Heng-Tung Hsu

INVESTIGATIONS OF ISOLATION IMPROVEMENT TECHNIQUES FOR MULTIPLE INPUT MULTIPLE OUTPUT (MIMO) WLAN PORTABLE TERMINAL APPLICATIONS

Various isolation improvement techniques for MIMO WLAN card bus applications consisted of three closely spaced loop antennas are presented and investigated both numerically and experimentally in this paper. The proposed techniques are easily implemented and proven effective to achieve high isolation among the antennas which is a must for MIMO terminals to receive uncorrelated signals with good system throughputs.

InAs Thin-Channel High-Electron-Mobility Transistors with Very High Current-Gain Cutoff Frequency for Emerging Submillimeter-Wave Applications

60 nm InAs high-electron-mobility transistors (HEMTs) with a thin channel, a thin InAlAs barrier layer, and a very high gate stem structure have been fabricated and characterized. The thickness of the channel, as well as that of the InAlAs barrier layer, was reduced to 5 nm. A stem height of 250 nm with a Pt-buried gate was used in the device configuration to reduce the parasitics. A high DC transconductance of 2114 mS/mm and a current-gain cutoff frequency (fT) of 710 GHz were achieved at VDS=0.5 V.

REDUCTION OF PEAK SAR IN HUMAN HEAD FOR HANDSET APPLICATIONS WITH RESISTIVE SHEETS (R-CARDS)

In this paper, reduction of peak speciflc absorption rate (SAR) for handsets with monopole type antenna through R-cards is investigated. While the numerical analysis was performed using flnite integration in time domain (FIT), real measurement has been made to validate the simulation results. Both the simulation and measurement results revealed that a minimum SAR Reduction Factor (SRF) of 60% was achieved. The good agreement between the simulation and measurement results has evidenced the efiectiveness of the proposed approach for SAR reduction in human head for handset applications.

Investigation of Impact Ionization in InAs-Channel HEMT for High-Speed and Low-Power Applications

An 80-nm InP high electron mobility transistor (HEMT) with InAs channel and InGaAs subchannels has been fabricated. The high current gain cutoff frequency (ft) of 310 GHz and the maximum oscillation frequency (fmax) of 330 GHz were obtained at VDS = 0.7 V due to the high electron mobility in the InAs channel. Performance degradation was observed on the cutoff frequency (ft) and the corresponding gate delay time caused by impact ionization due to a low energy bandgap in the InAs channel. DC and RF characterizations on the device have been performed to determine the proper bias conditions in avoidance of performance degradations due to the impact ionization. With the design of InGaAs/InAs/InGaAs composite channel, the impact ionization was not observed until the drain bias reached 0.7 V, and at this bias, the device demonstrated very low gate delay time of 0.63 ps. The high performance of the InAs/InGaAs HEMTs demonstrated in this letter shows great potential for high-speed and very low-power logic applications.

Effect of Gate Sinking on the Device Performance of the InGaP/AlGaAs/InGaAs Enhancement-Mode PHEMT

An enhancement-mode InGaP/AlGaAs/InGaAs pseudomorphic high-electron mobility transistor using platinum (Pt) as the Schottky contact metal was investigated for the first time. Following the Pt/Ti/Pt/Au gate metal deposition, the devices were thermally annealed at 325 degC for gate sinking. After the annealing, the device showed a positive threshold voltage (Vth) shift from 0.17 to 0.41 V and a very low drain leakage current from 1.56 to 0.16 muA/mm. These improvements are attributed to the Schottky barrier height increase and the decrease of the gate-to-channel distance as Pt sinks into the InGaP Schottky layer during gate-sinking process. The shift in the Vth was very uniform across a 4-in wafer and was reproducible from wafer to wafer. The device also showed excellent RF power performance after the gate-sinking process

A Koch-Shaped Log-Periodic Dipole Array (LPDA) Antenna for Universal Ultra-High-Frequency (UHF) Radio Frequency Identification (RFID) Handheld Reader

In this communication, we propose a printed log-periodic dipole array (LPDA) antenna covering the universal ultra-high-frequency (UHF) radio frequency identification (RFID) band of operation. Koch-shaped dipoles together with closely-coupled parasitic elements as directors were utilized for compactness in size and improvement in front-to-back ratio (F/B ratio). The experimental results demonstrate that the proposed antenna features good impedance match, moderate gain, better than 80% total radiation efficiency and excellent front-to-back ratio over the universal UHF RFID band from 840 MHz to 960 MHz. The antenna is well suitable for applications in RFID handheld readers.

Analysis of Wave Properties in Photonic Crystal Narrowband Filters with Left-Handed Defect

Optical wave properties for a photonic crystal narrowband filter with a left-handed defect are theoretically investigated. The filter is made of a defect layer together with two quarter-wave dielectric photonic crystals in a symmetric manner. It is found that a narrowband transmission filter can be achieved with the value of negative refractive index in defect being an even integer when the defect thickness equals the quarter wavelength. It also can be implemented when the value of negative refractive index is equal to an integer if the defect thickness is taken to be the half wavelength. As for the asymmetric one, there is only one selection rule for the quarter wavelength defect. Additionally, the effects of losses coming from the left-handed defect are also examined. The results establish guiding rules for the choice of the negative refractive index that are of technical use in designing such a filter with a left-handed defect.

A Multichanneled Filter in a Photonic Crystal Containing Coupled Defects

Optical flltering properties in a multichanneled transmis- sion fllter based on one-dimensional photonic crystal containing the coupled defects are theoretically investigated. The resonant transmis- sion peaks are designed to be located within the photonic band gap of a defect-free photonic crystal. The number of peaks is directly equal to the number of the coupled defects. The positions of resonant peaks can be tuned by varying the refractive index of the defect layer. In ad- dition, extremely resonant peaks can be produced by adding the Bragg mirrors at the front and rear sides of the structure.

A Novel Dipole Antenna Design With an Over 100% Operational Bandwidth

A novel design of a dipole antenna with ultrawideband characteristics is presented, which uses an additional shorter dipole as a matching element whose imaginary part (or susceptance) of input admittance compensates for that of the principal dipole in the frequency band of operation. Since both dipole elements efficiently radiate energy, the proposed twin-dipole antenna exhibits a higher efficiency compared to a conventional design. The measurement results have revealed that the twin-dipole is able to achieve a bandwidth of 131% (S11 <; -10 dB) from 1.6 GHz up to 7.7 GHz, with good radiation performance and more than 75% radiation efficiency.

InAs High Electron Mobility Transistors with Buried Gate for Ultralow-Power-Consumption Low-Noise Amplifier Application

An InAs/In0.7Ga0.3As composite channel high-electron-mobility transistor (HEMT) fabricated using the gate sinking technique was realized for ultralow-power-consumption low-noise application. The device has a very high transconductance of 1900 mS/mm at a drain voltage of 0.5 V. The saturated drain–source current of the device is 1066 mA/mm. A current gain cutoff frequency ( fT) of 113 GHz and a maximum oscillation frequency ( fmax) of 110 GHz were achieved at only drain bias voltage Vds = 0.1 V. The 0.08 ×40 µm2 device demonstrated a minimum noise figure of 0.82 dB and a 14 dB associated gain at 17 GHz with 1.14 mW DC power consumption.