This paper presents a method with an accurate control of threshold voltages (Vth) of AlGaN/GaN high-electron mobility transistors (HEMTs) using a fluoride-based plasma treatment. Using this method, the Vth of AlGaN/GaN HEMTs can be continuously shifted from -4 V in a conventional depletion-mode (D-mode) AlGaN/GaN HEMT to 0.9 V in an enhancement-mode AlGaN/GaN HEMT. It was found that the plasma-induced damages result in a mobility degradation of two-dimensional electron gas. The damages can be repaired and the mobility can be recovered by a post-gate annealing step at 400 degC. At the same time, the shift in Vth shows a good thermal stability and is not affected by the post-gate annealing. The enhancement-mode HEMTs show a performance (transconductance, cutoff frequencies) comparable to the D-mode HEMTs. Experimental results confirm that the threshold-voltage shift originates from the incorporation of F ions in the AlGaN barrier. In addition, the fluoride-based plasma treatment was also found to be effective in lowering the gate-leakage current, in both forward and reverse bias regions. A physical model of the threshold voltage is proposed to explain the effects of the fluoride-based plasma treatment on AlGaN/GaN HEMTs

Control of Threshold Voltage of AlGaN/GaN HEMTs by Fluoride-Based Plasma Treatment: From Depletion Mode to Enhancement Mode

This article presents the history of WPT and the various technologies and applications of this exciting technology. In the near future, standardization and regulation will be of importance to realizing WPT based products for commercial applications. The WPC has defined a standard for inductive coupling and members of this group have released conforming products. There are, as yet, no standards or regulation for resonant coupling and MPT technologies. In Japan, a technical forum known as Broadband Wireless Forum has been established to discuss the future of WPT. SPS researchers have also submitted a proposal for WPT to the International Telecommunication Union (ITU). In the IEEE MTT Society, the Technical Committee MTT-26 Wireless Energy Transfer and Conversion was established in June, 2011 to discuss the future of WPT.

Power without wires

Ieee transactions on antennas and propagation

A system of solar roof shingles and underlayment with wireless power transfer between the solar roof shingles (23) and the underlayment (16) is disclosed. Each roof shingle has a solar collector array (26) coupled to a wireless resonator (29). The solar collector array establishes a voltage in response to exposure to sunlight and the wireless resonator converts the voltage to a transmittable electromagnetic signal. The signal is transmitted to resonant devices embedded in the underlayment beneath the shingles The resonant devices may be resonant capture devices (17) that convert the received electromagnetic signal back to a usable voltage, or they may be wireless repeaters (41) that retransmit the electromagnetic signal to remote resonant capture devices (42), which then convert it to a voltage. This voltage is placed on an electrical grid (43) and made available at a remote location for use, storage, or placement on the public electrical grid.

Solar roof shingles and underlayment with wireless power transfer

Since their inception, micro-size light emitting diode (µLED) arrays based on III-nitride semiconductors have emerged as a promising technology for a range of applications. This paper provides an overview on a decade progresses on realizing III-nitride µLED based high voltage single-chip AC/DC-LEDs without power converters to address the key compatibility issue between LEDs and AC power grid infrastructure; and high-resolution solid-state self-emissive microdisplays operating in an active driving scheme to address the need of high brightness, efficiency and robustness of microdisplays. These devices utilize the photonic integration approach by integrating µLED arrays on-chip. Other applications of nitride µLED arrays are also discussed.

Nitride micro-LEDs and beyond--a decade progress review.

Copper (Cu) gate AlGaN/GaN high electron mobility transistors (HEMTs) with low gate leakage current were demonstrated. For comparison, nickel/gold (Ni/Au) gate devices were also fabricated with the same process conditions except the gate metals. Comparable extrinsic transconductance was obtained for the two kinds of devices. At gate voltage of -15 V, typical gate leakage currents are found to be as low as 3.5/spl times/10/sup -8/ A for a Cu-gate device with gate length of 2 /spl mu/m and width of 50 /spl mu/m, which is much lower than that of Ni/Au-gate device. No adhesion problem occurred during these experiments. Gate resistance of Cu-gate is found to be about 60% as that of NiAu. The Schottky barrier height of Cu on n-GaN is 0.18 eV higher than that of Ni/Au obtained from Schottky diode experiments. No Cu diffusion was found at the Cu and AlGaN interface by secondary ion mass spectrometry determination. These results indicate that copper is a promising candidate as gate metallization for high-performance power AlGaN/GaN HEMT.

