Norikazu Ito

Improved External Efficiency InGaN-Based Light-Emitting Diodes with Transparent Conductive Ga-Doped ZnO as p-Electrodes

Transparent conductive Ga-doped ZnO (ZnO:Ga) was fabricated to serve as p-contacts of InGaN-based light-emitting diodes (LEDs) using molecular-beam epitaxy. As-grown ZnO:Ga films typically have resistivities of ?=2-4?10-4 ??cm, and over 80% transparency in the near UV and visible wavelength ranges. The current-voltage characteristics between as-grown ZnO:Ga contacts and p-GaN layers were ohmic. The brightness of LEDs fabricated with ZnO:Ga p-contacts was nearly double compared to LEDs with conventional Ni/Au p-contacts. We obtained the external efficiency as high as 20.8% in the case of the near UV LED. The forward voltage at 20 mA was found not to increase even after the lamp LED with ZnO:Ga were kept for 80 h in high humidity and high temperature environments.

Recessed-Gate Enhancement-Mode GaN MOSFETs With a Double-Insulator Gate Providing 10-MHz Switching Operation

Recessed-gate GaN metal-oxide-semiconductor field-effect transistors with a double-insulator gate configuration demonstrate 10-MHz switching operation of which delay time is <;35 ns. The recess structure is fabricated by etching the Al0.19Ga0.81N layers to expose their underlying AlN layers. The devices include a thermally oxidized AlN layer onto which an Al2O3 film is formed by atomic layer deposition, which works as a gate insulator. This structure performs enhancement-mode operation with a typical threshold voltage of 1.4 V. A maximum drain current of 158.3 mA/mm is achieved at 6 V gate bias and maximum transconductance is 52.1 mS/mm at 10 V drain bias.

Microcrystalline Silicon Solar Cells Fabricated using Array-Antenna-Type Very High Frequency Plasma-Enhanced Chemical Vapor Deposition System

An array-antenna-type very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) system based on a multisubstrate deposition technique has been developed for the large area (>1 m2), high-throughput and low-cost production of thin-film silicon solar cells. A hydrogenated microcrystalline silicon ( µc-Si:H) solar cell fabricated at around 20 Pa showed an initial efficiency of 7.44% for a deposition rate of 0.32 nm/s with evaporated Ag as the back contact. The uniform formation of each layer was observed, and the quantum efficiency spectrum after exposure to air indicates high-stability high-quality film formation. The advantage of the novel array antenna method is discussed in terms of the material property as well as production cost.

Optical Gain and Optical Internal Loss of GaN-Based Laser Diodes Measured by Variable Stripe Length Method with Laser Processing

We demonstrate a new technique for the variable stripe length (VSL) method by which the optical gain and optical internal loss of GaN-based laser diodes (LDs) can be directly measured. In the technique, the laser processing is utilized for varying the excitation length. The excitation length of GaN-based LDs can be varied by directly processing its p-electrode with high-power laser irradiation. From the results of the measurements, it was revealed that the optical internal loss of GaN-based LDs was strongly affected by the layer structure.

A 13.56 MHz wireless power transmission systems with enhancement-mode GaN high electron mobility transistors

Enhancement-mode GaN-HEMT devices with a newly developed recessed-gate structure were fabricated. These devices were capable of operating at up to 30 MHz switching. A wireless power transmission (WPT) was adopted for a potential application of these GaN devices, because high-frequency (f) switching devices are expected to improve the power-transfer efficiency (η) of the WPT. A GaN-based E-class amplifier WPT system achieved 10W output power and η =63.5 % under the operating conditions of f =13.56 MHz, duty=50%, and a load resistance of 10Ω.