Jung Hui Tsai

Effects of the Use of an Aluminum Reflecting and an

A GaN-based light-emitting diode (LED) with an aluminum (Al) reflecting and an SiO2 insulating layers (RILs) deposited on the naturally textured p-GaN surface is fabricated and studied. The use of RIL could enhance the current spreading performance and reduce the photon absorption by the p-pad metal. The textured surface is used to limit the total internal reflection and increase photon scattering. In this paper, effects of the use of an Al RL and/or an SiO2 insulating layer on the performance of GaN-based LEDs are systematically studied and compared in detail. At 20 mA, as compared with a conventional LED with naturally textured (planar) p-GaN surface, the studied device exhibits 12.2% (55.5%) enhancement in light output power. Additionally, a 28.5% (95%) increment of luminous flux is achieved. The studied device also shows 15.6% light intensity improvement of far-filed pattern. Experimentally, although power consumption and junction temperature are slightly increased because of the insertion of RIL structure, these drawbacks could be surpassed by the mentioned optical improvements. Therefore, for conventional GaN-based LEDs, light extraction efficiency could be further improved by the employment of RIL structure.

{\rm SiO}_{2}

Enhanced light extraction efficiency (LEE) of high-power GaN-based light-emitting diodes (LEDs) is achieved by inserting a self-assembled SiO2 nanosphere monolayer between the substrate and backside reflectors. Due to the presence of concave surfaces and photonic crystal-like air voids, downward photons emitted from multiple quantum well toward 3-D backside reflectors, could be reflected, scattered, and redirected into arbitrary directions for light extraction. These textured 3-D backside reflectors with an SiO2 nanosphere monolayer could also extract the lateral light inside device into the normal direction and improve LEE. As compared with a conventional LED without a backside reflector and an LED with a planar hybrid backside reflector, at 350 mA, the studied device with a 3-D hybrid backside reflector exhibits 136.4% (165%) and 23.6% (27.4%) enhancements in light output power (luminous flux) without the degradation of electrical properties. Higher light intensities in light emission mapping image and far-field pattern are also obtained. These results show that a textured 3-D backside reflector could be easily formed by inserting an SiO2 nanosphere monolayer to significantly enhance the performance of high-power GaN-based LEDs.

Insulating Layers (RIL) on the Performance of a GaN-Based Light-Emitting Diode With the Naturally Textured p-GaN Surface

A high-power GaN-based light-emitting diode (LED) with an inductively coupled plasma (ICP)-transferred nanohemispherical hybrid backside reflector is studied. A self-assembled 100 ± 5 nm SiO2 nanosphere monolayer is drop-coated on the backside of a sapphire substrate as a mask to transfer nanohemispherical patterns onto the backside of the sapphire substrate by ICP. Nanohemispherical patterns could be transferred to the deposited backside reflector. Thus, reflected photons could be redirected and scattered into arbitrary directions for light extraction. As compared with a conventional LED without a backside reflector, at 350 mA, the studied device exhibits a 118.2% enhancement in light output power without the degradation of electrical properties. Note that the adhesion between an ICP-transferred sapphire substrate and the hybrid backside reflector is better than when directly inserting an SiO2 nanosphere monolayer in the device. Thus, the process yield could be enhanced for applying in the solid-state lighting.

Implementation of High-Power GaN-Based LEDs With a Textured 3-D Backside Reflector Formed by Inserting a Self-Assembled

Interesting pseudomorphic high electron mobility transistors using an electrophoretic deposition (EPD) approach are fabricated and studied. Due to the low-temperature deposited gate structure, the studied device exhibits enhanced performance with less thermal damages and improved Schottky contact properties by EPD approach. In comparison with a thermal evaporation (TE) device, the higher turn-on voltage, lower gate current, and lower interface state density are observed for the EPD device. For the gate dimension of 1 × 100 μm2 the EPD device shows the higher maximum drain saturation current of 242.2 (231.9) mA/mm and excellent maximum extrinsic transconductance of 151.6 (132.5) mS/mm at 300 (420) K. Besides, the EPD device presents a comparable RF performance as compared with the TE one. Due to the improved device performance and advantages of low cost, simple process, flexible deposition on varied substrate, and adjustable metal grain size, the reported EPD approach shows the promise for high-performance device applications.

