significant roles for the enhancement of the frequency characteristics [4] and proton radiation hardness [2]. From this point of view, diverse passivation materials and systems have been researched in GaN-based MIS-HEMTs [2, 5, 6]. However, the detailed impact of passivation layer on the device properties and the immunity to the proton irradiation is not clear in many cases, so far. In this work, we have prepared the SiN/Al2O3 bi-layer passivation system to study the impact of mechanical stress introduced by the passivation layer deposition on the device performance (Sample A: SiN = 20 nm, Sample B: SiN/Al2O3 = 5/15 nm, Sample C: SiN/Al2O3 = 5/35 nm). The correlation between the induced mechanical stress on the GaN channel and carrier transport properties in terms of carrier concentration and mobility at hetero-interface in GaN-based MIS-HEMTs was examined. The Al2O3 layer deposited by the atomic layer deposition (ALD) provided the tensile stress which compensated the compressive stress generated by the SiN layer deposited by the plasma enhanced chemical vapor deposition (PEDVD). When the compressive stress reduces through the deposition of Al2O3 on the SiN layer, the carrier concentration and mobility at hetero-interface were increased. With decreasing compressive stress, the piezoelectric field enlarges that enhances polarization charges [7] and carrier concentration. The carrier mobility was also improved with diminishing compressive stress since the mean free path related to the lattice scattering is decreased, enhancing transconductance as well. As a result, GaN-based MIS-HEMTs with slight tensile stress in Sample C exhibits the best device performance. The proton radiation hardness according to the passivation system was studied for various passivation systems. When we compared the device characteristics before and after the proton irradiation with the fluences of 1013 and 1014 cm-2 at an energy of 5 MeV, the device properties such as threshold voltage, drain current, and transconductance were changed. After the proton radiation, the threshold voltage was negatively shifted that is identical characteristics to the metal-oxide-semiconductor device [8], rather than GaN-based HEMTs [9] as relatively large amount of holes was trapped inside of the passivation system due to the lower hole mobility than electron. The alternation of the device characteristics depended on the passivation system. The ALD-deposited Al2O3 showed superior quality compared to the PECVD-deposited SiN that was revealed by the pulse-mode stress measurements. Therefore, the hole trap sites generated by the proton irradiation and located interior of the passivation system are less when the Al2O3 layer was employed as a passivation layer, showing smaller threshold voltage shift. Moreover, the carrier mobility deterioration caused by the Coulomb scattering between the trapped holes inside the passivation system and carriers at hetero-interface was improved with the Al2O3 passivation layer. The drain current and transconductance degradation were improved by applying Al2O3 passivation layer. Our systematic measurement results highlight that the optimized SiN/Al2O3 bi-layer passivation system is promising technique for the enhancement of the device performance and improvement of proton radiation hardness in GaN-based MIS-HEMTs. Acknowledgement This work was supported by the National Research Council of Science & Technology (NST) grant by the Korean government (MSIP) (No. CRC-19-02-ETRI). References [1] V. Kummer, W. Lu, R. Schwindt, et al., IEEE Electron Device Lett., vol. 23(8), pp. 455-457, 2002. [2] S. Ahn, B.-J. Kim, Y,-H. Lin, et al., J. Vac. Sci. Technol. B. vol. 34(5), 051202, 2016. [3] T. Palacios, A. Chakraborty, S. Rajan, et al., IEEE Electron. Device Lett., vol 26(11), pp. 781-783, 2005. [4] P. D. Ye, B. Yang, K. K. Ng, et al., Appl. Phys. Lett., vol. 86, 063501, 2005. [5] M. A. Khan, X. Hu, G. Simin, et al., IEEE Electron Device Lett., vol. 21(2), 99. 63-65, 2000. [6] K. Balachander, S. Arulkumaran, H. Ishikawa, et al., Phys. Status Solidi A., vol. 202(2), R16-R18, 2005. [7] S. Joglekar, M. Azize, E. J. Jones, et al., IEEE Trans. on Electron Devices, vol. 63(1), pp. 318-325, 2015. [8] K. Hayama, K. Takakura, H. Ohyama, et al., IEEE Trans. Nucl. Sci., vol 51(6), pp. 3795-3800, 2004. [9] X. Hu, A. P. Karmarkar, B. Jun, IEEE Trans. Nucl. Sci., vol 50(6), pp. 1791-1796, 2003. Figure 1

