Hon Kuan

DESIGN OF DUAL-BAND BANDPASS FILTERS USING A DUAL FEEDING STRUCTURE AND EMBEDDED UNIFORM IMPEDANCE RESONATORS

In this paper, a simple method and structure to design a dual-band bandpass fllter (BPF) by using a dual feeding structure and embedded uniform impedance resonator (UIR) is presented. In this structure, two passbands can be designed individually and several transmission zeros can be created to improve the band selectivity and stopband performance. The flrst passband is determined by the dual feeding structure and the second passband is determined by the UIR. Moreover, by using the inter coupling in the UIR, the performance of the second passband can be easily tuned without degrading the flrst passband. In order to verify the design concept, two fllter

A New Tri-Band Bandpass Filter for GSM, WiMAX and Ultra-Wideband Responses by Using Asymmetric Stepped Impedance Resonators

In this paper, a design of new tri-band bandpass fllter for the application of GSM (1.8GHz), WiMAX (2.7GHz) and UWB (3.3{ 4.8GHz) is proposed. The flrst two narrow passbands are created, and the bandwidth of the third passband can be tuned by properly selecting the impedance ratio (R) and physical length ratio (u) of the asymmetric stepped-impedance resonator. To improve passband performance and form the UWB passband, a U-shape defected ground structure and extra extended coupling lines are integrated with the asymmetric SIR. Due to the three transmission zeros appearing near the passband edges, the band selectivity of the proposed fllter is much improved. The fllter was fabricated, and the measured results have a good agreement with the full-wave simulated ones.

Amplified spontaneous emission from ZnO in n-ZnO/ZnO nanodots–SiO2 composite/p-AlGaN heterojunction light-emitting diodes

This study demonstrates amplified spontaneous emission (ASE) of the ultraviolet (UV) electroluminescence (EL) from ZnO at lambda~380 nm in the n-ZnO/ZnO nanodots-SiO(2) composite/p- Al(0.12)Ga(0.88)N heterojunction light-emitting diode. A SiO(2) layer embedded with ZnO nanodots was prepared on the p-type Al(0.12)Ga(0.88)N using spin-on coating of SiO(2) nanoparticles followed by atomic layer deposition (ALD) of ZnO. An n-type Al-doped ZnO layer was deposited upon the ZnO nanodots-SiO(2) composite layer also by the ALD technique. High-resolution transmission electron microscopy (HRTEM) reveals that the ZnO nanodots embedded in the SiO(2) matrix have diameters of 3-8 nm and the wurtzite crystal structure, which allows the transport of carriers through the thick ZnO nanodots-SiO(2) composite layer. The high quality of the n-ZnO layer was manifested by the well crystallized lattice image in the HRTEM picture and the low-threshold optically pumped stimulated emission. The low refractive index of the ZnO nanodots-SiO(2) composite layer results in the increase in the light extraction efficiency from n-ZnO and the internal optical feedback of UV EL into n-ZnO layer. Consequently, significant enhancement of the UV EL intensity and super-linear increase in the EL intensity, as well as the spectral narrowing, with injection current were observed owing to ASE in the n-ZnO layer.

Ultraviolet Electroluminescence From n-ZnO–SiO

Ultraviolet (UV) light-emitting diodes composed of n-ZnO:Al-SiO2-ZnO nanocomposite/p-GaN:Mg heterojunction were fabricated on the (0002) Al2O3 substrate. A SiO2 layer embedded with ZnO nanodots was prepared on the p-type GaN using spin-on coating of SiO2 nanoparticles together with atomic layer deposition (ALD). An n-type Al-doped ZnO layer was deposited also by ALD. The SiO2-ZnO nanocomposite layer accomplishes a role of the current blocking layer and also causes, by its low refractive index, the increase in the light extraction efficiency from n-ZnO. Significant UV electroluminescence from n-ZnO was achieved at a low forward-bias current of 1.8 mA. Strong UV emission arising from impact ionization in GaN, ZnO, and GaN:Mg states was also observed at reverse breakdown bias.

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Atomic layer deposition technique and subsequent rapid thermal annealing (RTA) were implemented to grow high-quality ZnO epilayers for the fabrication of n-ZnO/p-GaN heterojunction LEDs. The X-ray diffraction measurement reveals that the ZnO epilayer has high crystallinity with c axis orientation. Transmission electron microscopy images present that the ZnO layer is a single crystal, including only a few survivals of threading dislocations, which were generated in the GaN layer deposited by metal-organic chemical vapor deposition on the c-Al2O3 substrate and most of which were eliminated at the n-ZnO/p-GaN interface. An interfacial layer 4-5 nm thick caused by the RTA treatment was observed between the n-ZnO and p-GaN layers. Room temperature UV electroluminescence (EL) at 391 nm from ZnO was achieved at a low injection current about 10 mA. It is concluded that the competition between the ELs from the n-ZnO and p-GaN (around 425 nm) may be ascribed to the ZnO/GaN interface states coupled with the differences between the n-ZnO and p-GaN in carrier concentration and light emission efficiency.