{\rm SiO}_{2}

The Characteristics of GaN-based LEDs with hybrid microhole arrays and SiO2 microspheres (MSs)/ nanoparticles (NPs) are comprehensively studied. The SiO2 MSs/NPs antireflection coating, deposited by a rapid convection deposition, acts as a passivation layer of GaN-based LEDs. Since the critical angle could be enlarged by antireflection coating, Fresnel reflection could be reduced. In addition, due to the roughened surface of SiO2 MSs/NPs antireflection coating, the scattering effect could also be increased. Thus, the light extraction efficiency could be further enhanced. As compared with a conventional LED with a planar aluminum-doped zinc oxide current spreading layer (Device A), the studied device with the proposed hybrid structure and a sputtered SiO2 passivation layer (Device E) causes a suppressed leakage current and % enhancements on light output power, external quantum efficiency, and wall-plug efficiency performance.

Nanosphere Monolayer

Al0.25Ga0.75N/GaN enhancement-mode (E-mode) metal?oxide?semiconductor high-electron-mobility transistors (MOS-HEMTs) obtained by the ozone water oxidization method are investigated in this work. Decreased gate leakage and reduced channel depletion are obtained by forming the Al2O3 dielectric layer of the MOS gate structure by a cost-effective oxidization method. Pulse current?voltage (I?V), low-frequency noise, and Hooge coefficient measurements are compared to verify the interface quality improved by the oxide passivation effect. In comparison, a conventional Schottky-gate HEMT device is also fabricated on the same epitaxial sample. Enhanced device gain, current drive density, breakdown, on/off current ratio, and high-temperature stability up to 450 K are also investigated in this work.

Enhanced Light Extraction of a High-Power GaN-Based Light-Emitting Diode With a Nanohemispherical Hybrid Backside Reflector

A new and simple Al/aluminum-doped zinc oxide (AZO) composite structure is proposed to act as a transparent conductive layer (TCL) for GaN-based LEDs. The Al/AZO composite layers effectively improve the current spreading performance, compared with an AZO TCL. Experimentally, the specific contact resistance of the studied Al/AZO LED is reduced by about 27.6%, compared with an AZO LED. In addition, under an operating current of 20 mA, the forward voltage and light output power (LOP) of the studied Al/AZO LED are decreased by 0.15 V and increased by 19.5%, respectively. Uniform and enhanced intensities are also found in light emission mapping images of the studied Al/AZO LED. Consequently, based on benefits including simple structure and improved electrical and optical properties, the studied Al/AZO TCL is suitable for GaN-based LED applications.

Performance Enhancement on an InGaP/InGaAs PHEMT With an Electrophoretic Deposition Gate Structure

In this article, the characteristics of the GaAs homojunction camel-like gate field-effect transistors with and without the gate-to-source and gate-to-drain recesses structures are first investigated and compared. As to the device without the recesses structure, a second channel within the n+-GaAs cap layer is formed at large gate bias, which could enhance the drain output current and transconductance. Furthermore, a two-stage relationship between drain current (and transconductance) versus gate voltage is observed in the recesses structure. The simulated results exhibit a maximum drain saturation current of 447 (351 mA/mm) and a maximum transconductance of 525 (148 mS/mm) in the studied device without (with) the recesses structure. Consequentially, the demonstration and comparison of the variable structures provide a promise for design in circuit applications.

Characteristics of GaN-Based LEDs With Hybrid Microhole Arrays and SiO 2 Microspheres/Nanoparticles Structures

In this article, the influence of InGaAsP spacers inserted at base-collector (B-C) junction in the InP/In0.53Ga0.47As double heterojunction bipolar transistors is demonstrated by two-dimensional semiconductor simulation. Due to the addition of an InGaAsP spacer layer, two small potential spikes are formed at B-C junction and the current blocking effect is reduced. The results exhibit that the maximum current gain increases from 30 to 374 (375) as the thickness of InGaAsP spacer layer varies from 0 to 100 Å (300 Å). On the other hand, the device with a thicker spacer layer (300 Å) could effectively improve the knee effect of the current-voltage curves as compared the other devices. In addition, the collector-emitter offset voltages less than 10 mV are observed in the three devices.