Fabrication of normally-off field effect transistors (FETs) possessed uniform turn-on threshold voltage (Vth) is of special interests. In this work, they were fabricated using dry etching recess techniques under the gate region, with dry etching conditions of extremely low rate. We report how the recess depth under the gate area induced the Vth shift of normally-off FETs on AlGaN/GaN heterostructure, which were fabricated with a 1.5 nm/min etching rate. Chlorine-based inductively coupled plasma (ICP) was applied to perform the etching process for the AlGaN/GaN heterostructure. Devices were fabricated with different recess depths under the gate area, and examined to determine their performances, particularly the dependence of recess time and recess depth on Vth shift. The applied dry etching conditions resulted in a low-damaged and not-rough morphology on the etched surfaces of AlGaN/GaN. Fine controlled and well defined recess depth of the AlGaN/GaN heterostructure under the gate region was achieved with no etch-stop layers. Conventional fabrication processes were applied with the dry etching conditions of extremely low rate to fabricate normally-off MOSFETs of Al2O3/AlGaN/GaN. The achieved Vth of +5.64 V was high positive and the leakage current of off-state was measured as ~10−6 A/mm.

Ohmic contacts are formed by recess etching the source and the drain regions with line and dot patterns of a few hundred nanometers on an AlGaN/GaN heterostructure and treating the surface with tetramethylammonium hydroxide (TMAH) prior to ohmic metal deposition. Electrical characterizations show that ohmic behavior is obtained at a low annealing temperature of 700 ◦ C as the direct contact area to the two-dimensional electron gas (2DEG) is increased via the etched patterns, and the ohmic contact is further improved when the interface is treated with TMAH. The analyses based on transmission electron microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, and atomic force microcopy confirm the presence of an improved ohmic contact interface and morphology, suggesting a novel and viable route towards the fabrication of AlGaN/GaN-based devices with improved controllability and reliability.

High electron mobility transistors (HEMTs) and Schottky barrier diodes (SBDs) based on AlGaN/GaN heterostructure have been widely studied for high-frequency and/or high-power application. Widely distributed substrates for the high performance of RF applications are presently AlGaN/GaN on SiC, and those for high power performance are AlGaN/GaN on Si. Because the thermal conductivity of CVD diamond substrates is as high as 12 W/cm · K, devices on AlGaN/GaN on CVD diamond are one of the excellent alternatives for power and RF applications. In comparison, the thermal conductivity of AlGaN/GaN on SiC is 4.9 W/cm K, and that of AlGaN/GaN on Si is 1.3 W/cm · K. In this work, we report the fabrication of SBD devices with 163.8 mm Schottky channel length. We also compared the thermal properties of the fabricated large scale SBD devices on different substrates.

The COVID-19 pandemic has underscored the shortcomings in the deployment of state-of-the-art diagnostics platforms. Although several polymerase chain reaction (PCR)-based techniques have been rapidly developed to meet the growing testing needs, such techniques often need samples collected through a swab, the use of RNA extraction kits, and expensive thermocyclers in order to successfully perform the test. Isothermal amplification-based approaches have also been recently demonstrated for rapid severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) detection by minimizing sample preparation while also reducing the instrumentation and reaction complexity. In addition, there are limited reports of saliva as the sample source, and some of these indicate inferior sensitivity when comparing reverse transcription loop-mediated isothermal amplification (RT-LAMP) with PCR-based techniques. In this paper, we demonstrate an improved sensitivity assay from saliva using a two-step RT-LAMP assay, where a short 10 min RT step is performed with only B3 and backward inner primers before the final reaction. We show that while the one-step RT-LAMP demonstrates satisfactory results, the optimized two-step approach allows detection of only few molecules per reaction and performs significantly better than the one-step RT-LAMP and conventional two-step RT-LAMP approaches with all primers included in the RT step. We show control measurements with RT-PCR, and importantly, we demonstrate RNA extraction-free RT-LAMP-based assays for detection of SARS-CoV-2 from viral transport media and saliva clinical samples.

Point-of-care (POC) detection technologies that enable decentralized, rapid, sensitive, low-cost diagnostics of COVID-19 infection are urgently needed around the world. With many technologies approved for commercialization in the past 10 months, the field of COVID-19 POC diagnostics is rapidly evolving. In this Perspective, we analyze the current state of POC technologies for the diagnosis and monitoring of COVID-19 infection and discuss future challenges in COVID-19 diagnostics. As the COVID-19 pandemic becomes endemic, the advances gained during this past year will likely also be utilized for future prediction of emerging outbreaks and pandemics.

