A. Abdul Rahman

Dual-Functional On-Chip AlGaAs/GaAs Schottky Diode for RF Power Detection and Low-Power Rectenna Applications

A Schottky diode has been designed and fabricated on an n-AlGaAs/GaAs high-electron-mobility-transistor (HEMT) structure. Current-voltage (I–V) measurements show good device rectification, with a Schottky barrier height of 0.4349 eV for Ni/Au metallization. The differences between the Schottky barrier height and the theoretical value (1.443 eV) are due to the fabrication process and smaller contact area. The RF signals up to 1 GHz are rectified well by the fabricated Schottky diode and a stable DC output voltage is obtained. The increment ratio of output voltage vs input power is 0.2 V/dBm for all tested frequencies, which is considered good enough for RF power detection. Power conversion efficiency up to 50% is obtained at frequency of 1 GHz and input power of 20 dBm with series connection between diode and load, which also shows the device’s good potential as a rectenna device with further improvement. The fabricated n-AlGaAs/GaAs Schottky diode thus provides a conduit for breakthrough designs for RF power detectors, as well as ultra-low power on-chip rectenna device technology to be integrated in nanosystems.

Gateless-FET undoped AlGaN/GaN HEMT structure for liquid-phase sensor

A gateless field-effect-transistor (FET) device fabricated on undoped AlGaN/GaN high-electron-mobility-transistor (HEMT) structure is investigated as a liquid-phase sensor. Good gate controllability for typical current-voltage (I-V) characteristics of FET is observed. This result shows that an undoped-AlGaN surface at the open-gate area is effectively controlled by the isolated gate voltage via chemical solution. Stable pH sensing operation in aqueous solution is observed where this device exhibits a high linear sensitivity of 3.88 mA/mm/pH at drain-source voltage, VDS = 5 V. Due to the occurrence of large leakage current, the Nernstians like sensitivity is not observed. It is also found that the device is sensitive to changes in electrostatic boundary conditions of the polar liquids. This indicates that the change in dipole moment in each liquid causes the potential change at AlGaN surface.

RF characterization of planar dipole antenna for on-chip integration with GaAs-based schottky diode

The design and RF characteristics of planar dipole antenna facilitated with coplanar waveguide structure was presented. The dipole antennas were fabricated on semi-insulated GaAs substrates by using standard photolithography and lift-off process. As expected, it can be seen that the fundamental resonant frequency shift to higher frequency when the length of antenna decreases. Interestingly, the resonant frequencies of antenna are almost unchanged with the variation of antenna width and metal thickness. The width of dipole antenna and metal thickness only has an effect on the magnitude of return loss where the magnitude increases to more negative value with the increase of width and decrease of metal thickness. One of the most promising applications of our proposed dipole antenna is the capability to be integrated directly with AlGaAs/GaAs Schottky diode without any insertion of matching circuit between them.

Measuring service quality in online shopping: A case study of e-retailing in Iran

One of the key challenges in online businesses is how to measure service quality in order to have a better understanding of its consequences which holds a significant importance to customer satisfaction. In this paper, we ranked the dimensions of service quality that affect the customerspsila expectation in online shopping in Iran from the customerspsila perspective. The measurements used were based on the widely accepted SERVQUAL model which is the most common method for measuring service quality. A descriptive statistics analysis was used to evaluate the level of service quality of Iranianpsilas online shops from the customerspsila point of view. A comparison of service quality factors has also been done between Iranian and American customers.

Modeling, attitude estimation, and control of Hexarotor micro aerial vehicle (MAV)

This paper describes modeling, attitude estimation, attitude control, and altitude control for Hexarotor micro aerial vehicle (MAV). Hexarotor has high potential to carry more payload and high maneuverability compared to quadrotor. This paper proposes a new approach for attitude estimation in realtime system by using a nonlinear complementary observer based on special orthogonal group of rotation matrices SO(3), rather than conventional extended Kalman filter (EKF), to exploit the near-global convergence property of the observer and reduced computational complexity. The proposed attitude controller is based on an inner-outer loop structure PI+PID (proportional-integral plus proportional-integral-derivative) control method to reduce the overshoot effects, leads to faster response, improve robustness and transient response, whilst the proposed altitude controller is based on classical closed loop PID control system. Simulation results will be presented showing the proposed controller achieves excellent dynamical performance and finally, the experimental results, both in indoor and outdoor flying environment demonstrate the effectiveness of the proposed attitude observer and controller in real flight condition.

