Hyun-Chang Park

An RF-MEMS Switch With Low-Actuation Voltage and High Reliability

In this paper, we report a novel radio-frequency (RF) microelectromechanical systems switch with low-actuation voltage and long lifetime by adopting a design approach in which a freely moving contact pad structure opens and closes the switch through electrostatic actuation. Unlike electrostatically actuated suspended beam or bridge contact pad structures, the freely moving contact pad can be reliably operated at a low-actuation voltage of 4.5 V because actuation energy is not used in elastic deformation of a suspension. At a frequency of 50 GHz, an insertion loss of 0.5 dB and an isolation of 55 dB were obtained from the switch. Measured delay times for switch-on and switch-off were 120 and 130 ns, respectively. After 200 billion cyclic actuations with signal RF power of 0 dBm by cold switching, actuation voltages remained the same; insertion loss and isolation were maintained below 0.59 and 54 dB, respectively; and delay times for switch-on and switch-off have no change at 50 GHz. However, when the switch slants to any direction over 28deg, the actuation voltage increases over 5 V because movement of the movable lower contact pad is obstructed by friction between the movable contact pad and the guard poles

Facile Route to NiO Nanostructured Electrode Grown by Oblique Angle Deposition Technique for Supercapacitors

We report an efficient method for growing NiO nanostructures by oblique angle deposition (OAD) technique in an e-beam evaporator for supercapacitor applications. This facile physical vapor deposition technique combined with OAD presents a unique, direct, and economical route for obtaining high width-to-height ratio nanorods for supercapacitor electrodes. The NiO nanostructure essentially consists of nanorods with varying dimensions. The sample deposited at OAD 75° showed highest supercapacitance value of 344 F/g. NiO nanorod electrodes exhibits excellent electrochemical stability with no degradation in capacitance after 5000 charge-discharge cycles. The nanostructured film adhered well to the substrate and had 131% capacity retention. Peak energy density and power density of the NiO nanorods were 8.78 Wh/kg and 2.5 kW/kg, respectively. This technique has potential to be expanded for growing nanostructured films of other interesting metal/metal oxide candidates for supercapacitor applications.

94-GHz Log-Periodic Antenna on GaAs Substrate Using Air-Bridge Structure

We fabricated a compact, 94-GHz log-periodic planar antenna on GaAs substrate. We used millimeter-wave monolithic integrated circuit (MMIC) process technology to fabricate the on-chip antenna. An air-bridge crossover is used to replace the conventional bonded structure for crisscross connection, which is compatible with active devices and circuits. The fabricated antenna chip size is 1.2 times 2.6 mm2 . Measurement results show that this antenna can operate from 87 to 99.5 GHz with its return loss better than -10 dB. At 94 GHz, the log-periodic antenna has a return loss of -11 dB and 4.8 dBi of gain.

Optical absorption of Co2+ ions in In2S3 thin films

Abstract Polycrystalline thin films of undoped and Co-doped In 2 S 3 have been grown by the spray pyrolysis method and the flash evaporation method. The optical absorption spectra of these thin films have been investigated in the wavelength region from 500 to 2500 nm at 298 K. From the optical absorption measurements, the optical energy gaps of the thin films have been determined and the optical absorption of Co 2+ ions in In 2 S 3 thin films has been studied. In particular, the effects of cobalt doping on the optical properties of In 2 S 3 thin films have been presented.

A Novel 94-GHz MHMET-Based Diode Mixer Using a 3–dB Tandem Coupler

We report a high-performance 94-GHz monolithic millimeter-wave integrated-circuit diode mixer using metamorphic high-electron mobility transistor (MHEMT) diodes and a coplanar waveguide tandem coupler. A novel single-balanced structure of diode mixer is proposed in this paper, where a 3-dB tandem coupler with two sections of parallel-coupled line and air-bridge crossover structures are used for wide frequency operation. The fabricated mixer exhibits excellent local oscillator-radio-frequency (LO-RF) isolation, greater than 30 dB, in the 5-GHz bandwidth of 91-96 GHz. A good conversion loss of 7.4 dB is measured at 94 GHz. The proposed MHEMT-based diode mixer shows superior LO-RF isolation and conversion loss to those of the W-band mixers reported to date.