We investigate DC characteristics of AlGaN/GaN high-electron mobility transistors by using a source-bridged field plate and additional bottom plate (BP) structure. The analysis of experimental data was performed with a two-dimensional simulator. Source connected BP structure stabilized threshold voltage and transconductance regardless of various drain voltages. The effect of BP location was also analyzed, which had optimal DC values because of the dependence of breakdown voltage and drain current of the device on BP position between gate and drain. Finally, the optimum distance of 0.8 μm from drain side gate head edge to BP was achieved for optimum DC characteristics and the highest breakdown voltage of 341 V.

AlGaN/GaN heterostructures were successfully grown with the AlIn(Ga)N back barrier at 900 °C. However, the atomic composition of the AlIn(Ga)N layer was strongly dependent on the growth pressure, which resulted in a different lattice constant of the layer. The AlIn(Ga)N back barrier grown at 400 torr was almost lattice-matched to GaN layer. The AlGaN/GaN heterostructures with 10 and 15 nm-thick AlIn(Ga)N back barrier exhibited improved 2-DEG properties, compared to those of the conventional AlGaN/GaN heterostructure without the back barrier. The high electron mobility transistors (HEMTs) fabricated on the AlGaN/GaN heterostructure with a 15 nm-thick AlIn(Ga)N back barrier exhibited a very low off-state leakage current of ∼2 × 10−7 A/mm which is about 1 order lower in magnitude than the value of the device without the back barrier. The AlIn(Ga)N back barrier is a promising candidate as an alternative to conventional AlGaN and InGaN back barrier.

As the data transfer rates in various mobile systems increase, the needs for 5G wireless communication systems have also been correspondingly increased. For the enhancement of data transfer rates, 28 GHz, one of the high frequency bands proposed by Korea for global standards, was chosen for 5G systems. While GaN-based high-electron-mobility transistors (HEMTs) are very promising candidates for 5G network applications due to their usage in high output power amplifiers, typical GaN HEMTs only operate in a normally-on state. However, for successful operations of mobile handsets of 5G systems, a normally-off device operating under positive gate bias is required, which provides advantages of decreased circuit complexity and system cost, as well as potential for use in the domain of digital circuits. For positive gate bias operation, metal-insulator-semiconductor (MIS)-HEMT devices were fabricated by using the ETRI GaN MIS-HEMT process. We designed and fabricated MIS-HEMTs and characterized their performances. In addition to the adaptation of a gate recess technique, we employed an Al2O3 gate insulator to shift the threshold voltage in GaN MIS-HEMTs. The power amplifier (PA) monolithic microwave integrated circuit (MMIC) was designed and operated under a positive gate bias for 5G mobile handsets and exhibited a maximum output power of 29.5 dBm, a power gain of 11 dB, and a power added efficiency (PAE) of 11% at frequencies of 26 and 28 GHz.

In this paper, a direct-bonded copper (DBC) substrate with wire-bonding and a DBC substrate with clip-bonding are designed and fabricated to compare inductance as their structure. Through a comparison of them, it is proved that the clip for the semiconductor package is suitable for decreasing inductance and direct current resistance (DCR). Also, the clip which has reduced inductance and DCR is expected to improve switching characteristics and efficiency in the power conversion system.

We propose a low-temperature co-fired ceramic (LTCC)-based leadless surface mount package to improve switching and heat-dissipation properties of a power semiconductor. A silicon carbide (SiC) Schottky barrier diode (SBD) bare die is embedded to a cavity in the LTCC-based multi-layer substrate. Instead of conventional aluminum wires, a flat copper clip is used to reduce parasitic inductance and electrical resistance caused by bond wires. Multiple vias filled with silver are embedded in the multi-layer substrate to decrease the electrical resistance and to improve heat-dissipation of the package. The measured reverse recovery charge (Q rr ) was 18.02 nC at a reverse voltage of 300 V and $di/dt$ of $300\ \mathrm{A}/\mu\mathrm{s}$ . It exhibits 18.7% improvement in the Q rr as compared to the conventional TO-220 packaged product using the same bare die. The power loss and heat-dissipation performances of the proposed package was evaluated through a power factor correction (PFC) converter. In comparison with commercial products, we can see that the proposed package has low-loss and high heat-dissipation properties.

