Yasuo Ohno

Copper gate AlGaN/GaN HEMT with low gate leakage current

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.

Monolithic Blue LED Series Arrays for High‐Voltage AC Operation

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.

GaN Schottky Diodes for Microwave Power Rectification

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.

New application of microwave power transmission for wireless power distribution system in buildings

A wireless power distribution system (WPDS) in buildings is proposed as the application of a microwave power transmission (MPT). Problems against a commercial MPT system, which are frequency regulation and shortage of technically and economically effective application, can be solved with the WPDS in buildings. The microwave propagates in the deck plate waveguide. Total cost of the building can be reduced and the benefit of use of electricity with the WPDS is increased.

Evaluation of a Gate-First Process for AlGaN/GaN Heterostructure Field-Effect Transistors

In this study, we evaluated the annealing temperature and time-dependent electrical properties of AlGaN/GaN heterostructure field-effect transistors (HFETs) utilizing TiN/W/Au as the gate electrode. With the annealing temperature increasing from 750 to 900 °C for the annealing time of 1 min, the sheet resistance of TiN/W/Au films increased gradually while that of the ohmic contact was minimum (0.66 Ω mm) at 800 °C. From the current–voltage characteristics of the Schottky diode and HFETs, it is demonstrated that annealing at 800 °C showed the lowest on-resistance and highest maximum drain current. By prolonging the annealing from 0.5 to 10 min at 800 °C, good device performance was achieved when the annealing time was 1 and 3 min, while the device performance degraded showing an increased gate leakage current and gate resistance with increasing annealing time. These results demonstrated that the TiN/W/Au gate, which can withstand 800 °C annealing for a short time, is suitable for application in the gate-first process for AlGaN/GaN HFETs.

Investigation on Current Collapse of AlGaN/GaN HFET by Gate Bias Stress

The mechanism of current collapse of AlGaN/GaN heterojunction field-effect transistors (HFETs) was investigated by gate bias stress with and without illumination. It is clarified that there are two positions where negative charges accumulate, at the gate edge and in the bulk epi-layer. In the gate-edge mode, the charge comes either through the passivation film or the AlGaN layer, depending on the resistance of the films. Reduction of leakage current in the passivation film will be important to suppress the surface-related collapse.

Characterization of GaN MOSFETs on AlGaN/GaN Heterostructure With Variation in Channel Dimensions

GaN MOSFETs were developed on an AlGaN/GaN heterostructure 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. For devices with different types and sizes, discrepant field-effect mobilities were observed and the origins of the discrepancy were analyzed. One reason causing the discrepancy is the discrepancy of gate dimension between the design and fabrication. In devices with only mesa as the device isolation, the real channel width would be larger than the mesa width because a parallel channel might have formed in the isolation region just outside the device mesa. Boron ion implantation was found to be effective to cutoff the current path in the isolation region. Another reason causing the discrepancy is that the real channel length would be larger than the designed one owing to the lithography and gate dry recess process. To extract the correct mobility and effective channel length of the GaN MOSFET fabricated on AlGaN/GaN heterostructure with variation in the channel dimensions, several methods were proposed and compared basing on ring-type MOSFETs.

GaN Schottky Barrier Diode With TiN Electrode for Microwave Rectification

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.

S-Parameter Analysis of GaN Schottky Diodes for Microwave Power Rectification

A gallium nitride (GaN) Schottky diode on semi-insulating SiC has been developed for microwave power rectification. A 2 μm x 100 μm finger type diode shows ON resistance of 8.2 Ω and depletion capacitance of 0.36 pF at 0 V with breakdown voltage of 90 V. S-parameter analysis separated the ON resistance into the intrinsic part and the access region part, which will benefit device optimization. With a 10-finger diode, RF/DC conversion efficiency of 74.4% is obtained at input power of 5 W and frequency of 2.45 GHz.

Schottky Barrier Height Determination by Capacitance-Voltage Measurement on n-GaN with Exponential Doping Profile

A capacitance–voltage (C–V) method was developed to extrapolate the Schottky barrier height on n-GaN with exponential carrier concentration profile. The carrier concentration profile of the unintentionally doped GaN was determined by C–V measurement to be exponential. On the basis of this profile, one-dimensional Poissons equation was calculated to obtain the relation between bias voltage and depletion width. Schottky barrier height was obtained by fitting the curves of 1/C2 and V with the experimental one, using the Schottky barrier height itself as a fitting parameter.