A Novel 94-GHz MHEMT Resistive Mixer Using a Micromachined Ring Coupler

In this letter, we present a high performance 94-GHz millimeter-wave monolithic integrated circuit resistive mixer using a 70-nm metamorphic high electron mobility transistor (MHEMT) and micromachined ring coupler. A novel three-dimensional structure of a resistive mixer was proposed in this work, and the ring coupler with the surface micromachined dielectric-supported air-gap microstrip line structure was used for high local oscillator/radio frequency (LO-RF) isolation. Also, the LO-RF isolation was optimized through the simulation. The fabricated mixer has excellent LO-RF isolation, greater than 29 dB, in 2-GHz bandwidth of 93-95GHz. The good conversion loss of 8.9dB was measured at 94GHz. To our knowledge, compared to previously reported W-band mixers, the proposed MHEMT-based resistive mixer using a micromachined ring coupler has shown superior LO-RF isolation and conversion loss

Recovery of Dry-Etch Damage in Gallium-Nitride Schottky Barrier Diodes

We investigated the effects of recovery schemes on the characteristics of GaN Schottky diodes damaged by inductively coupled Cl 2 /H 2 /Ar/CH 4 plasma etching. The recovery schemes included N 2 plasma exposure, N 2 plasma plus annealing at 600°C in N 2 , and annealing at 700°C in N 2 . Ga-deficient GaN surface layers were observed after etching the GaN, and the Ga/N stoichiometry at the surface was most strongly affected by the Cl 2 flow rate among the process variables. The samples annealed at 700°C showed a clear improvement in diode characteristics and a complete restoration of the Ga/N ratio from ca. 0.87 to ca. 1.0 at the surface layers, as measured by Auger electron spectroscopy. For all other damage-recovery schemes, the samples showed no significant enhancement in diode characteristics and incomplete restoration of Ga/N stoichiometry at the surface layers.

Fabrication of new micromachined transmission line with dielectric posts for millimeter-wave applications

This paper describes a new GaAs-based surface-micromachined microstrip line supported by dielectric posts and with an air gap between the signal line and the ground metal. This new type of dielectric post and air-gapped microstripline (DAML) structure was developed using surface micromachining techniques to provide an easy means of air-bridge connection between the signal lines and to achieve low losses in the millimeter-wave frequency band with a wide impedance range. Each DAML is fabricated with a length of 5 mm. By elevating the signal lines from the substrate using micromachining technology, the substrate dielectric loss can be reduced. Compared with conventional microstrip transmission lines, which show over 10 dB cm−1 loss, the loss of a DAML can be reduced to 1.1 dB cm−1 at 50 GHz.

Synthesis of ZnO nanorods using different precursor solutions and their two terminal device characterization

Abstract ZnO nanostructures such as nanoparticles, nanorods, nanotubes and nanoplates were prepared by the novel chemical solution route using different combinations of precursors such as zinc nitrate [Zn(NO3)2·6H2O] and sodium hydroxide [NaOH], zinc nitrate [Zn(NO3)2·6H2O] and hexamethylenetetramine (HMT) (C6H12N4), and zinc acetate [Zn(CH3COO)2·2H2O] and NaOH at low temperature. The effects of temperature and bath concentration for the synthesis of various ZnO nanostructures have been studied. Synthesized powders were characterized by X-ray diffraction, scanning electron microscope and photoluminescence. The crystalline structure, size and shape of the synthesized particles have been analyzed. Energy band gap values and photoluminescence emissions of the nanostructures were found to change according to the kind of the particles. Rod shaped ZnO have been obtained from the combination of zinc nitrate and HMT. The synthesized ZnO nanorods were aligned between lithographically patterned finger type metal contacts and their current–voltage response were measured.

94 GHz CPW branch-line bandpass filter for planar integrated millimeter-wave circuits

We report the 94 GHz CPW branch-line bandpass filter for planar integrated millimeter- wave circuits. The basic idea is to use the branch-line coupler as a transversal filtering section by connecting the coupled ports to the open load stubs and taking the isolated port as the output node. The 94 GHz bandpass filter exhibits an insertion loss of 2.5 dB with an 11.7 % 3 dB relative bandwidth at a center frequency of 94 GHz and the return loss is better than -18 dB at a center frequency. The designed and fabricated 94 GHz bandpass filter shows the good performance for planar integrated millimeter-wave circuits.