–ZnO Nanocomposite/p-GaN Heterojunction Light-Emitting Diodes at Forward and Reverse Bias

White-light electroluminescence (EL) from n-type ZnO (n-ZnO)/p-type GaN (p-GaN) heterojunction light-emitting diodes operated at reverse breakdown bias was reported. The n-ZnO epilayers were grown by atomic layer deposition on p-GaN. The electron tunneling from the deep-level states near the ZnO/GaN interface to the conduction band in n-ZnO is responsible for the reverse breakdown. The EL spectrum was composed of the blue light at 450 nm and the broadband around 550 nm, which originated from the Mg acceptor levels in p-GaN and the deep-level states near the ZnO/GaN interface, respectively. The chromaticity coordinate of the EL spectrum was (0.31, 0.36), which is very close to (0.33, 0.33) of the standard white light.

UV Electroluminescence and Structure of n-ZnO/p-GaN Heterojunction LEDs Grown by Atomic Layer Deposition

In this work, use of localized Ti deposition associated with a transparent indium-zinc-oxide (IZO) layer is proposed to serve as Schottky current blocking and current spreading layer, respectively. In addition, an inductively coupled plasma (ICP) mesa etching on the surface layer (n-GaN) of regular vertical-conducting metal-substrate GaN-based light-emitting diodes (VM-LEDs) is also proposed to further enhance current spreading of the device. Through a two-dimensional device simulator, the calculated results indicate that significant avoidance of the current-crowding effect under cathode contact pad could be obtained once the n-GaN layer etching depth and width, IZO thickness, and Schottky current blocking width have been optimized. In experiments, 1000 m 1000 m GaN-based blue LEDs with an ICP mesa etching of 250 m in width and 2 m in depth on the surface n-GaN layer, 200 m in Schottky current blocking width, and a 300-nm-thick IZO layer have the been successfully fabricated. As compared to the regular VM-LEDs without the use of the present technology, typical improvement in light emission uniformity and light output power by about 6% and 38% at an injection current of 350 mA have been obtained.

White-Light Electroluminescence From n-ZnO/p-GaN Heterojunction Light-Emitting Diodes at Reverse Breakdown Bias

Through the use of tin (Sn) based solder balls and patterned laser lift-off technique, a metal substrate technology was proposed for the fabrication of vertical-structured metal substrate GaN-based light-emitting diodes (VM-LEDs). Advantages including reserving the merits of metallic substrate and simplifying the fabrication processes of vertical-structured GaN-based LEDs were demonstrated. As compared to conventional sapphire substrate GaN-based LEDs, the fabricated VM-LEDs with an emission area of 620×620μm2 show an increase in light output power about 145.36% at 350mA with a significant decrease in forward voltage from 4.51to3.46V.

Current Spreading and Blocking Designs for Improving Light Output Power from the Vertical-Structured GaN-Based Light-Emitting Diodes

This paper proposes a parallel coupled microstrip bandpass filter (BPF) with folded stepped impedance resonator (SIR) cells on the middle layer for spurious suppression. The folded SIR cells on the middle layer create a wide stopband by canceling the second and third harmonics of the parallel coupled microstrip BPF with suppression of over -50 and -30 dB, respectively. By selecting properly the impedance ratio (R) and physical length ratio (¿) of the folded SIRs, the parallel coupled BPF with a wide stopband can be implemented without causing a package problem of degrading the complete ground plane. The proposed BPF is designed and fabricated. Good agreement between the electromagnetic (EM) simulation and measurement is obtained.

A Sn-based metal substrate technology for the fabrication of vertical-structured GaN-based light-emitting diodes

The performance of vertical-structure metallic-substrate GaN-based light-emitting diodes (VM-LEDs) with a patterned SiO2 film as the current-blocking layer (CBL) was investigated. From theoretical calculations of current and light distributions and experimental results on current–voltage (I–V) and light output power–current (L–I) characteristics, we found that SiO2 CBL inserted under the n-pad electrode increases light output power by 35.4% at 20 mA as compared with VM-LEDs without CBL. Such an improvement is attributed to the insulated CBL structure, which provides better current spreading and less photon absorption and/or reflection at the n-electrode.