Copper gate AlGaN/GaN HEMT with low gate leakage current

GaN Schottky barrier diodes (SBDs) with low turn-on voltage are developed for microwave rectification. The diodes with reactively-sputtered TiN electrodes have a lower turn-on voltage compared with the diodes with Ni electrode, while the on-resistance, the reverse leakage current, and the reverse breakdown characteristics are comparable to each other. Theoretically, the SBDs with TiN electrodes can enhance the efficiency of a rectenna circuit at 2.45 GHz from 84% to 89% when the turn-on voltage decreases from 1.0 to 0.5 V.

GaN Schottky Barrier Diode With TiN Electrode for Microwave Rectification

Design and fabrication of monolithic blue LED series arrays that can be operated under high ac voltage are described. Several LEDs, such as 3, 7, and 20, are connected in series and in parallel to meet ac operation. The chip size of a single device is 150 μm x 120 μm and the total size is 1.1 mm x 1 mm for a 40 (20 + 20) LED array. Deep dry etching was performed as device isolation. Two-layer interconnection and air bridge are utilized to connect the devices in an array. The monolithic series array exhibit the expected operation function under dc and ac bias. The output power and forward voltage are almost proportional to LED numbers connected in series. On-wafer measurement shows that the output power is 40 mW for 40 (20 + 20) LED array under ac 72 V.

Monolithic Blue LED Series Arrays for High‐Voltage AC Operation

A GaN Schottky diode with a lateral structure for microwave power rectification was developed on a semi-insulating silicon carbide substrate. Device evaluation showed that the turn-on voltage was around 0.8 V. The on-resistance of the diode with one finger was 25.6 Ω, the breakdown voltages for those with the field plate reached 93 V, for the wafer with a doping level of 4.0 ×1016 cm-3. The forward and reverse characteristics became stabilized after surface etching. RF measurement at 2.45 GHz showed that the capacitance of the diode was about 0.29 pF at a bias of 0 V. The value is satisfactorily small for microwave rectification. If the superior material characteristics of GaN are fully utilized, GaN Schottky diodes will play key roles in microwave power transmission applications.

https://iopscience.iop.org/article/10.1143/JJAP.48.04C095/pdf

GaN Schottky Diodes for Microwave Power Rectification

GaN metal–oxide–semiconductor field-effect transistors (MOSFETs) with recessed gate on AlGaN/GaN heterostructure are reported in which the drain and source ohmic contacts were fabricated on the AlGaN/GaN heterostructure and the electron channel was formed on the GaN buffer layer by removing the AlGaN barrier layer. Negative threshold voltages were commonly observed in all devices. To investigate the reasons of the negative threshold voltages, different oxide thickness, etching gas and bias power of inductively-coupled plasma (ICP) system were utilized in the fabrication process of the GaN MOSFETs. It is found that positive charges of around 1 × 10 12  q/cm 2 exist near the interface at the just threshold condition in both silane- and tetraethylorthosilicate (TEOS)-based devices. It is also found that the threshold voltages do not obviously change with the different etching gas (SiCl 4 , BCl 3 and two-step etching of SiCl 4 /Cl 2 ) at the same ICP bias power level (20–25 W) and will become deeper when higher bias power is used in the dry recess process which may be related to the much serious ion bombardment damage. Furthermore, X-ray photoelectron spectroscopy (XPS) experiments were done to investigate the surface conditions. It is found that N 1s peaks become lower with higher bias power of the dry etching process. Also, silicon contamination was found and could be removed by HNO 3 /HF solution. It indicates that the nitrogen vacancies are mainly responsible for the negative threshold voltages rather than the silicon contamination. It demonstrates that optimization of the ICP recess conditions and improvement of the surface condition are still necessary to realize enhancement-mode GaN MOSFETs on AlGaN/GaN heterostructure.

Yasuo Ohno

Papers

Copper gate AlGaN/GaN HEMT with low gate leakage current

GaN Schottky Barrier Diode With TiN Electrode for Microwave Rectification

Monolithic Blue LED Series Arrays for High‐Voltage AC Operation

GaN Schottky Diodes for Microwave Power Rectification

Process dependency on threshold voltage of GaN MOSFET on AlGaN/GaN heterostructure