Fabrication and Characterization of Planar Dipole Antenna Integrated with GaAs Based-Schottky Diode for On-chip Electronic Device Application

The design and RF characteristics of planar dipole antennas facilitated with coplanar waveguide (CPW) structure on semi-insulated GaAs are performed and confirmed to work in super high frequency (SHF) range. As expected, the fundamental resonant frequency shifts to higher frequency when the length of antenna decreases. Interestingly, the resonant frequencies of antenna are almost unchanged with the variation of antenna width and metal thickness. It is shown experimentally that return loss down to −54 dB with a metal thickness of 50 nm is obtainable. Preliminary investigation on design, fabrication, and DC and RF characteristics of the integrated device (planar dipole antenna + Schottky diode) on AlGaAs/GaAs HEMT structure is presented. From the preliminary direct irradiation experiments using the integrated device, the Schottky diode is not turned on due to weak reception of RF signal by dipole antenna. Further extensive considerations on the polarization of irradiation etc. need to be carried out in order to improve the signal reception. These preliminary results provide a new breakthrough for on-chip electronic device application in nanosystems.

Fabrication and Characterization of GaN-Based Two Terminal Devices for Liquid Sensing

Gallium Nitride (GaN) based materials are highly suitable for liquid-phase sensor applications due to their chemical stability and high internal piezoelectric polarization. The sensitivity of GaN surfaces in aqueous solutions and polar liquids has been investigated. For this purpose, two terminal devices fabricated on bulk Si doped-GaN structures and undoped-AlGaN/GaN heterostructures with unpassivated open area are used to measure the responses to the changes of the H+ concentration in aqueous solutions and the dipole moment in polar liquids. The I–V characteristics show that the devices are able to distinguish the variations of pH. It is observed that the drain current decreases linearly with pH for both device structures. Evaluating the sensitivity in aqueous solutions at VDS = 2V, a quite large current change is obtained for both structures. For the response to polar liquids, it is also found that the drain current decreases with the dipole moments. The results indicate that both devices are capable of distinguishing molecules with different dipole moments.

Fabrication of open gate structure on GaN-based HEMT for pH sensing

In recent years, there has been demonstrated a rapid progress of the AlGaN/GaN system applications in optical and electronic device research due to its unique features. One of unique features of AlGaN/GaN system includes chemically stable where stability of surface is essentially important for liquid-phase sensor applications. Besides, AlGaN/GaN also allows highly sensitive detection of surface phenomena with the presence of a high-density two-dimensional electron gas (2DEG) in the high electron mobility transistor (HEMT) structure [1]. The material itself is found as environment-friendly and bio-friendly. Furthermore, the GaN material allows sensing operations at high temperatures due to large bandgap energies [2]. In future, realization of wireless sensor chips by on-chip co-integration of circuits for sensor signal processing and wireless communications also will be possible with the HFET technology. Regarding to these features, GaN-based HEMT motivates an interesting area to be studied particularly to be used in biochemical sensing applications.

Fabrication and Characterization of n-AlGaAs/GaAs Schottky Diode for Rectenna Device Application

Schottky diode was designed and fabricated on n-AlGaAs/GaAs high electron mobility transistor (HEMT) structure for rectenna device application. Rectenna is one of the most potential devices to form the wireless power supply which is really good at converting microwaves to DC. The processing steps used in the fabrication of Schottky diode were the conventional steps used in standard GaAs processing. Current-voltage (I–V) measurements showed that the device had rectifying properties with a barrier height of 0.5468 eV for Ni/Au metallization. The fabricated Schottky diode detected RF signals and the cut-off frequency up to 20 GHz was estimated in direct injection experiments. These preliminary results will provide a breakthrough for the direct integration with antenna towards realization of rectenna device application.