We propose a multi-layer ceramic based surface mount device (SMD) packaging for 1200 V and 1700 V silicon carbide (SiC) Schottky barrier diode (SBD) power semiconductors. The distance between anode and cathode on a ceramic substrate is 1.0 mm to achieve an isolation voltage performance over 2000 V. We implemented the proposed multi-layer ceramic based substrates with 8 mm x 8 mm size. Using the substrates, we fabricated three SMD typed power semiconductor packaging such as 1200 V/2 A, 1200 V/5 A, and 1700 V/5 A-class SiC SBD and evaluated forward/reverse static and dynamic performances. It is the world’s first result of SMD packaging using a multi-layer ceramic substrate for 1200 and 1700 V SiC SBD.

In this paper, the multilayer power inductor with the air-clip is designed and fabricated to improve the saturation current of power inductor for the power conversion system. It is proved that the air-clip is able to influence the saturation current of power inductor and the efficiency of the power conversion system. Also, the possibility of the power inductor with the air-clip in the power conversion system for improving rated current and efficiency is demonstrated.

An enhancement-mode AlGaN/GaN metal-insulator-semiconductor high-electron- mobility-transistor was fabricated using a recess gate and CF4 plasma treatment to investigate its reliable applicability to high-power devices and circuits. The fluorinated-gate device showed hysteresis during the DC current-voltage measurement, and the polarity and magnitude of hysteresis depend on the drain voltage. The hysteresis phenomenon is due to the electron trapping at the Al2O3/AlGaN interface and charging times longer than milliseconds were obtained by pulse I-V measurement. In addition, the subthreshold slope of the fluorinated-gate device was increased after the positive gate bias stress because of the two-dimensional electron gas reduction by ionized fluorine. Our systematic observation revealed that the effect of fluorine ions should be considered for the design of AlGaN/GaN power circuits.

The device performance deterioration mechanism caused by the total ionizing dose effect after the &gamma, ray irradiation was investigated in GaN-based metal-insulator-semiconductor high electron mobility transistors (MIS-HEMTs) for a 5 nm-thick SiN and HfO2 gate dielectric layer. The &gamma, ray radiation hardness according to the gate dielectric layer was also compared between the two different GaN-based MIS-HEMTs. Although HfO2 has exhibited strong tolerance to the total ionizing dose effect in Si-based devices, there is no detail report of the &gamma, ray radiation effects in GaN-based MIS-HEMTs employing a HfO2 gate dielectric layer. The pulsed-mode stress measurement results and carrier mobility behavior revealed that the device properties not only have direct current (DC) characteristics, but radio frequency (RF) performance has also been mostly degraded by the deterioration of the gate dielectric quality and the trapped charges inside the gate insulator. We also figured out that the immunity to the &gamma, ray radiation was improved when HfO2 was employed instead of SiN as a gate dielectric layer due to its stronger endurance to the &gamma, ray irradiation. Our results highlight that the application of a gate insulator that shows superior immunity to the &gamma, ray irradiation is a crucial factor for the improvement of the total ionizing dose effect in GaN-based MIS-HEMTs.

In this paper, a power inductor embedded pattern with multi-via is proposed to achieve the lower DCR. Also, the low temperature co-fired ceramic (LTCC) substrate for the power conversion module was fabricated to be interconnected with a proposed power inductor. A proposed power inductor can be interconnected to the bottom side of the LTCC substrate. Two types of power inductors were fabricated to compare electrical properties. One is with the single-stack multi-via pattern (SMP) and the other is with the dual-stack multi-via pattern (DMP). The DCR, inductance at the frequency of 1 MHz, and Q-factor at the frequency of 1 MHz for the fabricated inductor with SMP were 17.82 mΩ, 3.21 μH and 42.19, respectively. On the other hand, that of the fabricated inductor with DMP were 12.49 mΩ, 3.3 μH, and 56.77, respectively. Compared to the fabricated inductor with SMP, the inductor with DMP shows lower DCR and higher Q-factor while having a similar inductance. A LTCC based DC-DC converter as the power conversion module was designed to be interconnected with proposed inductors. A DC-DC converter with switching frequency of 1 MHz has input voltage of 12 V, output voltage of 5 V, and rated power of 35 W. The efficiency of the converter with SMP inductor at 35 W was 92.41 % and that of the converter with DMP inductor was 93.79 %. A proposed inductor was designed to minimize DCR and to improve the stability of fabrication. Based on the measured results, it is demonstrated that the DCR is decreased by a proposed structure. This is due to the multi-via pattern in the proposed inductor. It is also proved that a proposed inductor is very suitable for improving the efficiency of the power conversion module.